Allergic Conjunctivitis
Evidence-based assessment and management of allergic conjunctivitis. Comprehensive guide covering etiology, pathogenesis, classification across all subtypes, diagnosis, and treatment protocols for optometry practice.
Last updated: March 2026
Allergic conjunctivitis: diffuse bilateral conjunctival injection, chemosis, eyelid oedema, watery discharge, and fine papillary reaction on the palpebral conjunctiva
Allergic conjunctivitis is the most common form of ocular allergy, affecting an estimated 15–40% of the global population and representing one of the most frequent presentations in primary eye care. It encompasses a spectrum of IgE-mediated and, to a lesser extent, T-cell–mediated hypersensitivity disorders of the conjunctival mucosa, unified by the cardinal symptom of bilateral ocular pruritus and a papillary conjunctival reaction. The condition ranges from mild, self-limiting seasonal episodes — seasonal allergic conjunctivitis (SAC) and perennial allergic conjunctivitis (PAC) — to severe, sight-threatening forms such as vernal keratoconjunctivitis (VKC) and atopic keratoconjunctivitis (AKC). Contact allergic (delayed-type) reactions and giant papillary conjunctivitis (GPC) are mechanistically distinct but clinically overlap. The overwhelming majority of allergic conjunctivitis presentations in primary eye care represent SAC or PAC — both benign and manageable without corneal complications — but accurate subtype classification is essential as it directly determines the urgency of referral, the choice of therapeutic agent, and the long-term prognosis. Allergic conjunctivitis is frequently underdiagnosed when it presents as a component of allergic rhinoconjunctivitis, where nasal and ocular symptoms occur together and the ocular component is attributed to a cold or dry eye.
Seasonal Allergens (SAC)
- Tree pollen: Birch, oak, cedar, olive, and plane tree pollens are major triggers in temperate climates; release occurs from late winter through spring (February–May); birch is the dominant sensitiser in Northern Europe and Scandinavia
- Grass pollen: Timothy grass, ryegrass, Bermuda grass; peak season May–August; the most prevalent sensitising pollen globally; strongly associated with allergic rhinoconjunctivitis
- Weed pollen: Ragweed (Ambrosia species) dominant in North America (August–October); mugwort in Europe; Parietaria in Mediterranean region; tends to cause late-summer to autumn symptoms
- Fungal spores: Alternaria and Cladosporium are summer-autumn airborne spores that sensitise atopic individuals; Aspergillus and Penicillium are predominantly indoor moulds associated with PAC; often co-sensitised with pollen
Perennial Allergens (PAC)
- House dust mites (HDM): Dermatophagoides pteronyssinus and D. farinae; the most prevalent indoor perennial allergen worldwide; faecal particles (<10 µm) become airborne during bed-making, vacuuming, and in heated rooms; particularly relevant in Singapore’s hot, humid climate, where HDM colonisation of bedding is extensive year-round
- Animal dander and saliva: Cat (Fel d 1 — secretoglobin protein from sebaceous glands) and dog (Can f 1 — salivary lipocalin) are the most clinically important; allergen is sticky and persists on surfaces and clothing for months after animal removal; cat allergen is significantly more potent and pervasive than dog allergen
- Cockroach allergens: Blattella germanica (German cockroach) and Periplaneta americana; important in tropical urban environments including Singapore; faecal particles and cuticle fragments sensitise atopic individuals; associated with inner-city asthma and perennial rhinoconjunctivitis
- Indoor moulds: Aspergillus, Penicillium, and Fusarium species in damp indoor environments; less common but clinically relevant cause of PAC, particularly in humid tropical climates
Contact Allergens (Type IV — Delayed Hypersensitivity)
- Topical ophthalmic medications: Preservatives (benzalkonium chloride — BAC), aminoglycoside antibiotics (neomycin), topical glaucoma medications (brimonidine, latanoprost); a leading cause of contact allergic blepharoconjunctivitis in patients on chronic topical therapy
- Contact lens solutions: Thimerosal (now largely discontinued), chlorhexidine, and preserved multipurpose solutions; relevant in contact lens wearers presenting with chronic conjunctival irritation
- Cosmetics and skincare products: Eyeliner, mascara, eye shadow, nail polish (transferred to eyelid by finger contact), fragrances, and preservatives; periocular skin and conjunctival contact allergy
- Metals: Nickel (in spectacle frame nose pads and temples) and thimerosal; contact dermatitis of the periocular skin
IgE-Mediated (Type I) Hypersensitivity — SAC and PAC
- Sensitisation phase: On first allergen exposure, conjunctival dendritic cells (Langerhans cells) process and present allergen peptides to naïve CD4+ T-helper cells in the regional lymph nodes; under the influence of IL-4 from mast cells and basophils, T-cells differentiate into the Th2 phenotype, producing IL-4, IL-5, and IL-13; these cytokines drive B-cell class switching from IgM/IgG to allergen-specific IgE; secreted IgE binds to high-affinity FcεRI receptors on conjunctival mast cells (~50 million mast cells in the conjunctival substantia propria) and circulating basophils, priming the immune system for subsequent allergen challenge
- Early-phase allergic response (minutes): Re-exposure to the same allergen causes cross-linking of adjacent IgE molecules on mast cell surfaces; this triggers rapid mast cell degranulation, releasing preformed mediators stored in secretory granules: histamine (principal mediator of acute itch, hyperaemia, and tearing), tryptase, chymase, prostaglandin D2 (PGD2), cysteinyl leukotrienes (LTC4, LTD4, LTE4), and platelet-activating factor (PAF); symptoms appear within seconds to minutes: intense itch, conjunctival injection, chemosis, and watery discharge
- Histamine receptor signalling: Histamine binds H1 receptors on conjunctival sensory nerve endings, producing itch via C-fibre activation and a neurogenic flare response; H1 receptors on conjunctival vasculature produce vasodilation (hyperaemia) and increased vascular permeability (chemosis and oedema); H2 receptor activation amplifies vascular permeability; the conjunctiva has one of the highest densities of mast cells of any mucosal tissue
