Cherish the Gift of Sight

written by Dr Cheong Fook Meng on 21 October 2013
CHERISH THE GIFT OF SIGHT
 
The gift of sight has been considered the most precious of all our special senses. Many regard their eyes to be their “windows to the world”. The early detection of eye diseases is essential for the long-term preservation of good quality vision in the aging eye, for it to remain our window to the world for as long as possible.
 
GLAUCOMA: The silent stealer of sight
 
Glaucoma is one such eye disease that fits into this category of preventable blindness from early detection.
 
It is primarily a neurodegenerative disease of the optic nerve. In general terms, glaucoma represents a group of eye diseases defined by progressive optic neuropathy. It is characterized by progressive loss of neuroretinal fibres at the optic discs with distinctive patterns of visual field loss. The diagnosis of glaucoma is made on the basis of characteristic excavation and undermining of the neural and connective tissue elements of the optic disc, with eventual development of distinctive patterns of visual dysfunction. The intraocular pressure is no longer an essential criterion in the diagnosis of glaucoma in current practice. 
 
When darkness falls
The final common pathway of glaucoma progression is retinal ganglion cell damage. Axons of the retinal ganglion cells make up the 1.5 million fibres of the optic nerve that transmit photoreceptor impulses to the visual cortex. These axons are first gathered together to form the neuroretinal rim of the optic disc as they exit the eye. 
 
Death of the ganglion cell is primarily by apoptosis, a form of programmed cell death. Recent studies have indicated associated damage to the support cells of the retina, such as Muller cells and astrocytes, which are sensitive to intraocular pressure variations. Dysfunction in these cells results in the loss of normal protective functions making the ganglion cells susceptible to axoplasmic flow block and excitotoxicity damage.
 
Progressive damage to the ganglion cell population will result in dimming of vision, constriction of the field of vision and progressive loss of visual acuity.
 
 
 
Just enough and not too much
The intraocular pressure (IOP) is no longer the essential factor in the diagnosis of glaucoma in current practice. However, it still remains the most important modifiable risk factor and the main focus of glaucoma treatment. The IOP is determined by the equilibrium of aqueous fluid production in the ciliary processes in the posterior chamber and its outflow facility out of the eye through the trabecular meshwork and uveoscleral route. 
 
The traditional upper limit of normal IOP is 21mmHg, which was derived from large population-based epidemiologic studies, which found the mean IOP in the general population to be 15.5mmHg with a standard deviation (SD) of 2.6mmHg.
 
Some factors to be taken into account when evaluating the measured IOP are circadian cycle, central corneal thickness, blood pressure, intra-abdominal pressure, age, exercise, posture, fluid intake and diuretic effect of substances such as alcohol.
 
  
Ignorance is bliss?
Epidemiological analyses of the World Health Organization indicate that there are 104.5 million people worldwide with intraocular pressures above 21mmHg. The prevalence of glaucoma rises dramatically with increasing age, rising from 1%-4% of the population over 40 years of age to nearly 20% in certain ethnic groups over 80 years of age. It was estimated in 2010 that 8.4 million individuals were bilaterally blind from glaucoma worldwide. Predictions suggest that by 2020, over 11 million people will be bilaterally blind from glaucoma. Glaucoma was calculated to be responsible for 12.3% of blindness, making it the second leading cause of blindness worldwide, after cataract. Whilst cataract blindness is treatable, blindness from glaucoma is irreversible.
 
The many faces of glaucoma
There is a multitude of glaucoma sub-types that is beyond the scope of this article to list comprehensively. The wide variety of these sub-types can be classified into:
 
a) Open angle: Primary, secondary or normal tension
b) Angle closure: Primary or secondary, and other sub-categories such as acute, subacute or chronic
c) Developmental: Primary congenital or secondary associated with congenital anomalies
 
Of these, the more common and important sub-types in clinical practice are:
 
1. PRIMARY OPEN ANGLE GLAUCOMA (POAG) – the silent stealer of sight
This is the most common sub-type and occurs in adults with a normal (“open”) appearing anterior chamber angle on gonioscopy. It is usually insidious in onset, slowly progressive and painless. Because the central vision is relatively unaffected until late in the disease, patients will have developed severe visual fields loss by the time they present with visual symptoms. Late visual symptoms include reduced night vision, decreased peripheral vision (bumping into things), dimming or blurring of vision and “tunnel vision”. Risk factors are high IOP, advancing age, positive family history and ethnicity (more common in people of African and Hispanic descent). It is associated with high myopia (short-sightedness), retinal vein occlusion, diabetes mellitus and blood pressure variations.
 
