Describe primary open-angle glaucoma with its risk factors, pathogenesis and diagnosis.

Definition and Epidemiology

• It is the commonest form of glaucoma in most populations, and it is characteristically asymptomatic until it is advanced — which has earned it the name "the silent thief of sight". Central acuity is preserved until very late, the peripheral loss is gradual and binocular overlap conceals it, so patients present having already lost a great deal of field
• The consequence for practice is that POAG is a disease that must be sought rather than waited for — by examining the disc of every adult whose fundus is being viewed for any reason, and by screening those at risk

Pathogenesis

• The site of obstruction is the juxtacanalicular (cribriform) meshwork, immediately adjacent to Schlemm canal, where the great majority of the outflow resistance normally resides. The changes include thickening of the trabecular beams, accumulation of extracellular material, loss of trabecular endothelial cells and fusion of the beams — so the resistance rises while the angle remains anatomically open and normal-looking
• Damage at the optic nerve head is explained by two theories.
• The mechanical theory — raised pressure deforms and bows back the lamina cribrosa, the sieve-like plate through which the axons leave the eye; the pores become misaligned and directly compress the axon bundles, blocking axoplasmic flow and depriving the ganglion cell body of neurotrophic factors transported from the brain

Clinical Features and Diagnosis

• Symptoms — none, in the great majority, until the disease is advanced. Late symptoms are of missing objects to one side, difficulty in dim light, bumping into things and difficulty driving; and central acuity is normal until the very end, which is why acuity is a useless screening test for glaucoma
• The diagnosis rests on five examinations, and all five are required:
• 1. Tonometry — the pressure, with pachymetry to interpret it and diurnal phasing where there is doubt
• 2. Gonioscopy — to confirm that the angle is open, which is what makes it open-angle glaucoma, and to exclude secondary causes such as recession, neovascularisation or pigment
• 3. Optic disc assessment — ideally stereoscopically with a slit lamp and a 78 or 90 dioptre lens, looking for cupping, rim notching, violation of the ISNT rule, asymmetry between the eyes, disc haemorrhage and retinal nerve fibre layer defects
• 4. Perimetry — standard automated perimetry.
• 5. Imaging — optical coherence tomography of the retinal nerve fibre layer and ganglion cell complex, which detects structural loss before the field defect appears and provides objective, quantitative monitoring
• Normal-tension glaucoma deserves separate mention: typical glaucomatous damage with a pressure never above 21. It is associated with vasospasm, migraine, Raynaud phenomenon, nocturnal hypotension and sleep apnoea; disc haemorrhages are commoner; and the field defects tend to be deeper, steeper and closer TO fixation. Before accepting the diagnosis, exclude a missed pressure peak on diurnal phasing, previous steroid use, a past episode of raised pressure such as uveitis or trauma, and — critically — a compressive lesion of the optic nerve or chiasm. Neuroimaging is indicated where the patient is young, the acuity or colour vision is disproportionately reduced, the field defect respects the vertical midline, or there is pallor of the rim exceeding the cupping
• Central acuity is normal until the end, so visual acuity is useless as a screening test for glaucoma
• Binocular overlap conceals the field loss, so patients do not notice a defect until it crosses fixation or both eyes are affected
• Ask about a first-degree relative; it is the single most useful question in the history and multiplies the risk several-fold
• Tell the patient to bring their siblings; case-finding through families is the most efficient screening there is