Glaucoma, the silent sight stealer

According to the Ayurvedic texts, if a person was blind due to a white visible reflex, it was called ‘safed motia’, a colloquial term still used for cataract. However, if a person was blind without a visible whiteness, it was called ‘kala motia’, to describe what is currently known as ‘glaucoma’, a term from ancient Greek ‘glaukos’, meaning grey. Nearly 80 million people worldwide suffer from glaucoma, half of whom are unaware of its presence.

More than 200 years ago, a Parisian ophthalmologist, Pierre Demours, provided the first clear description of glaucoma, a leading cause of irreversible blindness today. A major treatment breakthrough occurred in 1857 when Albrecht von Graefe from Berlin surgically created a small hole on the periphery of the coloured iris diaphragm for the fluid to bypass the pupil and prevent angle closure. This treatment, highly relevant for our population, is still followed today, though a laser beam has replaced the surgical knife.

The eye maintains its nearly spherical shape due to a constant flow of a transparent fluid known as aqueous humor across the pupil, a hole in the centre of the iris diaphragm, at a pressure ranging from 10 to 21 mmHg. The aqueous fluid exits the eye to enter the bloodstream primarily through specialised channels at the angle located at the junction of the clear cornea and the iris to sustain homeostasis.

However, the eye pressure may rise over the years with or without an apparent block in the fluid flow. Ophthalmologists identify the former as ‘angle closure’ and the latter as ‘open angle’, the two major types of glaucoma, which equally affect glaucoma patients. Asians, including Indians, are more prone to angle closure because of the small shape of their eyes as compared to the white population.

In nearly 25 per cent of angle-closure patients, fluid flow may suddenly become obstructed, leading to a sharp increase in eye pressure and vision loss within days. Patients experience severe eye pain, listlessness, headaches and vomiting. They must seek emergency services for immediate relief from elevated eye pressure. Approximately 30 per cent of patients may face intermittent angle closure, with high pressure alleviated during sleep. Watching movies in the dark may trigger such attacks. During such an episode, individuals may experience blurred vision and see rainbow colours around lights. Nearly all of those suspected of angle closure or showing intermittent closure can be treated with a simple Von Graefe’s procedure. For the remaining patients, it prevents glaucoma’s progression. If left untreated, it may lead to insidious angle closure with increased eye pressure, often resulting in blindness without any symptoms.

The hemispherical back of the eye is lined with a tiered photosensitive layer where highly specialised cells called rods and cones convert light entering the eye into electrical signals through a cycle of sustainable chemical reactions. The process resembles how plants use a green pigment, chlorophyll, in photosynthesis. The electrical signals reach the retinal ganglion cells (RGC), varying in number from 0.7 to 1.5 million (average 1.2 million), via an intricate network of cells. Long fibres (axons) from the RGCs converge from each point in the retina to form the optic nerve at the back of the eye to carry signals to the brain for interpretation and future memory. Peripheral fibres entering the optic nerve are more susceptible to pressure changes in the eye.

For nearly 200 years, it has been known that high eye pressure can lead to the loss of optic nerve fibres, ultimately resulting in blindness. Presently, we understand that the RGCs, the parent cell bodies of the optic nerve fibres, are particularly vulnerable to elevated eye pressure. In one-third of patients, even at low or normal eye pressure — likely due to deprivation of neurotrophic growth factors — the RGCs undergo irreversible, controlled self-destruction, leading to the death of fibres. Even in healthy individuals, the RGCs decline at an average rate of 0.6 per cent per year. However, for unknown reasons, the RGCs may sustain viability for years even with raised eye pressure. Labelled ocular hypertension, they ought to be under close observation with or without treatment. Dr Sushmita Kaushik and Dr SS Pandav at the PGI, Chandigarh, found an 8-10 per cent loss of nerve fibre thickness among the suspected and glaucoma patients over six years who showed progressive visual function loss on perimetry.

The process in patients with open angles is slow and increases exponentially with age, not exhibiting symptoms until late. Undergoing routine eye examination at 40 and annually thereafter is the only way to detect the disease as early as possible, or even earlier if a family member has been diagnosed with glaucoma or if you wear myopic glasses (minus lenses for distant vision).

Besides myopia and hereditary factors, nocturnal obstructive sleep apnoea and smoking increase the risk. Racial factors significantly influence the occurrence of glaucoma, with an 8.5 per cent prevalence among the black population, which is over 2 to 4 times that of white Americans. In Ghana, one in 10 individuals is irreversibly blind, making it the leading cause of irreversible blindness. Surveys conducted in South India and other Asian countries indicate that the prevalence of glaucoma among people above 40 ranges from 2.6 per cent to 3.5 per cent. Four times as many individuals are suspected of having glaucoma and require detailed evaluation. Dr Srishti and Dr Pandav at the PGI, Chandigarh, identified a lack of awareness, illiteracy, and poor socio-economic status as factors contributing to late reporting for treatment; the nerve fibres had been extensively damaged by then.

For several decades, the primary method for assessing the severity of glaucoma has involved measuring eye pressure, evaluating the cavitation of the optic nerve head caused by the thinning of the nerve fibres, and assessing the loss of retinal sensitivity through automated perimetry. This technique entails random automated projections of tiny lights into the eye and charts the light intensity that the patient can perceive. However, by the time these tests show tangible results, nearly 25-35 per cent of the RGCs have already been lost. In the past 25 years, OCT, a quick, non-invasive tool measuring the thickness of nerve fibres around the optic nerve head, has emerged as a more sensitive and reliable technique than measuring the thickness of the RGC layer in the retina. In open angles, the treatment paradigm has shifted from reducing eye pressure through various eye drops or surgeries to administering neurotrophic factors into the eye to postpone inevitable blindness for as long as possible. In a breakthrough, Prof M Francesca Cordeiro from Imperial College, London, has developed a special fluorescent staining technique to determine the health of dying retinal cells to detect glaucoma at the earliest, hoping that emerging therapies can revive the dying cells.

According to a recent report by Dr NN Sood, a well-known glaucoma expert in India, 90 per cent of glaucoma patients were unaware of the type and severity of the disease they were experiencing. Only 3.6 per cent of patients from Karnal and 16 per cent from New Delhi had undergone the most basic yet critical examination to determine whether the drainage channels were open or closed. Furthermore, damage assessment to the optic nerve fibres had not been conducted for the vast majority. Ironically, younger patients were unaware that the disease can run in families.

— The writer is Professor Emeritus, PGI, Chandigarh

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