Heterochromia Iridis
A difference in iris colour between the two eyes (complete heterochromia) or within a single iris (sectoral heterochromia). May be congenital and benign, or acquired — signalling Horner syndrome, Fuchs heterochromic iridocyclitis, iris melanoma, or other pathology requiring urgent investigation.
Last updated: March 2026
Panel A — Anterior View: Complete Heterochromia (Acquired — Horner / Fuchs HCI)
Panel B — Iris Stromal Cross-Section: Melanocyte Comparison
Complete Heterochromia
Entire iris colour differs between eyes. Key causes: Horner, Fuchs HCI, Waardenburg.
Sectoral Heterochromia
One sector of iris differs. Key causes: iris melanoma, naevus, Brushfield spots, mosaicism.
Acquired = Investigate
New colour change demands workup: Horner (imaging), Fuchs (slit-lamp), melanoma (UBM/biopsy).
Panel A. Complete heterochromia: OD normal brown iris (A, F) vs OS lighter depigmented iris (B) with Horner signs — miosis (C), subtle ptosis (D) — and Fuchs HCI stellate KPs (E).
Panel B. Iris stromal cross-section comparing dense melanocyte population (brown, absorbs all wavelengths) vs sparse/absent melanocytes (blue/grey, Rayleigh scattering reflects short wavelengths). Fuchs HCI causes patchy PPE atrophy.
Heterochromia iridis (or iridium) refers to a difference in iris colouration, either between the two eyes (complete heterochromia) or within a single iris (sectoral or partial heterochromia). It is a sign, not a diagnosis, and its clinical significance spans from a benign constitutional variation to a marker of serious systemic or ocular disease.
Iris colour is determined primarily by the density and distribution of melanin-containing melanocytes within the anterior iris stroma. Greater melanin density produces brown irides; minimal melanin yields blue irides, which reflect short-wavelength light. Any process that alters melanin synthesis, melanocyte density, or iris architecture can produce heterochromia.
The key clinical distinction is between congenital heterochromia (present from birth or infancy, usually benign) and acquired heterochromia (new change from a previously symmetric iris colour, always requires investigation). A careful history — when was the colour difference first noticed, is it progressing, any relevant medical history — guides the diagnostic approach.
Important acquired causes include: Horner syndrome (lighter ipsilateral iris due to sympathetic denervation), Fuchs heterochromic iridocyclitis (lighter, depigmented affected eye), iris or uveal melanoma (darker sector), siderosis bulbi (iron-containing intraocular foreign body producing rust-brown iris discolouration), and prostaglandin analogue use (topical PGA drops causing ipsilateral iris darkening).
Congenital Causes
- Physiological (benign): Most common cause; idiopathic constitutional melanocytic variation; no associated pathology; stable for life; familial tendency.
- Waardenburg syndrome: PAX3 or MITF mutations; complete or sectoral heterochromia with white forelock (poliosis), congenital sensorineural hearing loss, and pigmentation anomalies of skin and hair. Autosomal dominant inheritance.
- Congenital Horner syndrome: Absent sympathetic innervation to melanocytes during the critical melanin deposition period (first 2 years of life); affects iris before melanin is fully deposited; causes lighter ipsilateral iris. Due to birth trauma, neuroblastoma, or cervical cord lesion.
- Neurofibromatosis type 1: Lisch nodules (melanocytic hamartomas) may create apparent focal pigmentation change; true heterochromia is uncommon.
Acquired Causes
- Fuchs heterochromic iridocyclitis (HCI): Unilateral, chronic low-grade anterior uveitis; inflammatory cytokines damage iris pigment epithelium causing progressive depigmentation of the affected (usually lighter) eye; stellate keratic precipitates throughout corneal endothelium.
- Acquired Horner syndrome: Sympathetic chain disruption at any level (hypothalamus, brainstem, cervical cord, apex of lung, carotid artery, orbit) causes slow iris depigmentation; associated ptosis, miosis, anhidrosis.
- Iris or uveal melanoma: Malignant melanocytic proliferation in one sector causes localised iris darkening; diffuse iris melanoma presents as acquired unilateral iris darkening with glaucoma — a diagnostic emergency.
- Siderosis bulbi: Iron-containing intraocular foreign body (retained metallic IOFB) releases ferrous ions that deposit in the iris epithelium and lens; produces rust-brown discolouration ipsilaterally.
- Prostaglandin analogue eye drops (PGA): Topical PGAs (latanoprost, bimatoprost, travoprost) stimulate melanogenesis in iridial melanocytes; iris hyperpigmentation develops ipsilaterally over months to years; irreversible; more pronounced in hazel/mixed irides.