2. ANGLE CLOSURE GLAUCOMA – the devastating masquerader
This type is only one third as common as POAG but is more visually devastating, causing nearly half of all glaucoma blindness. It is more common in the East Asian countries compared to other regions. There is a rapid increase of the IOP from acute obstruction of the anterior chamber drainage angle. Early warning symptoms are haloes around lights, transient blurring or aching of the eyes. Late symptoms are eye pain, blurred vision, headaches, nausea and vomiting. In an established attack, the affected eye will have a red, congested conjunctiva, hazy cornea and a fixed mid-dilated pupil. The presenting symptoms can be mistaken for that of acute gastroenteritis or a neurological aetiology, resulting in further delays in its diagnosis. Risk factors are race (higher risk in Inuits, Mongolians and Chinese), female gender (2-4X), family history (up to 6X) and hypermetropia (long- sightedness, irrespective of race due to the shorter eyeball length).
 
3. CORTICOSTEROID-INDUCED GLAUCOMA – the tragic iatrogenic risk
 There are a multitude of secondary causes of glaucoma. Perhaps the most preventable of these is the  corticosteroid-induced type. It is caused by prolonged use of topical, peri/intraocular, inhaled or systemic  corticosteroids. Approximately one-third of patients will demonstrate some responsiveness to corticosteroids. A  small percentage of these will have a clinically significant elevation in IOP, leading to glaucoma and visual loss.  The mechanism is thought to be an increased resistance to aqueous outflow in the trabecular meshwork. A high  percentage of patients with POAG demonstrate this response to topical corticosteroids. Systemic administration  may also raise IOP in some individuals but less frequently than topical administration does. Hence, the IOP needs  to be monitored regularly in all patients receiving prolonged corticosteroid treatment. This is especially true in  patients using topical corticosteroids for their allergy treatment.
 
For whom the bell tolls
Indiscriminate glaucoma screening of the general population has not been shown to be cost effective in various epidemiological models. However, targeted or opportunistic screening of at risk individuals is recommended. Individuals who are at a higher risk of developing glaucoma are:
 
1. Older age group. Prevalence of glaucoma rises with increasing age. Individuals over the age of 60 have a six fold increase in risk.
2. Relatives of glaucoma sufferers. A positive family history of glaucoma increases the risk four to nine times.
3. Short-sighted individuals. Individuals with a high degree of myopia are more prone to glaucoma. 
4. Individuals with diabetes mellitus and circulatory insufficiency. Individuals with poor diabetic control and compromised blood circulation, together with patients with hypertension (younger patients) and hypotension (older patients) have increased risk of glaucoma progression due to hypo-perfusion of the optic nerve head.
5. Individuals of African and Hispanic descent. It is six to eight times more common in African Americans and in Hispanics over the age of 60.
6. Asians. Individuals of Asian descent appear to be at some risk for angle-closure glaucoma, especially hypermetropic (long-sighted) females of oriental descent. This is partly due to their relatively shorter eyeballs.
 
Don’t wait, but rather, act!
The value of the clinical slit-lamp based examination has been proven by the test of time. The typical case of glaucoma can often be detected by clinical evaluation alone. However, in the quest for early diagnosis of glaucoma, the following investigative modalities have been incorporated into current practices of glaucoma assessment:
 
Pachymetry: This measures the central corneal thickness. Central corneal thickness has the potential to influence eye pressure readings. It has been shown to be a risk factor for subsequent progression to glaucoma in individuals with ocular hypertension.
 