- Late-phase allergic response (4–8 hours): Mast cell–derived chemokines (eotaxin, RANTES, MCP-1) recruit eosinophils, neutrophils, basophils, and Th2 lymphocytes to the conjunctival stroma; these cells amplify and sustain inflammation beyond the initial IgE-mediated trigger; IL-5 from Th2 cells promotes eosinophil survival and degranulation; the late phase is responsible for prolonged conjunctival inflammation persisting hours to days after allergen exposure
- Papillary reaction: Repeated allergen exposures and sustained Th2 inflammation lead to epithelial hyperplasia and inflammatory cell infiltration of the conjunctival substantia propria, elevating the conjunctival epithelium into fine papillae visible on the palpebral conjunctiva; in SAC/PAC, papillae remain small (<1 mm) and are histologically distinct from the giant papillae of VKC
- Goblet cell and tear film changes: Chronic allergic inflammation increases goblet cell density initially but then causes goblet cell apoptosis and mucin disruption with prolonged disease; IL-4 and IL-13 stimulate mucin overproduction early in disease, producing the characteristic stringy mucoid discharge; tear film instability compounds ocular surface inflammation
T-Cell–Mediated (Type IV) Hypersensitivity — Contact Allergic Conjunctivitis
Contact allergic blepharoconjunctivitis involves a delayed-type hypersensitivity reaction in which haptens (low-molecular-weight chemicals from topical medications or cosmetics) bind covalently to conjunctival epithelial proteins, forming complete antigens. Langerhans cells present these neoantigens to naïve T-cells in regional lymph nodes, generating allergen-specific memory CD4+ and CD8+ T-cells. On re-exposure, sensitised T-cells release IFN-γ, TNF-α, and IL-17, producing a lichenoid or eczematous inflammatory response with a latency of 24–72 hours. This mechanism is IgE-independent and does not respond to antihistamines alone; withdrawal of the offending agent and short-course topical steroid are the mainstays of treatment.
Clinical Subtypes
- Seasonal Allergic Conjunctivitis (SAC): The most common subtype; IgE-mediated; triggered by airborne seasonal allergens (pollen); bilateral; acute-to-subacute onset during pollen season; complete remission between seasons; no corneal involvement; no permanent sequelae; fine papillary reaction; responds well to topical antihistamines and mast cell stabilisers
- Perennial Allergic Conjunctivitis (PAC): Year-round low-grade symptoms with seasonal exacerbations; triggered by perennial allergens (HDM, pet dander, cockroach, moulds); milder per episode than SAC but persistent; may have subclinical conjunctival inflammation between symptomatic periods; same management approach as SAC
- Vernal Keratoconjunctivitis (VKC): Severe chronic recurrent bilateral disease predominantly in young atopic males; hot/dry climates; giant cobblestone papillae; Horner-Trantas dots; shield ulcer; sight-threatening; requires specialist management — covered in a dedicated section
- Atopic Keratoconjunctivitis (AKC): Severe chronic bilateral disease in adults (20–50 years) with atopic dermatitis; inferior fornix scarring; corneal neovascularisation; stellate anterior subcapsular cataract; risk of keratoconus; does not remit — covered in a dedicated section
- Giant Papillary Conjunctivitis (GPC): Mechanically triggered; contact lens or prosthesis wearers; large tarsal papillae; mucoid discharge; lens intolerance; responds to lens holiday and mast cell stabilisers — covered in a dedicated section
- Contact Allergic Blepharoconjunctivitis: Type IV (delayed) hypersensitivity; history of topical medication, cosmetic, or lens solution exposure; eczematous eyelid skin; inferior papillary or follicular conjunctival reaction; onset 24–72 hours after contact; IgE-independent; responds to allergen removal and short-course topical steroid
Subtype Comparison
| Subtype | Age / Sex | Pattern | Cornea | Severity |
|---|---|---|---|---|
| SAC | Any age; M=F | Seasonal | Spared | Mild |
| PAC | Any age; M=F | Perennial | Spared | Mild–Moderate |
| VKC | 5–15 yrs; M > F | Seasonal / perennial | Shield ulcer, SEIs | Severe |
| AKC | 20–50 yrs; M > F | Perennial, chronic | Neovascularisation, scarring | Severe |
| GPC | CL wearers; any | Perennial (while wearing) | Spared | Moderate |
| Contact allergic | Any; medication users | After exposure (24–72 h) | Spared (usually) | Mild–Moderate |
- Personal history of atopic disease: Atopic dermatitis, allergic rhinitis, and asthma are strongly associated with allergic conjunctivitis; the atopic march — eczema in infancy progressing to rhinoconjunctivitis and asthma in childhood — is the prototypical risk trajectory; concurrent rhinitis is present in over 80% of SAC patients (allergic rhinoconjunctivitis)
- Family history of atopy: Polygenic inheritance of atopic diathesis; having one atopic parent approximately doubles the risk; two atopic parents increases risk to 40–60%; key genes include the IL-4/IL-13 locus on chromosome 5q31, the IgE receptor gene on chromosome 11q, and HLA-DR loci
- Elevated total serum IgE: Elevated baseline serum IgE reflects the overall burden of atopic sensitisation; values >150 IU/mL in adults are associated with higher allergen-specific IgE titres and more severe ocular symptoms; normal IgE does not exclude allergic conjunctivitis
- Allergen sensitisation: Specific IgE or positive skin prick test to at least one environmental allergen is required for IgE-mediated disease; polysensitisation (sensitisation to three or more allergens) is associated with more severe and perennial disease
- Urban living and air pollution: Diesel exhaust particles (DEP) and ground-level ozone enhance allergen immunogenicity and directly irritate the conjunctival mucosa; climate change — resulting in longer and more intense pollen seasons, increased atmospheric CO2 (which increases pollen production), and higher temperatures — is increasing the global prevalence of allergic rhinoconjunctivitis
- Tropical climate (Singapore-specific): Singapore’s hot, humid, year-round climate creates conditions for persistently high house dust mite (HDM) loads; HDM is the dominant perennial sensitiser in Singapore and Southeast Asia; PAC from HDM is the predominant form of allergic conjunctivitis in the region, in contrast to pollen-dominated SAC in temperate climates
- Contact lens wear: Disrupts the conjunctival epithelial barrier and tear film; creates a mechanical substrate for allergen accumulation; increases mast cell reactivity; predisposes to GPC; worsens pre-existing allergic conjunctivitis symptoms
