Definition
Coats' disease is an idiopathic vascular disease of unknown origin. It is usually one-sided and it leads to multiple changes to small vessels (capillaries) in the retina (in particular, aneurysmal dilatation of small vessels in the retina known as telangiectasias). These changes can be further complicated by intra- and sub-retinal exudation (the seepage of blood through the wall of the vessel). The disease was initially described by George Coats in 1908. It is also sometimes reported under the name of "Leber's miliary aneurysm".
Epidemiology
Coats' disease usually affects boys between age 6 and 10 but it can occur at any age in males (1 month to 83 years). It is very rare in girls or women.
Clinical presentation
In babies or very young children, Coats' disease is difficult to diagnose and diagnosis may take a long time. It is often reached during a consultation for indirect signs of the disease e.g. strabismus or leukocoria (white pupil). Diagnosis is therefore often made at an advanced stage of the disease, sometimes even at a very late stage (neovascular glaucoma). Visual acuity is often very poor at the time of diagnosis (the child can often no longer count the fingers on a hand with the damaged eye). Diagnosis is based on an examination of the fundus.
In older children and adults, when the diagnosis is not made early, the clinical presentation is often less severe. The diagnosis is easier and the prognosis better as regards the patient's sight.
Diagnosis (criteria - method)
In the majority of cases, the disease is one-sided, affecting only one eye. There are no other signs of the disease. An examination of the fundus allows the practitioner to reach the diagnosis and shows : 1) retinal capillary telangiectasias, aneurysmal dilatations of capillary veins in the retina, 2) associated vascular abnormalities such as a lack of capillary veins or retinal tortuosities, 3) lipid exudates (or intra-retinal exudation) which are almost always present on diagnosis and are often associated with a detachment of the exudative retina. In Coats' disease, the vascular lesions are situated on the periphery of the retina.
The lipid exudates and retinal detachment extend to the central area of the retina (the macula) and are the main cause of loss of central vision and decrease in visual acuity in the affected eye. Coats' disease is sometimes observed in other diseases such as retinitis pigmentosa or Turner Syndrome.
Photographing the fundus to diagnose the disease is a simple, quick, non-invasive procedure. It shows the type and extent of lesions, especially the retinal detachments and lipid exudates.
Fluorescein angiography highlights the vascular and capillary changes in the retina and helps to determine their extent. Damage to the walls of blood vessels (with rupturing of the hemato-retinal barrier) is shown by the spread of the colouring agent in the wall of the vessels. This is present from the early stages on angiography and persists at the latest stages. In some cases, there is a more or less extensive loss of the capillary bed. This loss may be very extensive, leading to the total absence of blood supply to the retina. The angiography also indicates the areas of treatment of the retinal abnormalities, using laser photocoagulation.
An OCT examination (optical coherence tomography) is used to show changes within the retina such as the presence of a macular edema or serous retinal detachments. It also measures them accurately. This is an essential tool for patient monitoring since it quantifies the amount of subretinal liquid and provides an objective measurement of the effectiveness of treatment.
In young children, Coats' disease should be distinguished from the following: retinoblastoma, retinal detachment for other reasons, Norrie disease, congenital cataract, persistance of the primary vitreous and familial exudative vitreo-retinopathy. In adults, Coats' disease may be confused with Eales disease, an inflammatory vasculopathy, a vaso-proliferative tumor, idiopathic macular telangiectasia or any other vascular diseases of the retina leading to exudation.
Pathophysiology - Etiology
Coats' disease affects the walls of the retinal capillary vessels. Normally, the wall of the capillaries is impermeable; it does not allow serum to seep outside the vessels. In Coats' disease, the vascular wall is abnormal and has lost a number of cells (endothelial cells, pericytes). Some cells (endothelial cells) may also be fenestrated. The natural barrier between blood and the retinal tissue provided by the wall of the blood vessel becomes permeable, allowing liquids to pass through. These liquids lead to exudates within the retina (lipid exudates) and under the retina, causing retinal detachment.
The abnormalities in the wall of the retinal blood vessels are also probably the cause of malformations in small vessels and dilatations in the shape of small aneurysms leading to telangiectasias.
The term "Coats' Syndrome" is used when there are retinal vascular lesions similar to those found in Coats' disease associated with another systemic disease (e.g. Norrie disease).
Genetics
At present, there is nothing to suggest that this disease is hereditary. Usually, no family history of Coats' disease is found and there is no preponderance of the disease in specific populations or ethnic groups.
Development - Prognosis
The development of the disease is variable, but often progressive. Periods of remission usually alternate with periods of relapse. The disease is more aggressive in babies and young children. A few rare cases of spontaneous remission have been described, usually in adults. In most cases, in the absence of treatment, the disease naturally develops into massive retinal exudation with detachment of the retina, then to neovascular glaucoma and ocular phthisis (complete loss of the eye). Treatment is effective and, in most cases, leads to remission. However, relapse is frequent and the disease requires lifelong ophthalmological and retinal monitoring every six months.
Treatment
At present, treatment is based mainly on laser photocoagulation. The aim of this treatment is to destroy the retinal vascular abnormalities responsible for exudation using a laser beam. The treatment is very effective and has the advantage of being carried out during a normal consultation, without the need for hospitalization or anesthetic. The vascular abnormalities responsible for exudations are destroyed by several medium-sized impacts of the laser beam. The treatment is applied even in severe forms of the disease responsible for retinal detachments. In complicated forms, when treatment by laser photocoagulation is impossible because of lesions that are too extensive, too peripheral or difficult to see, cryotherapy or endo-ocular surgery may be offered. These forms of treatment require an anesthetic and are usually carried out in an operating theatre.
