What is primary glaucoma and how is it classified?
Glaucoma is a group of optic neuropathies in which retinal ganglion cells and their axons are progressively lost, producing a characteristic cupped optic disc and visual field defects. Raised intraocular pressure (IOP) is the main modifiable risk factor, but glaucoma is defined by the nerve damage, not by the pressure alone.
Primary means there is no other ocular or systemic disease causing the rise in pressure (unlike secondary glaucoma from uveitis, steroids, trauma or a swollen lens). Primary glaucoma is split by the state of the iridocorneal angle on gonioscopy:
| Feature | Primary open-angle (POAG) | Primary angle-closure (PACG) |
|---|---|---|
| Angle on gonioscopy | Open (wide, about 20–45°) | Narrow or closed; iris touches the trabecular meshwork |
| Site of resistance | Trabecular meshwork | Iris blocks access to the meshwork (pupillary block) |
| Onset | Insidious, painless — 'silent thief of sight' | May be acute (painful red eye) or chronic |
| Typical patient | Older adults; family history | Women, hyperopes (short eyes, crowded anterior segment) |
| Definitive treatment | Drops, laser trabeculoplasty, trabeculectomy | Laser peripheral iridotomy, both eyes |
What is normal IOP and how is glaucoma diagnosed?
Normal IOP is about 10 (some texts 12) to 21 mmHg. Goldmann applanation tonometry is the gold standard for measuring it. A diagnosis of glaucoma rests on three pillars examined together:
- Tonometry — IOP (Goldmann applanation).
- Optic disc assessment — cupping, rim thinning, haemorrhages (fundoscopy, imaging).
- Perimetry — characteristic visual field defects.
- Gonioscopy — to decide whether the angle is open or closed, which decides the whole treatment pathway.
What optic disc changes are seen in glaucoma?
Glaucoma thins the neuroretinal rim, so the central cup enlarges. The healthy rim follows the ISNT rule: it is thickest Inferiorly, then Superiorly, Nasally and thinnest Temporally. Early glaucoma tends to break this rule by thinning the inferior and superior poles first.
| Sign | What it means |
|---|---|
| Cup-to-disc ratio above 0.5 | Suspicious enlargement of the cup |
| Asymmetry of 0.2 or more between the two eyes | Suggests glaucoma in the eye with the larger cup |
| Disc (splinter) haemorrhage | Flame-shaped bleed at the disc margin; a marker of progression |
| Bayonetting of vessels | Vessels bend sharply as they dip over the undermined rim edge |
| Violation of the ISNT rule | Focal rim thinning, usually inferior or superior |
What visual field defects occur in glaucoma?
Field defects follow the arcuate course of the retinal nerve fibres, which arch around the fovea and meet at the horizontal raphe on the temporal side. So defects respect the horizontal midline and spread from near the blind spot.
| Defect | Description |
|---|---|
| Paracentral scotoma | Small isolated defect within the central 10–20°; often the earliest |
| Roenne's nasal step | Step along the horizontal meridian on the nasal side |
| Seidel's scotoma | Sickle-shaped extension from the blind spot |
| Bjerrum (arcuate) scotoma | Arc from the blind spot curving around fixation to the raphe |
| Ring (double arcuate) scotoma | Upper and lower arcuate defects joining |
| Tubular vision with temporal island | End stage; a small central island and a temporal island remain |
Which drugs lower IOP and how do they work?
Drugs either reduce aqueous production (ciliary body) or increase outflow (trabecular or uveoscleral). The approximate IOP reductions below are from the StatPearls chapter on open-angle glaucoma.
| Class (example) | Main action | IOP fall | Key adverse effects / cautions |
|---|---|---|---|
| Prostaglandin analogues (latanoprost) | Increase uveoscleral outflow | 25–33% | Eyelash growth, iris and eyelid darkening; once daily in the evening |
| Beta-blockers (timolol) | Reduce aqueous production | 20–25% | Contraindicated in asthma, COPD and heart block; can mask hypoglycaemia |
| Alpha-2 agonists (brimonidine) | Reduce production, increase uveoscleral outflow | 20–25% | Allergic conjunctivitis; apnoea and bradycardia in infants and young children |
| Carbonic anhydrase inhibitors — topical (dorzolamide) | Reduce aqueous production | 15–20% | Stinging, bitter taste |
| Carbonic anhydrase inhibitor — oral (acetazolamide) | Reduce aqueous production | Used short term | Paraesthesia, hypokalaemia, metabolic acidosis, renal stones; avoid in sulfonamide allergy |
| Miotics (pilocarpine) | Contract ciliary muscle, opening the trabecular meshwork | 20–25% | Brow ache, induced myopia, small pupil with poor night vision, retinal detachment risk |
How is primary open-angle glaucoma treated?