- Topical ophthalmic medication use: Patients on chronic multi-drop therapy (glaucoma, post-operative regimens) are at increased risk of contact allergic blepharoconjunctivitis from preservatives (BAC) or drug antigens (brimonidine, neomycin); the inferior palpebral conjunctiva is most affected
- Hygiene hypothesis: Reduced early childhood exposure to microbial antigens (farm environments, large sibships, infectious diseases) is associated with higher atopic sensitisation rates; supports the notion that excessive environmental hygiene shifts immune responses towards Th2 polarisation
- Stress and sleep deprivation: Psychological stress promotes mast cell degranulation and reduces the threshold for allergic responses; sleep deprivation elevates inflammatory cytokines; both are relevant triggers for acute allergic flares in sensitised individuals
External Examination
- Periocular and eyelid oedema: Bilateral palpebral oedema and erythema during acute allergic episodes; eyelids appear swollen and pink; may be asymmetric; Dennie-Morgan infraorbital folds (extra infraorbital skin creases) are a chronic atopic sign seen in PAC and AKC patients
- Allergic shiner: Bilateral infraorbital darkening due to venous stasis secondary to chronic nasal and periorbital congestion; a non-specific chronic atopic sign; more pronounced in patients with co-existing allergic rhinitis
- Periocular skin changes (contact allergic): Eczematous, erythematous, scaling, or lichenified periocular skin in contact allergic blepharoconjunctivitis; may extend to the lower eyelids and cheeks; skin is involved before conjunctiva in delayed-type reactions
- Watery or mucoid discharge: Clear to faintly mucoid discharge, bilateral; in acute SAC, profuse watery lacrimation from mast cell–mediated vasodilation; in chronic PAC or AKC, mucoid stringy discharge from goblet cell stimulation; discharge is not mucopurulent (absence of pus distinguishes allergic from bacterial aetiology)
Slit Lamp Findings
- Diffuse conjunctival hyperaemia: Bilateral conjunctival injection involving the bulbar and palpebral conjunctiva; typically less intense than bacterial conjunctivitis; predominantly pink rather than red; does not blanch entirely with topical phenylephrine (unlike episcleral injection)
- Chemosis: Conjunctival oedema producing a gelatinous appearance of the bulbar conjunctiva; can be mild (subtle elevation adjacent to the limbus) to severe (prolapsing conjunctiva between the eyelid margins); particularly prominent in acute SAC and drug-induced allergic reactions (e.g. topical anaesthetic reactions)
- Fine papillary reaction: Small (<1 mm), flat-topped elevations on the palpebral conjunctiva — predominantly inferior but may involve superior tarsus; each papilla has a central vascular core (distinguishing papillae from follicles, which are avascular); in SAC/PAC, papillae are fine and uniform; giant papillae (>1 mm, cobblestone) indicate VKC or GPC
- Conjunctival injection and hyperaemia grading: Bulbar conjunctival injection typically graded 1+ (trace) to 4+ (marked); use standardised grading scales (Efron, CCLRU) for longitudinal monitoring; in SAC, injection rapidly resolves after allergen removal or antihistamine therapy
- Cornea (typically spared in SAC/PAC): The cornea is normal in uncomplicated SAC and PAC — its involvement (SPK, pannus, shield ulcer, neovascularisation) indicates VKC or AKC and should prompt ophthalmology referral; contact allergic conjunctivitis may produce fine inferior SPK from chronic surface inflammation
- No preauricular lymphadenopathy: Absence of preauricular lymphadenopathy is an important negative sign distinguishing allergic from viral (adenoviral, herpetic) conjunctivitis; its presence in a presumed allergic case should prompt reconsideration of the diagnosis
Cardinal Ocular Symptoms
- Bilateral ocular pruritus (itch): The single most diagnostically important symptom of allergic conjunctivitis; absent or minimal in bacterial, viral, dry eye, and non-allergic conjunctivitis; mediated by histamine acting on conjunctival H1 receptors and trigeminal sensory nerve fibres; severity ranges from mild intermittent itch (SAC between pollen peaks) to constant, disabling pruritus (VKC, AKC); the patient invariably rubs their eyes, perpetuating the itch-rub cycle
- Watery or mucoid lacrimation: Profuse clear tearing in acute SAC (early phase); thicker mucoid or stringy discharge in PAC and AKC from chronic goblet cell stimulation; patients report constantly watery, runny eyes; discharge does not mat lids on waking (unlike bacterial conjunctivitis)
- Bilateral redness: Diffuse bilateral conjunctival redness; symmetrical or nearly symmetrical; onset correlates with allergen exposure (outdoor activity, petting animals, dusty environments); in SAC, redness develops within minutes of allergen contact
- Burning and foreign body sensation: Surface discomfort from papillary irritation, tear film instability, and inflammatory cytokine effects on ocular surface sensory nerves; burning is secondary to itch and is not the primary complaint (unlike dry eye, where burning predominates over itch)
- Eyelid swelling: Puffiness and heaviness of the upper and lower eyelids during acute episodes; resolves with antihistamines; patients may present for eyelid swelling without prominent ocular redness in early acute SAC
- Photophobia: Mild-to-moderate in SAC/PAC; significant photophobia raises concern for corneal involvement (VKC/AKC) or other diagnoses requiring referral
- Blurred vision: Transient, clears with blinking — attributable to excessive tearing and mucoid discharge on the ocular surface; persistent or non-clearing blur warrants slit lamp assessment for corneal pathology
Associated Systemic Symptoms
- Nasal symptoms (allergic rhinoconjunctivitis): Sneezing, rhinorrhoea (clear), nasal congestion, nasal itch, and postnasal drip are present concurrently in >80% of SAC patients; the term “allergic rhinoconjunctivitis” describes this combined presentation; ocular symptoms often worsen after nose-blowing (nasolacrimal spread of nasal allergens)
- Skin symptoms: Urticaria (hives), atopic dermatitis flare, or contact eczema of the periocular skin may accompany or precede ocular symptoms; Dennie-Morgan folds and allergic shiners reflect chronic atopic skin involvement
- Respiratory symptoms: Cough, wheeze, and chest tightness in patients with concomitant asthma; exacerbated during pollen seasons; systemic antihistamines provide modest benefit for ocular and nasal symptoms but minimal bronchodilation
