7 Primary Glaucoma
CLASSIFICATION OF GLAUCOMA
Glaucoma may be classified as primary when the cause of the disease is unknown, or secondary when an increase in IOP occurs secondary to another ocular disease. Primary glaucomas may be further subdivided, as shown in Table 7.1.
INTRAOCULAR PRESSURE
The concept of ‘normal’ IOP is based on a population survey in Europe where readings were assumed to be normally distributed and two standard deviations above the mean gave a normal upper limit of 21 mmHg, implying that only 2.5% of normal people would be expected to have ‘increased’ IOP. However, ‘normal’ IOP is not normally distributed but skewed to the right and as a result a greater proportion of the normal population has an IOP exceeding 21 mmHg than was predicted initially. This right skew increases with age and varies by race; for example, mean IOP in Japan is 11.6 mmHg but that in Barbados is 18.1 mmHg. IOP tends to be higher in older people. Measurement of IOP by methods that applanate the cornea (see Ch. 1) is affected by central corneal thickness which varies between people. The Goldmann applanation tonometer assumes a central corneal thickness of 520 μm; applanation underestimates IOP with thinner corneas and overestimates IOP with thicker corneas. As a rule increased corneal thickness of 10 μm artefactually increases the IOP by 1 mm and similarly underestimates IOP in thin corneas. This is of considerable importance after laser corneal refractive surgery. The factors that regulate IOP are those that alter the rate of aqueous production or outflow resistance.
Fig. 7.1 In a population survey of 2000 Caucasian males aged over 40 years, the normal mean IOP was found to be 16.0 mmHg with a standard deviation of 2.5 mmHg. Two standard deviations from the mean is 21 mmHg, which is usually regarded as the (statistical) upper limit for normal IOP; however, the distribution is skewed with a longer tail to the right, and most of these people have ocular hypertension. Glaucoma patients with an IOP lower than 21 mmHg are regarded as having normal pressure glaucoma; above this level they have either ocular hypertension or glaucoma.
Fig. 7.2 Diurnal pressure curves show that IOP is dependent on posture and time of day. IOP is always higher when supine in comparison to an erect posture. This graph shows that during the day IOP is always lower when erect than when supine. (Night time measurements of IOP were only taken in the supine position). IOP tends to be higher in the mornings than later in the day. IOP also varies seasonally, being slightly higher in winter. Whatever the mechanisms regulating IOP the end result is that the two eyes of an individual usually have a similar IOP.
Fig. 7.3 Variation in IOP during the day is often exaggerated in patients with POAG, clearly illustrated by the 2-hourly pressure readings of the right eye of this patient. Note the ‘normal’ levels from 17.00 to 19.00 hours and the rapid pressure rise between 19.00 and 21.00 hours. It is obviously necessary to have frequent IOP measurements to manage patients with disease progression despite ‘controlled’ IOP. Glaucoma surgery significantly dampens diurnal curves.
AQUEOUS HUMOUR FORMATION AND OUTFLOW
Fig. 7.4 Aqueous is secreted by the ciliary epithelium and flows past the equator of the lens through the posterior chamber and the pupil to reach the anterior chamber. Aqueous leaves the anterior chamber and enters the canal of Schlemm by the trabecular meshwork. It then passes into collector channels and aqueous veins to reach the episcleral veins. Resistance to flow is greatest at the trabecular meshwork. A proportion of aqueous also leaves the eye by draining into the suprachoroidal space and is known as the uveoscleral or nonconventional outflow pathway.
Fig. 7.5 Aqueous humour passes through the canal of Schlemm to drain into collector channels (in the sclera) which empty into the conjunctival veins. This anastomosis can be seen as ‘aqueous’ veins in the conjunctiva.