- Trauma: Localised anterior segment trauma may damage iris pigment epithelium, causing focal lightening or darkening.
Iris colour is principally determined by the number, activity, and melanin content of melanocytes in the anterior iris stroma. Unlike skin melanocytes, iris melanocytes do not transfer melanosomes to keratinocytes; colour depends entirely on in-situ melanin density within the stromal melanocytes.
Sympathetic innervation and melanin deposition: During early postnatal life, sympathetic fibres reaching the iris via the superior cervical ganglion and ciliary nerves play a critical role in stimulating melanocyte activity and melanin deposition. Disruption of this pathway (Horner syndrome) — whether congenital or acquired — reduces trophic stimulation, resulting in lighter iris colour on the denervated side. In congenital Horner syndrome, this effect is pronounced because melanin deposition has not yet been completed at birth.
Fuchs HCI: The precise pathomechanism remains debated. Rubella virus has been implicated (high antibody titres in anterior chamber); cytomegalovirus (CMV) has been identified in some cases. Chronic inflammation damages the iris pigment epithelium and stroma, releasing melanin granules into the anterior chamber (heterochromia cyclitis), progressive iris atrophy, and pigment cell loss.
Prostaglandin analogues: Prostaglandin F2α receptors on iris melanocytes stimulate tyrosinase activity and increased melanin synthesis; this is a direct pharmacological effect and is dose- and duration-dependent.
Siderosis: Iron ions (Fe²⁺) from a retained metallic IOFB undergo Fenton reactions, producing reactive oxygen species that are toxic to the iris pigment epithelium, trabecular meshwork, lens, and retina. Iron deposits in epithelial cells cause rust-brown discolouration and progressive cell death.
Complete (Total) Heterochromia
The two eyes are entirely different colours. Most striking form. May be congenital (physiological or syndromic) or acquired (Horner, Fuchs HCI, melanoma).
Sectoral (Partial) Heterochromia
A distinct sector of one iris differs in colour from the rest. May be congenital (melanocytic naevus, Waardenburg) or acquired (sector melanoma, iris atrophy).
Central Heterochromia
A ring of different colour surrounds the pericentral zone of the pupil in one or both eyes. Usually benign; represents variation in melanin density between anterior and posterior stromal layers. Common in green/hazel irides.
Congenital vs Acquired
The most clinically critical distinction. Any new-onset, progressive, or adult-acquired iris colour change requires systematic evaluation to exclude Horner syndrome, Fuchs HCI, melanoma, siderosis, and PGA use.
- Family history of heterochromia: Congenital physiological heterochromia has a familial pattern.
- PAX3/MITF mutation carrier: Waardenburg syndrome — autosomal dominant; hearing loss and pigmentation anomalies in family members.
- Horner syndrome risk factors: Apical lung tumour (Pancoast), carotid artery dissection, neck surgery, birth trauma (brachial plexus injury), neuroblastoma (children) — all interrupt the sympathetic chain.
- HLA associations (Fuchs HCI): HLA-A28 association reported; more common in young adults; may follow CMV or rubella exposure.
- Prostaglandin analogue use: Patients on latanoprost, bimatoprost, or travoprost are at risk of ipsilateral iris hyperpigmentation — especially relevant if used unilaterally for unilateral glaucoma.
- Occupational or hobby-related IOFB risk: Metalwork, hammering, grinding without protective eyewear — siderosis from retained iron IOFB.
- Light iris colour, fair skin: Risk for iris melanoma (darker lesion more apparent against a lighter background).
General
- Visible colour difference between the two irides (complete heterochromia) or within one iris (sectoral).
- Document whether the affected eye is lighter or darker — guides the differential diagnosis.
Horner Syndrome — Ipsilateral
Fuchs Heterochromic Iridocyclitis
- Cosmetic concern: Most common presenting complaint, particularly in congenital cases; patients or parents notice the colour difference.
- Asymptomatic (physiological): Congenital physiological heterochromia causes no visual symptoms; discovered incidentally.
- Photophobia: In Horner syndrome, the mydriatic fellow eye may be relatively photophobic; in Fuchs HCI, low-grade photophobia from chronic anterior chamber activity.
- Blurred vision: Fuchs HCI → posterior subcapsular cataract (common, develops in ~70%); Horner → usually no direct effect on vision.
- Ptosis: Functional drooping of the eyelid in Horner syndrome; may cause functional field loss superiorly.
- Pain: Typically absent in heterochromia itself; headache or neck pain may accompany Horner syndrome from carotid dissection or malignancy.
- Floaters or visual field loss: If iris melanoma with vitreous seeding or associated cataract.