Ocular Complications
- Corneal complications (rare in SAC/PAC; more relevant in VKC/AKC): Uncomplicated SAC and PAC do not produce corneal disease; however, failure to diagnose underlying VKC or AKC — or inadequate management of severe allergic disease — can allow shield ulcer formation, corneal scarring, or neovascularisation to develop; any corneal involvement is a red flag for more severe allergic subtype
- Secondary bacterial conjunctivitis: Chronic eye rubbing, tear film disruption, and conjunctival epithelial barrier breakdown in prolonged allergic conjunctivitis predispose to secondary bacterial superinfection; the transition from clear watery discharge to mucopurulent discharge signals bacterial co-infection
- Dry eye disease (DED) co-morbidity: Chronic allergic inflammation disrupts goblet cell function and tear film stability; IL-4 and IL-13 impair mucin production; prolonged mast cell activation reduces aqueous tear secretion; the combination of allergic conjunctivitis and DED is common and therapeutically challenging, as antihistamines (anticholinergic effect) can worsen aqueous tear deficiency
- Keratoconus: Chronic forceful eye rubbing — a reflex driven by intense allergic itch — is an established risk factor for keratoconus; eosinophil-derived proteases (in VKC/AKC) contribute to stromal collagen degradation; patients with allergic conjunctivitis and progressive corneal irregularity should undergo corneal topography
- Conjunctival scarring (contact allergic): Chronic contact allergic blepharoconjunctivitis — particularly from long-term preserved topical medications — can produce inferior fornix shortening, symblepharon, and cicatrising changes mimicking ocular cicatricial pemphigoid; early identification and withdrawal of the offending agent is critical
Treatment-Related Complications
- Steroid-induced ocular hypertension: Short-course topical corticosteroids are occasionally used for severe acute SAC flares; even brief steroid courses can raise IOP in susceptible individuals (steroid responders — approximately 5% of the general population); IOP should be checked before commencing and monitored during steroid therapy
- Steroid-induced posterior subcapsular cataract: Risk with prolonged or repeated topical steroid courses; particularly relevant in children with severe allergic eye disease (VKC/AKC) where prolonged steroid use is more likely; monitoring for lens opacification is required
- Anticholinergic dry eye from systemic antihistamines: First-generation antihistamines (chlorphenamine, diphenhydramine) have significant anticholinergic side effects including reduced aqueous tear secretion; second-generation agents (cetirizine, loratadine, fexofenadine) are substantially better tolerated; relevant in patients with co-existing dry eye
- Rebound hyperaemia from topical vasoconstrictors: OTC decongestant drops containing oxymetazoline or naphazoline produce symptomatic redness relief but cause rebound hyperaemia and tachyphylaxis with frequent use; chronic use worsens conjunctival injection and is not recommended for allergic conjunctivitis management
The Atopic March and Co-morbid Conditions
- Allergic rhinitis (Allergic rhinoconjunctivitis): Co-exists in >80% of SAC patients; the two conditions share allergen sensitisation, nasal-lacrimal mucosal continuity (allowing nasal allergen deposition to provoke conjunctival symptoms via the nasolacrimal duct), and systemic Th2-driven inflammation; combined therapy targeting both nasal and ocular components — intranasal corticosteroid spray plus topical ocular antihistamine — is more effective than treating either organ in isolation; allergic rhinitis is a major cause of impaired sleep quality and reduced quality of life
- Asthma: Present in approximately 30–40% of patients with allergic rhinoconjunctivitis; the unified airway model recognises that allergic inflammation of the upper and lower respiratory mucosa share a common immunological aetiology; inadequately controlled rhinoconjunctivitis is associated with more difficult-to-control asthma; allergen-specific immunotherapy (ASIT) targeting nasal allergens may reduce bronchial hyperreactivity
- Atopic dermatitis (eczema): Periocular atopic dermatitis — characterised by Dennie-Morgan folds, eczematous eyelid skin, and Hertoghe sign (lateral eyebrow thinning from chronic rubbing) — commonly accompanies PAC and AKC; the shared Th2 cytokine milieu (IL-4, IL-13) links these conditions; dupilumab (anti-IL-4Rα monoclonal antibody), approved for moderate-to-severe atopic dermatitis, has shown promise for ocular surface manifestations including AKC
- Food allergies: IgE-mediated food hypersensitivity (peanut, tree nuts, milk, eggs, shellfish) may co-exist in polysensitised atopic individuals; ocular symptoms (acute bilateral itch and chemosis) can be a component of food-induced allergic reactions; anaphylaxis from food allergy can produce bilateral conjunctival injection and periorbital oedema as part of the systemic response
- Urticaria and angioedema: IgE-mediated urticaria (hives) and angioedema (deep tissue swelling, including periorbital) can accompany or follow acute allergic conjunctivitis episodes in highly sensitised individuals; severe angioedema involving the periorbital tissues may require systemic antihistamine or oral corticosteroid
- Sinusitis: Chronic allergic rhinitis predisposes to ostial obstruction and recurrent or chronic rhinosinusitis; sinus pressure from ethmoid sinusitis can present with periorbital pain and pressure, which may be confused with ocular disease; ENT assessment is appropriate in patients with prominent sinus symptoms
- Anaphylaxis: Severe IgE-mediated systemic allergic reaction; conjunctival injection and lacrimation are early ocular signs of systemic anaphylaxis triggered by insect venom, foods, latex, or medications; patients with a history of anaphylaxis should be assessed for sensitisation to relevant allergens and carry an epinephrine auto-injector
Clinical Diagnosis
Allergic conjunctivitis (SAC/PAC) is primarily a clinical diagnosis requiring no routine laboratory investigation. The combination of bilateral itch, papillary conjunctival reaction, watery or mucoid discharge, absence of preauricular lymphadenopathy, and a consistent atopic history with seasonal or allergen-exposure pattern is sufficient for confident diagnosis and empirical treatment in the majority of cases.
