In 1906, Austrian ophthalmologist Ernst Fuchs reported 38 cases presenting with iris heterochromia, cyclitis, and cataract. Fuchs heterochromic iridocyclitis (FHI) is also called Fuchs uveitis syndrome (FUS). It is a unilateral uveitis characterized by the triad of iris heterochromia, iridocyclitis, and cataract, commonly occurring in the 20s to 40s with no gender difference.
FHI is reported to account for 2–11% of all uveitis cases 2). In a survey from the Uveitis Clinical Practice Guidelines (3,055 cases), it accounted for 0.5% (15 cases) 3). The prevalence may be underestimated; in Japanese individuals, heterochromia is less noticeable due to brown irises, leading to delayed diagnosis 7). Most cases are unilateral, with bilateral involvement in only about 10–13% 2).
FHI itself follows a benign course unless secondary glaucoma develops, and often does not require treatment. However, secondary glaucoma is the most vision-threatening complication of FHI, and its incidence increases with longer follow-up.
Because Japanese individuals have brown irises, differences in iris color between eyes are less apparent. Instead of simple heterochromia, attention should be paid to the iris collarette and iris crypts becoming less distinct compared to the healthy eye, and to diffuse iris atrophy centered on the minor iris circle. Comparing with the fellow eye before dilation is a key diagnostic point.
Unlike other forms of anterior uveitis, FHI patients do not experience pain, redness, or photophobia. Many patients remain asymptomatic for years before seeking medical attention.
Heterochromia (Iris Atrophy)
Present in 75–90% of patients. Usually, the lighter-colored eye is the affected one. In Japanese individuals, due to brown irises, it appears as diffuse iris atrophy. It is important to compare with the fellow eye without dilation.
Stellate Keratic Precipitates (KPs)
White, fine stellate keratic precipitates diffusely distributed over the entire corneal endothelium. Unlike other types of uveitis, they are evenly distributed up to the superior cornea. This is the most specific finding for diagnosis.
Iridocyclitis and Cataract
Anterior chamber inflammation is chronic and mild. Cataract progresses faster than in other uveitis. Posterior synechiae do not occur (most important differentiating feature). Koeppe/Busacca iris nodules may be present.
That is correct. In FHI, posterior synechia does not occur as a rule, and the presence of posterior synechia is an important finding against FHI. If posterior synechia is observed, differentiation from other diseases such as sarcoidosis, herpetic iritis, and CMV iridocyclitis is necessary.
The exact etiology of FHI remains unknown and is considered multifactorial. Historically, many hypotheses have been proposed, including degenerative disease, Toxoplasma/HSV infection, immune abnormality, and vascular abnormality (from Amsler sign), but most are now refuted.
In recent years, the association with rubella virus has gained attention as the most plausible hypothesis. A review by Mohamed & Zamir (2005) suggested that persistent low-grade intraocular infection may occur after fetal or childhood rubella infection7). de Groot-Mijnes et al. (2006) detected rubella virus RNA (genome) by PCR in aqueous humor of FHI patients and confirmed intraocular antibody production using the Goldmann-Witmer coefficient. Intraocular antibody production against RV was found in 48 of 64 European cases (75%)8). The declining incidence of FHI in regions with rubella vaccination also supports the rubella hypothesis.
In Asia, CMV infection has been reported as FHI-like inflammation. Chee & Jap (2008) compared CMV-positive FHI-like anterior uveitis with CMV-negative FHI and Posner-Schlossman syndrome, showing that high intraocular pressure and anterior chamber cells are common, but FHI findings (stellate KP, iris atrophy) are similar to CMV-negative cases9). The International Task Force on CMV Anterior Uveitis (TITAN 2024) emphasized that identification of CMV-DNA by aqueous humor PCR is useful for differentiation4).
The reported frequency of secondary glaucoma associated with FHI ranges widely from 10% to 59%, with higher rates in long-term follow-up 2). The uveitis clinical practice guidelines report 10–20% 3). The following factors have been reported to increase risk:
The diagnosis of FHI is based on a combination of clinical findings. There is no definitive diagnostic test. The diagnostic criteria proposed by La Hey et al. (1994) 10) are often referenced. Diagnosis is made only after excluding other diseases.
| Disease | KP | Posterior synechiae | IOP | Features |
|---|---|---|---|---|
| FHI | Stellate/diffuse | None | Chronic elevation (10–59%) | Asymptomatic/monocular |
| Posner-Schlossman syndrome | Few/white | None | Paroxysmal (40–70 mmHg) 2) | Recurrent/acute |
| CMV iridocyclitis | Few | None | Elevated | CMV-PCR positive 4) |
| Sarcoidosis | Mutton-fat/granulomatous | Present | Normal to elevated | Bilateral, systemic findings |
| Herpetic iridocyclitis | Few | Possible | Elevated | Corneal hypoesthesia, sector atrophy |
Other differential diagnoses are as follows:
Although not mandatory in all cases, anterior chamber PCR is useful when accompanied by high intraocular pressure, when differentiation between steroids and antiviral drugs is needed for treatment, or when differentiation between CMV and FHI is difficult9). Particularly in Asians, there are many reports of CMV-positive FHI-like inflammation, and differentiation can change treatment strategy.