The aim is to lower IOP enough to stop further nerve damage. NICE NG81 recommends offering 360° selective laser trabeculoplasty (SLT) as first-line treatment for newly diagnosed ocular hypertension with IOP of 24 mmHg or more and for COAG that is not advanced. A generic prostaglandin analogue is offered when SLT is declined, unsuitable or not enough.
- Laser: selective laser trabeculoplasty improves trabecular outflow.
- Medical: start with a prostaglandin analogue; add or switch to a beta-blocker, alpha-2 agonist or topical CAI if the target pressure is not reached.
- Surgery: trabeculectomy creates a guarded fistula from the anterior chamber to a subconjunctival bleb. Reported success ranges from 31% to 88%, and is higher when antimetabolites (mitomycin C or 5-fluorouracil) are used to limit scarring.
How does acute angle-closure glaucoma present and how is it treated?
In a short eye with a shallow anterior chamber, a mid-dilated pupil (dim light, dilating drugs) increases contact between iris and lens. Aqueous builds up behind the iris (pupillary block), pushes the peripheral iris forward and closes the angle. IOP can rise to 50–80 mmHg within hours.
| Symptoms | Signs |
|---|---|
| Severe eye pain and headache | Red eye with ciliary (circumcorneal) flush |
| Blurred vision with coloured haloes around lights | Hazy, oedematous cornea |
| Nausea and vomiting (may mimic an abdominal emergency) | Fixed, mid-dilated, oval pupil; shallow anterior chamber |
| Often starts in dim light or after dilating drops | Stony-hard eye; after the attack, glaukomflecken (lens opacities) |
- Lower the pressure fast: acetazolamide 500 mg (IV or oral), topical timolol 0.5%, apraclonidine 1%; IV mannitol 1–2 g/kg if the pressure stays high.
- Pilocarpine 1–2% once IOP falls below about 40 mmHg — the ischaemic iris sphincter does not respond at higher pressures.
- Definitive: laser peripheral iridotomy (LPI) is the treatment of choice; it gives aqueous a new path to the anterior chamber.
- Prophylactic LPI in the fellow eye, which shares the same anatomy and risk.
What are PACS, PAC and PACG?
Angle closure is graded along a spectrum (American Academy of Ophthalmology terminology, as summarised in StatPearls):
| Stage | Angle | IOP / synechiae | Optic nerve |
|---|---|---|---|
| PACS — primary angle-closure suspect | Iridotrabecular contact present | Normal IOP, no peripheral anterior synechiae | Normal |
| PAC — primary angle closure | Iridotrabecular contact | Raised IOP or peripheral anterior synechiae | Normal |
| PACG — primary angle-closure glaucoma | Iridotrabecular contact | Raised IOP and/or synechiae | Glaucomatous optic neuropathy |
What are the common exam traps in glaucoma questions?
- Raised IOP is not glaucoma by itself — with a normal disc and field it is ocular hypertension; damage at normal pressures is normal-tension glaucoma.
- Gonioscopy, not tonometry, separates open-angle from angle-closure disease. The angle decides whether the patient needs an iridotomy.
- Prostaglandin analogues act on uveoscleral outflow; pilocarpine acts on trabecular outflow. Beta-blockers, carbonic anhydrase inhibitors and alpha-2 agonists reduce aqueous production.
- Brimonidine is avoided in infants and young children because it can cause apnoea and bradycardia.
- Pilocarpine does not work while the IOP is very high in acute angle closure — the iris sphincter is ischaemic. Lower the pressure first.
- Treat the fellow eye with a prophylactic laser iridotomy after acute angle closure.
- Acetazolamide is a sulfonamide — check for sulfa allergy, and expect paraesthesia, hypokalaemia and metabolic acidosis.
- Visual acuity can be normal in advanced glaucoma; a normal Snellen chart does not exclude it.