- History (key elements): Onset and duration of symptoms; seasonal or allergen-exposure pattern; bilateral vs. unilateral; cardinal symptom of itch; co-existing rhinitis, asthma, or eczema; family atopic history; allergen exposure history (pets, dusty environments, new medications or cosmetics, swimming pool); prior treatment and response; contact lens wear history; previous episodes and seasonal pattern
- Visual acuity: Normal in uncomplicated SAC/PAC; reduced VA warrants corneal assessment and reconsideration of VKC/AKC; serial VA monitoring needed in children on steroid therapy
- Upper lid eversion: Essential to exclude VKC (giant cobblestone papillae) and GPC (large papillae in contact lens wearers); SAC/PAC shows only fine papillae on the inferior or superior tarsal conjunctiva; failure to evert the upper lid risks missing the most diagnostically critical finding
- Slit lamp biomicroscopy: Assess papillae size and distribution (fine vs. giant); chemosis degree; discharge character; cornea with fluorescein (normal in SAC/PAC); limbal assessment (Trantas dots indicate VKC); exclude preauricular lymphadenopathy and vesicular lid lesions (viral aetiology)
- IOP measurement: Baseline before any steroid prescription; essential for monitoring during steroid therapy; non-contact tonometry appropriate for primary care; Goldmann applanation for accurate readings
Investigations
- Conjunctival scraping cytology (Giemsa stain): Eosinophils (>2 per high-power field) and free eosinophil granules are characteristic of allergic conjunctivitis; useful when diagnosis is uncertain or to distinguish allergic from bacterial (neutrophils) and viral (lymphocytes) conjunctivitis; typically reserved for atypical or refractory cases
- Skin prick testing (SPT): Gold standard for identifying IgE-mediated sensitisation to specific allergens; a wheal >3 mm larger than the negative control indicates sensitisation; panels include common regional aeroallergens (grass pollen, HDM, cat, dog, cockroach, moulds); performed by allergist; must withhold antihistamines for 3–7 days before testing; guides allergen avoidance counselling and ASIT selection
- Specific IgE serology (ImmunoCAP / RAST): Measures allergen-specific IgE in serum; values expressed in kUA/L (class 0–6); can be performed without withholding antihistamines; used when SPT is impractical (severe eczema precluding skin testing, high anaphylaxis risk, dermographism); correlates reasonably with SPT for common aeroallergens
- Total serum IgE: Elevated (>150 IU/mL adults) supports atopic diagnosis; not diagnostic in isolation (normal in 20–30% of atopic individuals; elevated in parasitic infection, ABPA, hyper-IgE syndrome); useful contextual information when assessing atopic burden
- Tear IgE and histamine (research / specialist): Elevated tear IgE (>0.35 kUA/L) is highly specific for ocular IgE-mediated disease; tear tryptase and histamine are elevated acutely during allergen challenge; used in research and specialist allergy clinics; not routinely available in primary eye care settings
- Conjunctival allergen provocation test (CAPT): Controlled instillation of dilute allergen solution onto the conjunctival surface; objective assessment of conjunctival allergen sensitivity; used in research and ASIT efficacy trials; not routine clinical practice
- Patch testing: For suspected contact allergic blepharoconjunctivitis (Type IV hypersensitivity); performed by dermatologist; panels include cosmetic preservatives (BAC, thimerosal, parabens), fragrance mix, neomycin, nickel; essential for identifying the causative contact allergen
Singapore Optometry Scope Note: Optometrists in Singapore may perform comprehensive anterior segment assessment including slit lamp biomicroscopy, upper lid eversion, fluorescein staining, and non-contact tonometry for IOP monitoring. Non-contact fundus assessment using slit lamp biomicroscopy with a condensing lens or approved diagnostic imaging equipment is within scope — dilation fundus examination is not performed by optometrists in Singapore. Topical mast cell stabilisers and dual-action antihistamine/mast cell stabiliser drops are within optometric scope for mild-to-moderate SAC and PAC. Topical corticosteroids require initiation and monitoring by an ophthalmologist. Any case with corneal involvement, reduced BCVA, suspected VKC or AKC, or inadequate response to non-steroidal therapy should be referred to an ophthalmologist.