Fig. 7.6 The trabecular meshwork has an inner lamellated and an outer nonlamellated cribriform (juxtacanalicular) region. The lamellated meshwork is further divided into a uveal portion (between scleral spur and iris root) and a corneoscleral portion (between cornea and scleral spur). The lamellated region is made up of connective tissue plates with a core of elastic and collagen fibres covered by trabecular cells. The juxtacanalicular region has no collagen beams and consists of an elastic network and layers of cells (cribriform cells) within an extracellular matrix. The ciliary muscle tendon inserts into the inner meshwork and scleral spur.
Table 7.2 lists the factors that potentially cause an increase in IOP; these include increased ciliary epithelial production of aqueous, an altered blood–retinal barrier and, more commonly, increased resistance of the conventional outflow channels. Table 7.3 shows factors that may cause a decrease in IOP: decreased aqueous production, structural alterations in the conventional outflow channels, and an increase in outflow by nonconventional routes.
PRIMARY OPEN ANGLE GLAUCOMA
PATHOGENESIS OF GLAUCOMA
Trabecular meshwork
Fig. 7.8 (Left) Scanning electron micrograph showing an en face view of the normal meshwork from the anterior chamber. (Right) Transmission electron micrograph showing details of the meshwork bordering Schlemm’s canal (SC)in a normal eye. Aqueous enters the canal as a result of the formation of giant vacuoles (GV) that rupture into the canal.
Reproduced with permission from Johnson J. Glaucoma 2001, pp 55–67.
Optic nerve damage
A vascular mechanism may have several components relating to systemic blood pressure (BP), local vascular damage and autoregulation. The concept of ‘perfusion pressure’ (mean BP minus IOP) is important in describing the potential effect of either raised IOP or low BP, particularly in the nocturnal hypotension that affects some patients. A primary vessel defect or poor autoregulation involving the short posterior ciliary arteries and the circle of Zinn–Haller that supply the laminar region (see Ch. 17) could cause local vascular insufficiency. Vascular insufficiency may directly damage neurones through ischaemia, hypoxia, or indirectly by activation of astrocytes.
Fig. 7.10 A coronal digest preparation demonstrates the pores in the lamina cribrosa through which the retinal axons pass. The pores have a larger diameter with less supporting collagenous tissue superiorly and inferiorly; this is the area in which glaucomatous visual field damage initially occurs.
Reproduced from Arch Ophthalmol 1990; 108: 51–143.
ASSESSMENT OF THE EYE WITH PRIMARY OPEN ANGLE GLAUCOMA
Key components for assessment of the glaucomatous eye are: IOP measurement, gonioscopy, examination of the optic disc and retinal nerve fibre layer and visual field examination. The techniques of IOP measurement and gonioscopy are covered in Ch. 1.
Gonioscopy
Table 7.4 describes angle grading, derived from Scheie and Shaffer.
Angle grade | Angle width | Description |
---|---|---|
4 | 35–45° | Wide open |
3 | 20–35° | Open |
2 | 20° | Apex of angle not visible, scleral spur visible |
1 | 10° | Posterior half of meshwork not visible, spur not visible, Schwalbe’s line visible |
0 | 0° | No angle structures seen |
Fig. 7.12 Different systems of grading ‘openness’ of the angle have been suggested. In clinical practice it is necessary to know whether: (i) the angle is open and incapable of closure; (ii) the angle is open, but could potentially close (i.e. is narrow); (iii) the angle is closed; and (iv) if peripheral anterior synechiae (PAS) are present in part or throughout. This composite diagram correlates the gonioscopic and microscopic appearances of the angle of the anterior chamber when both ‘open’ and ‘closed’ and shows the relationship of the trabecular meshwork to the surrounding cornea, ciliary body and canal of Schlemm
Fig. 7.13 A goniophotograph of the angle of the anterior chamber in a patient with pigmentary glaucoma shows extensive pigment deposition at the trabecular meshwork so that the angle details are more clearly visible. The trabecular meshwork extends from the anterior pigment deposition (uppermost in this picture) to the ciliary band (anterior iris root). There is a sharply defined midtrabecular band of pigment.