Fuchs heterochromic iridocyclitis (FHI) is a chronic, usually unilateral, non-granulomatous uveitis of insidious onset. It is characterized by iris depigmentation, accounting for approximately 0.5–7% of all cases with uveitis [1–3]. However, FHI is commonly misdiagnosed because of its easily neglected and diverse manifestations . The typical characteristics of FHI are keratic precipitates (KPs), iris atrophy, non-persistent inflammatory manifestations, vitreous infiltration, lens opacification, and secondary glaucoma with symptoms such as blurred vision . Medium- or star-shaped KPs could be distributed in the cornea of the triangle area, pupil area, or diffusely posterior in patients with FHI. Ocular examination can also show light anterior chamber flare, a small number of aqueous cells, iris depigmentation, or iris atrophy. FHI patients are prone to have Koeppe nodules, posterior capsule opacification, ocular hypertension, opacity, and cells in the anterior vitreous. In addition, clinical manifestations such as abnormal corneal spots or corneal endothelium, abnormal blood–humor barrier, and non-persistent inflammation can be found in FHI [5–11].
At present, the complex and varied symptoms of FHI make it difficult to diagnose. Misdiagnosis can result in unnecessary corticosteroid treatment or misinformed expectations about the progress of the condition. Many risk factors for FHI, including inflammation, tumor, trauma, surgery, congenital syndromes, nerve dysfunction, autoimmunity, and infection, have been studied . However, viral pathogens are more regarded as potential causes of FHI; these include the herpes simplex virus (HSV and VZV) , cytomegalovirus (CMV) [14, 15], and rubella [16–18]. The predominant mechanisms involved in viral anterior uveitis are chronic inflammatory processes , which are accompanied by changes in cytokine levels  and immunopathology .
Rubella virus infection highly correlates with FHI [16–18], with 68% of FHI patients infected with the rubella virus . Rubella virus, a member of the Riboviriad family, Matonaviridae [23, 24], is the etiologic agent of rubella, which can induce congenital birth defects, miscarriage, and stillbirth via transplacental transmission [24–26]. Rubella occurs worldwide during epidemics . Although the global incidence of rubella has decreased with the development of vaccines, the number of congenital rubella syndrome cases is approximately 100,000 every year . Rubella virus can persistently infect the eye for years .
In eyes with Fuchs’ uveitis syndrome, significantly higher levels of inflammatory factors (IL-6, IL-8, and IL-10) than those in senile cataract-affected eyes were detected because immune mediators are crucial to specific viral intraocular inflammation . IL-6 and IL-8 have the potential to act as markers of inflammation in the aqueous humor in FHI.
The role of the intraocular presence of rubella during the development of FHI is controversial [18, 29], and the mechanism of rubella infection in FHI is not clear. In this study, we aimed to investigate the viruses involved in FHI, such as CMV, HSV, VZV, and rubella, and determine the correlation between cytokine levels and virus status in the aqueous humor of patients with FHI, especially in the case of rubella.