Step 1 — Allergen Avoidance and Environmental Control
- Pollen avoidance (SAC): Monitor pollen forecasts; stay indoors on high-pollen-count days (particularly mid-morning and afternoon when pollen dispersal peaks); keep windows closed during peak season; shower and change clothes after outdoor activity; wraparound sunglasses significantly reduce direct ocular pollen exposure
- HDM reduction (PAC — Singapore-relevant): Use dust-mite impermeable mattress and pillow encasements; wash bedding weekly at 60°C; reduce indoor humidity to <50% (air conditioning, dehumidifiers); HEPA air purifiers; minimise carpeting and soft furnishings; replace stuffed toys or wash in hot water
- Pet allergen avoidance: Most effective measure is animal removal from the home; if not possible, keep pets from bedroom and upholstered furniture; use HEPA vacuum cleaners; allergen levels remain elevated for 4–6 months after animal removal due to persistence of Fel d 1 on surfaces and clothing
- Cold compresses: Cold pack or refrigerated flannel applied to closed eyelids for 5–10 minutes several times daily; vasoconstriction reduces histamine-mediated hyperaemia and chemosis; reduces mast cell degranulation threshold; most effective and safest non-pharmacological intervention; substitutes for eye rubbing
- Preservative-free lubricating drops: Frequent instillation (4–8 times daily) dilutes allergen load in the tear film and reduces allergen-epithelial contact time; reduces surface friction and inflammatory mediator concentration; preferred over preserved drops to avoid compounding surface toxicity; chilled lubricants provide additional vasoconstrictive comfort benefit
- Eye rubbing cessation: Rubbing physically degranulates mast cells (mechanically cross-linking IgE receptor clusters) and introduces additional allergen from hands; patients must be explicitly counselled on the itch-rub amplification cycle and strategies to resist rubbing (cold compress substitution, oral antihistamine to reduce systemic itch drive)
Step 2 — Topical Pharmacotherapy
Dual-Action Antihistamine / Mast Cell Stabilisers (First-Line):
- Olopatadine 0.1% eye drops (BD) or 0.2% (once daily): The most widely used and evidence-supported first-line agent; combined selective H1-antihistamine and mast cell stabiliser; rapid onset of itch relief within 3–5 minutes of instillation; sustained anti-inflammatory effect over 12 hours; the 0.2% once-daily formulation improves adherence; excellent safety profile in children and adults; preferred first-line in Singapore optometric practice
- Ketotifen 0.025% eye drops (BD): Dual H1-antagonist and mast cell stabiliser with additional eosinophil inhibition; widely available OTC in many markets; effective for both SAC and PAC; mild stinging on instillation; cost-effective alternative to olopatadine
- Azelastine 0.05% eye drops (BD): Selective H1-antagonist and mast cell stabiliser with anti-PAF and anti-leukotriene activity; rapid onset itch relief; useful in patients with leukotriene-driven components; mild bitter taste when drops reach nasopharynx via nasolacrimal duct
- Epinastine 0.05% eye drops (BD): H1-antagonist with mast cell stabilising activity; combined effect on histamine, leukotrienes, and prostaglandins; approved for SAC
Dedicated Mast Cell Stabilisers (Prophylactic / Maintenance):
- Lodoxamide tromethamine 0.1% (QID): Most potent dedicated mast cell stabiliser; inhibits mast cell degranulation and eosinophil activation; requires 2–4 weeks of regular use before full benefit; most effective when commenced 2–4 weeks before the expected pollen season; not effective for acute itch relief but highly effective for chronic suppression; superior to cromoglicate in clinical trials
- Sodium cromoglicate 2–4% (QID): Classic mast cell stabiliser; prophylactic only — no acute antihistamine effect; requires pre-season commencement; safe for long-term use including in children and pregnancy; less potent than lodoxamide but widely available and inexpensive; still useful as monotherapy in mild PAC with HDM sensitisation
- Nedocromil sodium 2% (BD-QID): Dual mast cell stabiliser and anti-inflammatory; inhibits eosinophil, neutrophil, and mast cell mediator release; useful adjunct in moderate PAC
Topical NSAIDs (Adjunct):
- Ketorolac tromethamine 0.5% (QID): Inhibits COX enzymes, reducing prostaglandin D2 and E2 production; approved for relief of ocular itching and redness in allergic conjunctivitis; evidence weaker than for antihistamines; used as adjunct therapy when antihistamines alone are insufficient; limited to short-term use given risk of corneal melting with prolonged use in compromised ocular surfaces
Topical Corticosteroids (Severe Acute Flares — Ophthalmologist-Initiated):
- Fluorometholone (FML) 0.1% (QID): Preferred steroid for allergic conjunctivitis; reduced anterior chamber penetration minimises IOP risk; short course (1–2 weeks) for severe SAC flares unresponsive to dual-action antihistamines; must taper to avoid rebound inflammation; IOP monitoring mandatory
- Loteprednol etabonate 0.2–0.5% (QID): Retrometabolic steroid; metabolically inactivated in the anterior segment; lower IOP-raising potential than conventional steroids; evidence for allergic conjunctivitis management; appropriate when IOP sensitivity is a concern
