Interferons are glycoproteins which have antiviral, antiproliferative, and immunomodulatory functions. They are used widely for the treatment of many conditions like Hepatitis C, cancers, and immune-mediated disorders like multiple sclerosis. Interferon was discovered by Issacs and Lindenmann in 1957 while studying the phenomenon of virus interference. The FDA approved the use of Interferon alpha-2a and alpha-2b for the treatment of hairy cell leukemia in 1986. As time progressed, the use of interferons (IFN) has expanded to include a broad spectrum of conditions. However, their use is associated with numerous adverse effects like fatigue, influenza-like syndrome, toxicities related to the central nervous system, the gastrointestinal tract, endocrine system, and cardiovascular, renal, and musculoskeletal systems. The first report of ocular toxicity of IFN therapy was made by Ikebe and associates in 1990 in a 39-year-old patient who developed retinal hemorrhages and cotton wool spots after receiving intravenous IFN.
Interferons are a family of proteins which are released by a variety of cells in response to infections caused by viruses. There are several classes of IFN including IFN alpha, beta, and gamma. IFN alpha has been used for management of hairy cell leukemia, malignant melanoma, follicular lymphoma, condylomata acuminata, AIDS-related Kaposi sarcoma, and chronic hepatitis B and C. IFN beta has been used for multiple sclerosis, and IFN gamma has been used for chronic granulomatous disease and malignant osteopetrosis. In ophthalmology, IFN has been used to manage subretinal neovascularisation of age-related macular degeneration, glaucoma filtering surgery, and ocular cicatricial pemphigoid. The ophthalmic complications of IFN are mainly limited to the optic disc and retina, and the characteristic finding is a vascular retinopathy.
The reported incidence of IFN-associated retinopathy ranges from 18% to 86%, according to various studies. Higher incidences are seen in some parts of Japan where values of more than 50% are common. Increased incidences of retinopathy were seen with older age and in patients with systemic conditions like hypertension and diabetes mellitus and are probably related to the dose of IFN as well. IFN retinopathy can occur at any time between four to 28 weeks of initial treatment, and various studies report the average time of presentation at around 20 weeks. Another peculiar phenomenon reported in some studies is that nonresponsiveness to IFN therapy was more prevalent in those developing retinopathy.
The exact pathogenesis of the development of IFN retinopathy is not clear. Some authors suggest ischemia as the cause of the vascular changes. Others implicate immune complex deposits in the blood vessel walls which can cause a reduction in retinal capillary perfusion and formation of cotton wool spots. It is also suggested that inflammatory cytokine deposition in the blood vessel wall may be the underlying causative factor for this condition. Some studies have found an association between IFN retinopathy and elevated serum vascular endothelial growth factor (VEGF) levels. Vascular endothelial growth factor is associated with neovascularization, but the correlation between IFN retinopathy and elevated serum vascular endothelial growth factor levels is not clear. Studies were conducted by Abe and associates about the long-term effect of IFN on mice. They discovered that the retinal vascular bed was occluded in mice pretreated with urethane and not in those without pretreatment. They concluded that a diseased retina predisposed to the development of IFN retinopathy.
It has been suggested by different authors that there are certain risk factors which predispose the patient to the development of IFN retinopathy. They include systemic hypertension, diabetes mellitus, the age of the patient, and the dose of interferon. Concomitant administration of drugs like ribavirin may predispose patients to the development of retinopathy. Arterial sclerosis, erythrocyte count, leukocyte count, platelet count, serum glutamic-pyruvic transaminase (SGPT) levels, serum glutamic-oxaloacetic transaminase (SGOT) levels, hemoglobin, triglycerides, and total cholesterol have been examined as risk factors, but so far, the results are inconclusive.
The classical presenting complaint in the typical IFN retinopathy is a defective vision, but many patients may be asymptomatic. Typical ocular findings include cotton wool spots, and retinal hemorrhages in the posterior pole, particularly around the optic disc. 
Atypical ocular side effects have been seen in some patients. These include central retinal artery occlusion, central retinal vein occlusion, anterior ischemic optic neuropathy, optic disc edema, neovascular glaucoma, vitreous hemorrhage, Vogt-Koyanagi-Harada-like disease, retinal detachment, subconjunctival hemorrhage, optic disc edema, and panophthalmitis. Common to these conditions are severe and permanent visual impairment, which persists despite discontinuation of IFN therapy and instituting appropriate ophthalmological management. Some authors believe that IFN may not be entirely responsible for these complications but could be related to pre-existing ocular pathology in these patients.
Most authors claim that there is no difference in the clinical features of patients receiving different types of IFN.
Any patient complaining of any ocular symptoms while receiving IFN treatment should be examined thoroughly. One should especially be vigilant in older patients, those receiving higher doses of IFN or combination with other medicines, and patients with pre-existing morbidities like hypertension and diabetes mellitus. Visual acuity testing, pupil examination, and dilated fundus examination are mandatory. Fluorescein angiography may reveal areas of capillary dropout, hemorrhages and cotton wool spots may cause blocked fluorescence. Optical coherence tomography may help to diagnose macular edema, especially in subtle cases.
It is recommended that all patients receiving IFN therapy should have a thorough ophthalmological examination before starting treatment. Any preexisting retinopathy should be ruled out. Monthly monitoring of patients should be done while receiving treatment if there is preexisting retinopathy and every three months in asymptomatic patients unless they complain of defective vision.
If a patient presents with an ophthalmological complication while receiving IFN therapy, the general recommendation is to stop IFN treatment. However, some authors suggest discontinuing treatment only in those with severe retinopathy or those with preexisting diseases like diabetes mellitus. A good outcome is seen in many cases after stopping IFN.
IFN retinopathy is a relatively new entity, but it requires attention since a large number of patients, especially those suffering from Hepatitis C receive IFN as a part of their treatment. All patients should undergo a comprehensive ocular examination before starting treatment and regular follow up is important, especially in the first six months when most complications arise.