Nerve Fibre Layer Imaging: This modality measures the thickness of the nerve fiber layer and creates a three dimensional representation of the optic nerve head. It is potentially a sensitive method for evaluating progression with sequential scans. Three main types of these are Optical Coherence Tomography (OCT), Scanning Laser Polarimetry (GDx) and Confocal Scanning Laser Ophthalmoscopy (HRT)
 
Perimetry: This is a visual field test that demonstrates the state of optic nerve function and visual sensitivities. Newer perimetric strategies and algorithms have been introduced to detect changes at an earlier stage.
 
Preserving sight and improving vision
The aims of glaucoma management are to maintain good functional vision throughout the patients’ lifetime with minimal negative impact on the quality of life. The key element of successful treatment lies in the early detection of the disease. The goal of treatment is the reduction of risk factors to prevent disease progression or to slow its rate of deterioration. This involves IOP reduction, elimination of angle closure risk and treatment of associated pre-disposing factors such as diabetes mellitus and blood pressure control.
 
 
 
Although it is no longer an essential diagnostic criterion, the intraocular pressure remains the most important modifiable risk factor and is the primary focus of glaucoma treatment. A reduction of between 25 – 40% is generally required to prevent progression. The actual target IOP reduction depends on baseline IOP, stage of disease, estimated progression rate, other risk factors and life expectancy of the patient. The treatment modalities available are medical, laser therapy and surgery.
 
Medical treatment: There is now a good selection of different classes of topical medications, which have resulted in a significant reduction in the need for surgery in patients with glaucoma. The medications have different mechanisms of action and can be used in combination for greater effect. They are generally well tolerated by patients. In addition, there are now an increasing number of fixed combination eye drops to improve patient compliance.
 
Laser therapy: A variety of laser based treatment options are available.
Laser trabeculoplasty involves the application of laser energy to the trabecular meshwork to enhance aqueous outflow. This is useful for short to medium term IOP control.
Laser peripheral iridotomy uses laser energy to create a small hole in the peripheral iris to relieve pupil block in primary angle closure glaucoma.
Laser peripheral iridoplasty is beneficial in both acute and chronic forms of primary angle closure glaucoma. The laser spots are applied circumferentially to the far peripheral iris to cause contraction of the iris tissue from the drainage angle.
Laser cycloablation reduces aqueous production by applying laser to the ciliary processes that produces the aqueous fluid. It is a destructive procedure and is used mainly in severely damaged eyes with advanced secondary glaucoma.
 
Surgery: This is usually considered if glaucoma continues to progress despite maximum medical therapy. It is also indicated as a primary intervention in the younger patient who presents with severe glaucoma damage. The principle of surgery is that of a trabecular meshwork “by-pass” surgery, in which an alternative aqueous outflow route is fashioned to reduce the IOP to a significantly lower level. Trabeculectomy is still the gold standard for glaucoma drainage surgery, with good long term results and low complication rates. Alternatives are the non-penetrating drainage surgery such as viscocanalostomy and deep sclerectomy. A recent addition to glaucoma surgery is the new mini shunt implant that can be inserted into the eye through a smaller incision. Its long-term results are being evaluated. When traditional drainage surgery fails, a tube implant may be necessary to maintain aqueous outflow. The silicone tube can be left in the eye indefinitely. In eyes with angle closure glaucoma and a cataract, phacoemulsification of the cataract will result in a significant decrease in IOP and will also eliminate the risk of acute closure in future. As an added bonus, the unaided visual quality can also be improved at the same time from the use of modern intraocular lenses.
 
And finally, for brighter and better…
Glaucoma is primarily a neurodegenerative disease of the optic nerve resulting in visual loss.
Glaucoma blindness is preventable.
Key to prevention is early detection of the disease process.
Intraocular pressure remains the most important modifiable risk factor and is the main focus of treatment.
Effective treatment modalities for reducing IOP are available to help preserve vision and maintain a better quality of  life.
 
 
AUTHORS:
 
1) Dr. Cheong Fook Meng,  FRCS(Edin), MRCOphth(Lond), MBBS(London), AMM
 Consultant Ophthalmologist, a Member of Harley Street Group
 
2) Ms. Teoh Shu Kheng, B.A. (Malaya)
 Research Assistant