- Prednisolone acetate 0.5–1% (QID): Reserved for severe refractory acute flares or severe VKC/AKC; most potent IOP-raising potential; short-course pulse only; strictly ophthalmologist-supervised
Step 3 — Systemic Therapy
- Oral second-generation antihistamines: Cetirizine 10 mg OD, loratadine 10 mg OD, or fexofenadine 120–180 mg OD; provide systemic anti-allergic effect covering both ocular and nasal symptoms; preferred over topical-only therapy when rhinoconjunctivitis is the dominant presentation; second-generation agents have minimal sedation and anticholinergic effects compared with first-generation; onset within 1–2 hours; cetirizine has the strongest evidence for ocular symptom reduction among oral antihistamines
- Intranasal corticosteroid sprays: Fluticasone propionate, mometasone furoate, or budesonide; first-line for moderate-to-severe allergic rhinitis with co-existing ocular symptoms; provide both nasal and secondary ocular symptom relief via reduction of nasal-lacrimal allergen load; superior to oral antihistamines for nasal congestion; minimal systemic absorption; the combination of intranasal corticosteroid + topical ocular antihistamine achieves the best overall rhinoconjunctivitis control
- Oral antihistamine + intranasal steroid combination: For moderate-to-severe allergic rhinoconjunctivitis; combination therapy produces greater symptom reduction than either agent alone; a practical step-up approach in primary care when topical ocular drops alone are insufficient
Step 4 — Disease-Modifying Therapy
- Allergen-specific immunotherapy (ASIT): The only treatment that alters the underlying allergic immune response; subcutaneous immunotherapy (SCIT) or sublingual immunotherapy (SLIT) administered over 3–5 years; induces tolerance via Treg and IL-10/TGF-β pathways, shifting immune response from Th2 to Th1; approved for grass pollen, HDM, cat, and selected other allergens; reduces both nasal and ocular symptom severity; may prevent development of new allergen sensitisations and the development of asthma in rhinitis patients; requires confirmed specific allergen sensitisation (SPT or specific IgE) and allergy specialist referral
- Dupilumab (anti-IL-4Rα): Monoclonal antibody blocking the shared IL-4 and IL-13 receptor subunit; approved for moderate-to-severe atopic dermatitis, asthma, and chronic rhinosinusitis with nasal polyps; emerging evidence for AKC and severe PAC; specialist prescription
Contact Allergic Blepharoconjunctivitis Management
- Identify and remove the offending agent: The most critical and often curative step; review all topical ophthalmic medications, cosmetics, contact lens solutions, and skincare products; substitute preserved drops with preservative-free formulations; replace contact lens solutions with peroxide-based systems
- Short-course topical steroid: FML 0.1% QID for 1–2 weeks to resolve the acute inflammatory response after allergen withdrawal; antihistamines have limited efficacy in Type IV reactions (IgE-independent)
- Patch testing referral: To dermatologist or allergist to confirm the specific contact allergen and guide long-term avoidance
Stepped Treatment Summary
| Severity | Treatment | Referral |
|---|---|---|
| Mild SAC/PAC | Allergen avoidance; cold compress; PF lubricants; topical olopatadine or ketotifen BD; oral antihistamine if rhinitis co-exists | Not required |
| Moderate SAC/PAC | Above + lodoxamide QID (maintenance); intranasal steroid spray; oral cetirizine; consider ASIT referral if polysensitised | Allergy specialist for ASIT; ophthalmology if not improving |
| Severe SAC flare | Short-course FML 0.1% QID ×1–2 weeks + antihistamine + mast cell stabiliser; IOP check before and during steroid | Ophthalmology for steroid initiation and IOP monitoring |
| Suspected VKC / AKC | Commence supportive measures; refer; do not initiate steroid without ophthalmologist | Urgent ophthalmology |
| Contact allergic | Withdraw offending agent; PF lubricants; short-course FML QID ×1–2 weeks; patch test referral | Dermatology/allergy for patch testing |
Refer to Ophthalmology for:
Urgent:
- Corneal involvement — SPK, macroerosion, shield ulcer, pannus, neovascularisation
- Reduced visual acuity not clearing with blink
- Severe photophobia or blepharospasm
- Suspected VKC (giant cobblestone papillae) or AKC (periocular eczema, inferior scarring)
Routine:
- Steroid initiation for moderate-to-severe acute flare
- IOP elevation on topical steroid
- Failure to respond to dual-action antihistamine and mast cell stabiliser after 4–6 weeks
- Suspected keratoconus on corneal topography
Overall Prognosis
The prognosis for SAC and PAC is excellent — both conditions are benign and produce no permanent structural changes to the cornea or conjunctiva in the vast majority of patients. Symptoms are manageable with appropriate pharmacotherapy and allergen avoidance. However, both conditions may persist throughout life as long as allergen sensitisation persists, and quality of life during symptomatic periods can be substantially impaired. The natural history of allergic sensitisation in adults is variable — some patients develop new sensitisations over time (polysensitisation), while others lose sensitivity with advancing age or after prolonged allergen avoidance. Allergen immunotherapy remains the only intervention proven to alter the natural course of the disease and may induce long-term remission in a significant proportion of patients.
