Glaucoma: The slow thief of sight

 Dr. Femi Emmanuel  Babalola. Mbchb. FWACS, FMCophthal

Consultant ophthalmologist, and president, Ophthalmological Society of Nigeria.

Director, Rachel Eye Center,

Gimbiya street.

Garki, Abuja.

www.racheleye.com

Introduction:

The Glaucomas are a group of diseases characterized by increased intraocular pressure, optic nerve disease and visual field loss.

There are basically two groups. These are

• The Primary glaucomas

• The Secondary glaucomas.

Essentially for the primary glaucomas, there is no directly attributable cause. These in turn can take two forms.

• Primary open angle glaucoma (POAG)

• Primary angle closure glaucoma.

The primary angle closure glaucomas can be acute or chronic. The acute forms are associated with sudden increase in the intraocular pressure, redness and pain. This is an ocular emergency that often needs early surgical intervention.

Most of this write up will focus on the Primary open angle glaucoma (POAG), as this is the entity that tends to cause blindness and is referred to colloquially as the ‘slow thief of sight’. It is a ‘thief’ because it is often symptomless.  It is usually diagnosed by chance when the patient comes for a refraction or presents with some unrelated minor eye complaint. Sometimes patients come only after they have lost nearly all vision in one of the eyes when it is too late to reverse visual loss. Only few patients are health-conscious enough to come for a glaucoma screening. Sometimes a patient might appear to need frequent changes in his refraction and then a look at the retina indicates glaucomatous change. Since POAG tends to become more common after the age of 40, all persons in this age group are advised to come for screening at least once a year, particularly if there is a positive family history.

However it must be noted that glaucoma can set in at any age. We thus have also congenital glaucoma, infantile glaucoma, and juvenile glaucoma

Next to cataracts, glaucoma is probably the second commonest cause of blindness in Nigeria. One out of every two hundred adults over 40 may have glaucoma.

Signs of glaucoma.

There is a tripod of signs on which the diagnosis of glaucoma rests:

• Raised intra-ocular pressure

• Changes in the optic nerve head appearance

• Changes in the central visual fields.

The intra-ocular pressure: Aqueous is mostly formed in the ciliary body and transported from the posterior chamber through the pupil into the anterior chamber of the eye. It helps to maintain the tension inside the eye which is after all a hollow viscera.  The aqueous escapes through the angle of the anterior chamber particularly through the trabecular meshwork and subsequently into the collector channels and Schlem’s canal. The intra-ocular pressure is measured in millimeters of mercury (MMhg) using instruments referred to as tonometers of which there are various types. The most modern variety are the so called Non-Contact tonometers (NCT) which rely on a puff of air to measure the pressure.  The upper limit of normal is traditionally taken as 20mmhg, while the range is often between 10-15. However there is nothing magical about these figures as some may have low intraocular pressure and yet demonstrate glaucomatous optic nerve damage, (so called Low tension glaucoma) , while some individuals may have a high pressure and yet demonstrate little or no glaucomatous damage (so called ocular hypertensives). Obviously the ocular hypertensives are a group that need to be monitored closely for evidence of optic nerve damage and treatment ought to be instituted if there is any evidence of optic nerve damage. Some ophthalmologists prefer to institute treatment even when there is no overt damage, in the belief that it is likely that damage may occur. The OHTS group (ocular hypertension treatment study group) attempted to look at this in a systematic way through a series of randomized control trials and concluded that  50% of glaucoma can be prevented by early treatment of ocular hypertension.²

Optic nerve damage is probably the key element in the patho-physiology of glaucoma. The sentinel nerve fibres of the retina aggregate into the optic nerve which ultimately link to the visual cortex. Damage to the optic nerve means that proportionately less electrical information is conducted to the visual cortex are reduced.

The optic nerve head is what is visible on ophthalmoscopy. There are three components to the optic nerve head. These are the margin, the rim and the cup. A simple analogy to describe these components is the common toilet seat found in many water cisterns. The margin is the edge of the seat. It is supposed to be sharply defined and not blurred. The rim is the area on which the individual seats, and the cup is the central depression into which waste is passed. A healthy rim is supposed to be wide and pink, taking up between 60 to 90 percent of the total surface area. The cup inversely takes up only 10 to 40 percent. The ratio of the width of the cup to the entire disc is a reflection of this relationship and is referred to as the Cup to Disc ratio (or CD ratio for short). A normal  C/D ratio is usually taken as 0.4 or less. A ratio of between 0.4 to 0.6 is suspicious while a ratio of 0.7 and above is clearly pathological. Obviously there are some exceptions to these rules where you have so called ‘healthy cupping’ but by and large these are the ranges used as guides in clinical practice.