The anterior chamber symptoms of FHI hardly respond to steroid eye drops, and long-term use increases the risk of steroid-induced cataract and steroid-induced glaucoma. The uveitis treatment guidelines state that “observation without steroid eye drops” is the principle3). Since posterior synechiae do not occur, mydriatics are also unnecessary.
Cataracts associated with FHI have better surgical outcomes compared to other uveitis. The uveitis treatment guidelines state that “even if active inflammation is present, cataract surgery rarely exacerbates inflammation”3).
The prevalence of secondary glaucoma ranges from 10% to 59%, and 73% of cases do not respond to maximum medical therapy and require surgery 2).
| Drug | Use | Notes |
|---|---|---|
| Beta-blockers (e.g., timolol 0.5% twice daily) | First-line 2, 6) | Suppresses aqueous humor production |
| Carbonic anhydrase inhibitors (e.g., dorzolamide) | Second-line 2, 5) | Eye drops or oral |
| Alpha-2 agonists (e.g., brimonidine 0.1%) | Second-line 2) | Suppresses aqueous production and enhances uveoscleral outflow |
| PGA preparations | Administer with caution | Risk of inflammation exacerbation and CME2) |
| Miotics (e.g., pilocarpine) | Contraindicated2) | Inflammation exacerbation, ciliary spasm |
When intraocular pressure control is insufficient with medication, surgical treatment is necessary. The 5th edition of the Japanese Glaucoma Practice Guidelines recommends trabeculectomy (with antimetabolites) or tube shunt surgery for uveitic glaucoma5).
Surgical outcomes for uveitic glaucoma including FHI are shown below.
| Procedure | Target | Outcome |
|---|---|---|
| Trabeculectomy with MMC (1 year) | Uveitic glaucoma | Success rate 58–90.9%2) |
| Trabeculectomy with MMC (4 years) | Uveitic glaucoma | Success rate 62.3%2) |
| Baerveldt (2 years) | Uveitic glaucoma | Success rate 91.7%2) |
| Ahmed valve (2 years) | Uveitic glaucoma | Success rate 68.4%2) |
In principle, they should not be used. Mild anterior chamber reaction in FHI hardly responds to steroid eye drops, and long-term use increases the risk of steroid-induced cataract and steroid-induced glaucoma. Since posterior synechiae do not occur, mydriatics are also unnecessary. The Uveitis Clinical Practice Guidelines also state that “unnecessary treatment should not be performed” 3).
The theory that persistent low-grade infection is established in the anterior chamber after rubella infection during the fetal or childhood period is currently the most supported 7, 8). Evidence includes detection of rubella virus RNA in aqueous humor, demonstration of intraocular antibody production by the Goldmann-Witmer coefficient, and a decreasing trend of FHI in regions where rubella vaccination is widespread 8).
Secondary glaucoma associated with FHI is of the open-angle type, and the following mechanisms contribute to elevated intraocular pressure 2).
In FHI, posterior synechiae, peripheral anterior synechiae (PAS), and pupillary block are usually absent, so angle-closure mechanism is not common; the main cause is trabecular meshwork damage in open-angle glaucoma 5). Typically, glaucoma in FHI tends to persist even after inflammation subsides and does not respond to steroids 2). This helps differentiate it from steroid-induced glaucoma.
Gonioscopy reveals an open angle and neovascularization crossing the trabecular meshwork 2). Iris fluorescein angiography may show leakage from iris vessels or ischemic changes.
For reference, the following mechanisms may also be involved in intraocular pressure elevation associated with uveitis 2):
Among these, posterior synechiae, PAS, and pupillary block are usually not involved in FHI; trabeculitis and structural changes are the main causes of intraocular pressure elevation.
Chronic low-grade inflammation causes degeneration of lens epithelial cells, leading to posterior subcapsular cataract (PSC). About three-quarters of lens opacities in FHI patients are posterior subcapsular, and progression is faster than in other types of uveitis. Cataracts progress even without steroid treatment, so regular follow-up is necessary.
Yuksel Elgin & Hepokur (2025) reported GATT for FUS-related glaucoma in 4 cases. All surgeries were unsuccessful, with 3 cases requiring trabeculectomy with MMC and 1 case requiring diode laser cyclophotocoagulation after an average of 6 months 1). In FUS glaucoma, scarring of the posterior trabecular meshwork is the main cause, suggesting that GATT may not achieve sufficient intraocular pressure reduction.
Advances in multiplex PCR technology have enabled accurate detection and differentiation of rubella virus, CMV, and HSV in the anterior chamber. The usefulness of antiviral therapy (ganciclovir eye drops, valganciclovir oral) is being investigated in Asian CMV anterior uveitis 4).
Implant devices such as iStent and Hydrus are at risk of occlusion by inflammatory debris, but preliminary promising results have been reported for canaloplasty. Establishing a MIGS algorithm specific to FHI glaucoma remains a challenge.
Continuous monitoring of epidemiological changes in FHI associated with rubella vaccination is ongoing, with reports of a decreasing trend in FHI incidence among younger generations 7).
At present, the effectiveness of MIGS for FHI glaucoma is considered limited. In a report of 4 GATT cases, all were unsuccessful 1). In FHI glaucoma, scarring of the posterior trabecular meshwork may be involved, and angle surgery alone may not provide sufficient effect. Implant devices such as iStent and Xen are at risk of occlusion by inflammatory debris.