Prognosis by Subtype
| Subtype | Visual Prognosis | Disease Course | Key Determinant |
|---|---|---|---|
| SAC | Excellent; no corneal sequelae | Seasonal; remits between seasons | Allergen avoidance; pre-season mast cell stabiliser |
| PAC | Excellent; no corneal sequelae | Chronic; may persist for decades | HDM/dander reduction; maintenance antihistamine |
| VKC | Generally good; risk of corneal scarring | Remits by early adulthood | Early diagnosis; prevent shield ulcer; steroid monitoring |
| AKC | Guarded; corneal neovascularisation, cataract risk | Chronic, non-remitting | CsA; dupilumab; steroid minimisation |
| Contact allergic | Excellent once offending agent removed | Resolves within days–weeks of withdrawal | Identify and remove causative allergen |
| Post-ASIT (all IgE-mediated) | Significantly reduced symptoms; sustained remission possible | Disease-modifying; 3–5 year course | Allergen confirmation; compliance with full course |
| Condition | Key Distinguishing Features | Differentiating Clue |
|---|---|---|
| Viral conjunctivitis (adenoviral EKC) | Acute watery discharge; follicular (not papillary) conjunctival reaction; preauricular lymphadenopathy; concurrent URTI; itch minimal; subepithelial infiltrates in EKC; unilateral onset | Preauricular LAP; follicles not papillae; minimal itch; watery discharge |
| Bacterial conjunctivitis | Mucopurulent or purulent discharge; papillary reaction; lid matting on waking; no significant itch; no atopic history; responds to topical antibiotics; no chemosis; unilateral onset | Mucopurulent discharge; lid matting; no itch; responds to antibiotics |
| Vernal Keratoconjunctivitis (VKC) | Young atopic male; hot climate; giant cobblestone papillae (>1 mm) on upper lid eversion; Horner-Trantas dots at limbus; ropy mucoid discharge; shield ulcer; severe photophobia; seasonal predominance | Giant cobblestone papillae; Trantas dots; shield ulcer; young male in tropical/hot climate |
| Atopic Keratoconjunctivitis (AKC) | Adult (20–50 yrs); severe periocular atopic dermatitis; inferior fornix scarring and symblepharon; corneal neovascularisation; stellate anterior subcapsular cataract; keratoconus; chronic non-remitting; Staphylococcal blepharitis co-existing | Adult age; periocular eczema; inferior fornix scarring; chronic non-remitting course |
| Giant Papillary Conjunctivitis (GPC) | Contact lens or ocular prosthesis history; large superior tarsal papillae; mucus and lens intolerance; itch less severe than VKC; no Trantas dots; no limbal involvement; resolves with lens holiday | Contact lens wear; large superior tarsal papillae; resolves with lens holiday |
| Dry Eye Disease (DED) | Bilateral burning and grittiness; reduced TBUT; interpalpebral SPK; no significant itch; minimal discharge; worse in dry/airconditioned environments; diurnal variation (worse end of day); Schirmer reduced; normal palpebral conjunctiva | Burning > itch; no papillae; reduced TBUT; responds to lubricants |
| Toxic / Medicamentosa conjunctivitis | History of chronic topical medication use (multi-drop therapy, BAC-preserved drops); inferior papillary or follicular reaction; diffuse inferior SPK; no atopic history; itch absent; improves on withdrawal of offending agent | Topical medication history; inferior pattern; no itch; improves on cessation |
| Episcleritis | Sectoral or diffuse episcleral injection; mild boring ache; no discharge; no itch; normal vision; blanches with topical phenylephrine 2.5%; may be associated with systemic disease (IBD, RA, gout) | Sectoral non-blanching redness; no itch; no discharge; aching pain |
| Chlamydial conjunctivitis | Chronic course (>4 weeks); prominent inferior fornix follicles; mucopurulent discharge; fails topical antibiotics; STI history; superior pannus in trachoma; preauricular LAP may be present | Chronicity; follicles not papillae; STI history; fails topical antibiotics |
| Anterior uveitis (iritis) | Deep aching pain; photophobia; ciliary (perilimbal) flush; keratic precipitates; anterior chamber cells and flare; miosis; no discharge; reduced VA; associated with HLA-B27 conditions | Ciliary flush; AC cells/flare; photophobia; no discharge — refer urgently |
| Contact allergic blepharoconjunctivitis | History of topical medication, cosmetic, or CL solution exposure; eczematous periocular skin; inferior papillary reaction; delayed onset 24–72 hours; no specific allergen sensitisation (IgE-negative); resolves with allergen removal | Topical medication/cosmetic history; periocular skin eczema; delayed onset; IgE negative |
- Bilateral itch is the cardinal symptom of allergic conjunctivitis — if a patient does not report itch as their primary or significant complaint, the diagnosis of allergic conjunctivitis should be reconsidered; bacterial conjunctivitis produces no itch, viral produces minimal itch, and dry eye produces burning rather than itch; the presence and severity of itch is the single most diagnostically discriminating symptom in red eye triage
- Always evert the upper lid — this is the single most important clinical examination step that distinguishes SAC/PAC (fine papillae) from VKC (giant cobblestone papillae >1 mm) and GPC (large papillae in contact lens wearers); failure to evert the upper lid is the most common reason for delayed diagnosis of VKC and GPC in primary care
- In Singapore, HDM is the dominant perennial allergen, not pollen — unlike temperate climates where seasonal grass pollen is the predominant trigger for SAC, Singapore’s tropical year-round climate favours persistently high HDM loads; PAC from HDM is the most common form; allergen avoidance counselling should focus on dust mite reduction strategies (impermeable encasements, hot washing, air conditioning) rather than pollen avoidance
- Cold compresses are the most underused and most effective non-pharmacological intervention — cold temperature inhibits mast cell degranulation, reduces histamine-mediated vasodilation, and substitutes for eye rubbing; instructing patients on cold compress technique (5–10 minutes, refrigerated flannel or gel eye mask) before prescribing medications can significantly reduce topical drop requirements and drug costs
- Olopatadine once-daily 0.2% significantly improves adherence — the twice-daily dosing of conventional antihistamine drops is a major adherence barrier in paediatric and working adult populations; the once-daily 0.2% formulation provides 24-hour coverage and halves the dosing burden; adherence is the most common reason for treatment failure with topical antihistamines
- Mast cell stabilisers must be commenced before allergen exposure for maximum effect — sodium cromoglicate and lodoxamide work by preventing mast cell degranulation and are not effective for acute relief; patients should be advised to commence mast cell stabiliser therapy 2–4 weeks before the anticipated pollen season (or HDM season in perennial patients) for optimal prophylactic protection
- Check and document baseline IOP before every topical steroid prescription — short-course FML for a severe SAC flare is appropriate management, but the prescribing clinician must document a pre-steroid IOP and arrange follow-up IOP check within 2–4 weeks; a steroid responder who develops significant IOP elevation from a short course prescribed without follow-up is a preventable adverse outcome
- Allergen immunotherapy is underutilised in ophthalmology practice — for patients with confirmed specific IgE sensitisation whose SAC/PAC is inadequately controlled with pharmacotherapy, or who require recurrent steroid courses, ASIT referral to an allergist is appropriate and evidence-based; it is the only intervention that produces long-term remission and prevents new sensitisations
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