- Glaucoma (Fuchs HCI): Secondary open-angle glaucoma in approximately 10–15% of patients; mechanism includes trabecular obstruction by inflammatory cells and fibrin; may be refractory.
- Cataract (Fuchs HCI): Posterior subcapsular cataract develops in approximately 70% of patients; cataract surgery typically has excellent outcomes in Fuchs HCI (unlike other uveitic conditions).
- Metastasis (iris melanoma): Diffuse iris melanoma carries a ~20% 5-year metastatic rate; circumscribed iris melanoma ~3–5%; metastases preferentially to liver.
- Amblyopia (congenital Horner): Severe unilateral ptosis in infancy causes visual deprivation amblyopia; requires prompt recognition and ptosis surgery.
- Siderosis complications: Progressive retinal toxicity (electroretinogram changes), lens opacity (iron deposits — rusty subcapsular cataract), and secondary glaucoma from iron-laden trabecular cells; can lead to phthisis bulbi.
- Systemic malignancy (acquired Horner): Newly acquired adult Horner syndrome may be the first sign of Pancoast lung tumour, carotid artery dissection (stroke risk), or cervical lymphadenopathy from lymphoma. Urgent systemic workup is mandatory.
Horner Syndrome
Interruption of three-neuron sympathetic arc. First-order: hypothalamus to ciliospinal centre (brainstem/cervical cord lesion — demyelination, tumour). Second-order: ciliospinal centre to superior cervical ganglion (Pancoast tumour, thyroid carcinoma, neck surgery). Third-order: superior cervical ganglion to iris (carotid dissection, cluster headache, cavernous sinus lesion). Hydroxyamphetamine test localises to third-order neuron.
Waardenburg Syndrome
PAX3 (types 1 & 3) or MITF (type 2) mutations; autosomal dominant. Features: complete or sectoral heterochromia, sensorineural hearing loss (unilateral or bilateral), white forelock (poliosis capitis), premature greying, lateral displacement of inner canthi (types 1 & 3). Hearing loss is the most functionally significant feature.
Oculodermal Melanocytosis (Naevus of Ota)
Congenital hyperpigmentation of periocular skin and ipsilateral iris (deeper pigmentation). Associated with increased risk of ipsilateral uveal melanoma and glaucoma. More prevalent in Asian and African populations. Not true heterochromia but causes ipsilateral iris darkening.
BAP1 Tumour Predisposition Syndrome
Germline BAP1 mutation; predisposes to uveal melanoma (including iris), cutaneous melanoma, mesothelioma, and renal cell carcinoma. If iris melanoma is detected, BAP1 mutation screening is indicated.
- Slit-lamp biomicroscopy with retroillumination: Characterise iris colour, stroma texture, keratic precipitates (stellate = Fuchs HCI), transillumination (albinism, siderosis), lesion morphology (melanoma).
- Anterior segment photography: Document iris colour and any lesions for serial comparison; essential for melanoma monitoring.
- Pharmacological pupil testing for Horner syndrome:
- Cocaine 4–10% (or apraclonidine 1%): Confirms Horner by demonstrating lack of dilation or reversal of anisocoria (apraclonidine).
- Hydroxyamphetamine 1%: Distinguishes pre- vs post-ganglionic: dilates pupil if 1st/2nd order lesion; fails to dilate if 3rd order (postganglionic).
- Phenylephrine 1%: Dilates a postganglionic Horner eye (denervation supersensitivity).
- MRI/CT of the sympathetic chain: If Horner syndrome is confirmed; investigate from hypothalamus to orbit. CT chest for Pancoast tumour; MRI neck/brain for carotid dissection, brainstem lesion.
- Dilated fundus examination: Posterior pole assessment; vitreous cells (Fuchs); retinal pigment changes (siderosis).
- UBM and AS-OCT: Depth and extent of iris melanoma; ciliary body involvement; cyst vs solid differentiation.
- Fluorescein angiography (FFA): Intrinsic vascularity of iris melanoma; absent in benign naevus.
- B-scan ultrasound: IOFB localisation (siderosis); melanoma extraocular extension.
- ERG: Reduced b-wave amplitude (siderosis — characteristic early finding before visible retinal damage).
- Serology / genetic testing: Rubella antibodies (Fuchs HCI), CHD7/PAX3 sequencing (Waardenburg), BAP1 testing (melanoma with family history).
Congenital Physiological Heterochromia
Reassurance and no treatment required. Annual review with slit-lamp to confirm stability. Patient education on distinguishing normal variation from acquired change.
Horner Syndrome
- Confirm with pharmacological testing (apraclonidine or cocaine).