Other changes in the optic nerve head which may be suspicious are the notching of the rim, nasalization of optic vessels, bayonet sign and lamina dot sign, in which the lamina cribrosa becomes very prominent. A grooving of the Nerve fibre layer is a sign that is also often sought. Damage to the optic nerve is not just attributable to increased IOP alone, but is really a factor of the micro-nutrition and vascular supply. The factors responsible for these changes are yet to be fully understood but apoptotic mechanisms have been blamed.

The Visual Fields are a way of quantifying the damage done to the optic nerve. There are several ways in which the visual fields are assessed, using varying levels of sophistication, ranging from the simple Bjerrum screen, to the state of the art Computerised visual field systems such as Humphrey’s or Kowa. The Motions Sensitivity test is also useful. Visual field changes cannot be properly quantified clinically without instrumentation until the very late stages, so these instruments are important. There are typical changes in the central visual fields which tend to be diagnostic, but by and large, the peripheral vision of the patients gets progressively reduced until the patient gets what is called ‘tunnel vision’. This usually entails that the visual field has been reduced to about 10 degrees or less from central rest.

Management.

The management of glaucoma is aimed at reducing the intra-ocular pressure. These days the concept of target pressure has gained grounds. It means that the particular pressure at which a patient no longer loses visual fields is determined and steps are taken to ensure that this target is attained and maintained. Pressure control can be medical or surgical. There are several medical approaches to the control of glaucoma. The more commonly used ones include the Beta antagonists such as Timolol maleate, the miotics such as Pilocarpine, and the carbonic anhydrase inhibitors such as Diamox. The latter comes in tablet form, while the former two are topical in form of eye drops. Newer medicaments include the Dorzolamide, which are also carbonic anhydrase inhibitors in topical form, and the Prostaglandin analogues such as Xalatan and Travatan, which have revolutionized pressure control but tend to be very expensive.

Surgical treatment is manly in the form of filtration procedures that afford an alternative pathway for escape of aqueous. The most common procedure done is called trabeculectomy.  Sometimes a valve, such as the Ahmed valve, is inserted in difficult cases.

Risk factors:

Glaucoma tends to run in families, but the inheritance pattern does not necessarily follow a simple Mendellian pattern.  Children of glaucoma patients have a 4.6% chance of developing the disease, while siblings are at a higher risk of about 10%. The HLA glc1a gene is more common however in glaucoma patients than in the general population.²  African descent is an important risk factor. The prevalence is generally higher and the prognosis worse.

Diabetics also have a higher prevalence of POAG than non-diabetics.

Elevated systolic blood pressure is also associated with POAG in untreated patients, and treatment does reduce the risk.

The role of the General Practitioner or non-ophthalmic specialist.

1. Know the basic facts about glaucoma as outlined above.

2. Recognise when a patient is at risk and advise accordingly. For example,  positive family history, diabetes, hypertension, age over forty. Advise on an ophthalmic consult accordingly.

3. Recognise ‘narrow field behaviour’ in your patients.  For instance a patient can see well straight ahead, but bumps into things by the side.

4. The fundus does not bite and it is certainly not a NO GO area for non-ophthalmologists! Peek into the fundus from time to time, particularly in patients above 40, and know how a normal disc should appear.

5. Sit down with your classmate who is an ophthalmologist on a lazy afternoon and revise the basics of fundoscopy. You will be surprised how simple it really is!

6. It is only when we all share a high index of suspicion that we can begin to make an impact on this ‘slow thief of sight!’

7. Good luck.

Figure 1. Top Normal view from a non-glaucoma eye. Bottom. Same view as seen through the eye of a glaucoma patient with field defects.

Figure 1                                                                        Figure 2

Figures 2: Top-  Normal disc on funduscopy and in cross section. Note pink healthy rim and a C/D ratio of about 0.25.

Bottom- Glaucomatous disc and cross section. Note large C/D of about 0.95, bean pot cupping, Bayonette sign of vessels entering cup, and a positive lamina dot sign.

References:

1. Wallace LM, Alward MD, John HF et al.  Clinical features associated with mutations in the chromosome 1 open-angle glaucoma gene. (GLC1a). The New Eng. J. Med. 1998. 338:1022-1027 number 15.

2. Heuer DK, Higginbotham EJ, Kass MA et al. and the Ocular Hypertension Treatment Study Group. Is there a penalty for delayed treatment in ocular hypertensives? Evidence from the Ocular Hypertension Treatment Study (OHTS). Presentedat ARVO, May 4-9, 2003.