- Urgent neuroimaging — MRI brain + neck + chest CT — if acquired in adult; time-sensitive if carotid dissection (anticoagulation) or Pancoast tumour.
- Congenital Horner: neuroblastoma workup in infants (urinary catecholamines, MIBG scan); ptosis surgery if causing visual deprivation.
- The iris colour difference itself does not require treatment.
Fuchs Heterochromic Iridocyclitis
- Topical corticosteroids: rarely indicated; Fuchs HCI is relatively steroid-unresponsive and the low-grade nature usually does not warrant long-term steroid use.
- Monitor for posterior subcapsular cataract (develop in ~70%); phacoemulsification has excellent outcomes in Fuchs HCI — low risk of fibrin or posterior synechiae compared to other uveitis.
- Monitor and treat secondary glaucoma (10–15%); topical IOP-lowering agents; filtering surgery for uncontrolled IOP.
- Antiviral therapy (valganciclovir): emerging evidence for CMV-associated Fuchs HCI.
Iris Melanoma / PGA-Induced / Siderosis
- Iris melanoma: surveillance photography; sector iridectomy or brachytherapy for growing lesions; enucleation for diffuse type.
- PGA-induced hyperpigmentation: switch medication class if unilateral use with cosmetically unacceptable asymmetry; pigmentation is irreversible.
- Siderosis: urgent surgical removal of IOFB; ERG monitoring post-removal; manage secondary glaucoma and cataract.
Singapore Optometry Scope Note: Optometrists should carefully assess all patients presenting with heterochromia, distinguishing congenital from acquired causes through history and slit-lamp examination. Any adult with a new or progressive iris colour change requires prompt investigation — do not dismiss as cosmetic. Perform and document pharmacological pupil testing (apraclonidine) where Horner syndrome is suspected and refer urgently for neuroimaging if confirmed in an adult. Assess the anterior chamber for stellate KPs and vitreous cells (Fuchs HCI) at every slit-lamp examination. Examine and photograph any pigmented iris lesion and refer for UBM and ophthalmology assessment. Counsel patients on prostaglandin analogue-related iris darkening when initiating IOP-lowering therapy. In Singapore, Horner syndrome presenting in adults without obvious cause (e.g., ipsilateral shoulder/arm pain, smoker) warrants urgent CT chest to exclude apical lung pathology.
Prognosis is entirely dependent on the underlying cause. Congenital physiological heterochromia carries an excellent prognosis with no impact on vision or systemic health.
Horner syndrome: Visual prognosis is excellent for the eye itself; the overall prognosis depends on the underlying cause — carotid dissection may have stroke risk, and Pancoast tumour has a variable oncological prognosis depending on staging.
Fuchs HCI: Good visual prognosis overall; approximately 70% develop visually significant cataract requiring surgery (outcomes excellent), and 10–15% develop glaucoma requiring long-term management. Vision loss from uncontrolled glaucoma is the main concern.
Iris melanoma: Circumscribed iris melanoma has a 5-year survival of approximately 95% and a metastatic rate of only 3–5%. Diffuse iris melanoma has a substantially worse prognosis (~20% metastatic rate). BAP1 mutation portends a higher systemic risk.
Siderosis: Prognosis depends on speed of IOFB removal; delayed removal leads to irreversible retinal and trabecular damage. Early removal after identification is associated with good functional outcomes.
| Condition | Key Differentiator |
|---|---|
| Iris melanoma vs benign naevus | Melanoma: >3 mm base, >1 mm height, documented growth, intrinsic vascularity on FFA, feeder vessels, ectropion uveae. Naevus: flat or minimally elevated, stable, no vascularity, no growth. |
| Fuchs HCI vs herpes simplex uveitis | Herpes simplex: sector iris atrophy (fan-shaped), patchy keratic precipitates, elevated IOP at presentation, history of recurrences. Fuchs: stellate KPs across whole endothelium, vitreous cells, no posterior synechiae. |
| Fuchs HCI vs glaucomatocyclitic crisis (Posner-Schlossman) | PSS: recurrent unilateral acute IOP spikes (often 40–60 mmHg), mild flare, very few KPs, no heterochromia, normal IOP between episodes. Fuchs: chronic, stellate KPs, vitreous cells. |
| Horner syndrome vs pharmacological miosis | Pharmacological miosis: bilateral if systemic; history of medication; no ptosis; dilation with tropicamide; no heterochromia (unless chronic). |
| Central heterochromia vs uveitis | Central heterochromia: stable, symmetric within the iris, no KPs, no flare, no cells in AC. Uveitis: cells/flare in AC, KPs, ciliary injection. |
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