Glaucoma, Primary Open-Angle

BASICS

• Chronic, progressive optic neuropathy with optic nerve rim and RNFL loss.
• Associated with increased IOP, but can occur with normal IOP.
• Normal IOP range: 10-21 mm Hg.
• Early disease difficult to detect due to preserved central vision.

PREGNANCY CONSIDERATIONS

• Avoid prostaglandin analogues during pregnancy.

EPIDEMIOLOGY

• Prevalence in adults >40 years: ~3.5%.
• Second leading cause of blindness in US.
• Leading cause of blindness in African descent populations.

ETIOLOGY AND PATHOPHYSIOLOGY

• Aqueous humor produced by ciliary epithelium in posterior chamber.
• Flows through pupil to anterior chamber, drains via trabecular meshwork (TM) into Schlemm canal, then episcleral veins.
• 5-10% drains via uveoscleral pathway.
• IOP depends on balance between production and outflow; impaired TM outflow causes elevated IOP.
• IOP fluctuates diurnally and with BP, respirations.
• Elevated IOP damages optic nerve causing vision loss, starting peripherally.

GENETICS

• TMCO1 genotype linked to glaucoma risk in non-Hispanic whites.
• MYOC gene first associated with POAG.

RISK FACTORS

• Myopia
• Diabetes mellitus, hypothyroidism, hypertension
• African descent
• Family history of glaucoma
• Prolonged corticosteroid use
• Systemic calcium channel blockers
• High coffee intake
• Prior pars plana vitrectomy
• Obstructive sleep apnea

GENERAL PREVENTION

• Diet high in nitrates and green leafy vegetables linked to lower POAG risk, possibly via nitric oxide effects on circulation.

DIAGNOSIS

HISTORY

• Painless, slowly progressive visual loss, usually unnoticed until late.
• Central vision preserved until advanced.

PHYSICAL EXAM

• Visual acuity and field testing essential.
• Ophthalmoscopy shows optic nerve cupping.
• Tonometry measures IOP, may be normal or elevated.
• Cup-to-disc ratio (CDR) >0.5 suspicious (ISNT rule: inferior ≥ superior ≥ nasal ≥ temporal).
• Early visual field defects: paracentral scotomas, nasal steps.

DIFFERENTIAL DIAGNOSIS

• Normal-tension glaucoma
• Optic nerve pits
• Anterior ischemic optic neuropathy
• Compressive optic neuropathy

DIAGNOSTIC TESTS

• Optical coherence tomography (OCT): detects RNFL thinning.
• OCT angiography (OCTA): shows decreased macular vessel density correlating with progression.
• Visual field perimetry to assess functional loss.
• Tonometry: assess IOP, consider corneal thickness influence.

TREATMENT

GENERAL MEASURES

• Early treatment delays progression.
• Target IOP reduction critical to slow damage.

MEDICATION

• Multiple agents often required with different mechanisms.
• Prostaglandin analogues (first line): latanoprost, travoprost, bimatoprost, latanoprostene bunod.
• β-Adrenergic antagonists: timolol, betaxolol (selective safer in lung disease).
• α2-Adrenergic agonists: brimonidine (avoid in young children).
• Carbonic anhydrase inhibitors (oral and topical): acetazolamide, dorzolamide, brinzolamide (avoid in sulfa allergy and cirrhosis).
• Rho kinase inhibitors: netarsudil (may cause corneal verticillata).
• Parasympathomimetics (miotics): pilocarpine (can cause eye pain, myopia, iridocyclitis).
• Hyperosmotic agents (acute): mannitol IV, glycerin oral (caution in diabetics and cardiac patients).

SURGERY/OTHER PROCEDURES

• Argon laser trabeculoplasty (ALT).
• Selective laser trabeculoplasty (SLT): as effective as ALT, repeatable.
• Trabeculectomy: filtering surgery for advanced or refractory cases.
• Tube shunts (Molteno, Ahmed): for difficult cases.
• Ciliary body ablation: for poor visual potential or unfit patients.
• Minimally invasive glaucoma surgery (MIGS): often combined with cataract surgery.
• Cataract extraction can reduce IOP in ocular hypertension.

ONGOING CARE

FOLLOW-UP

• Monitor vision and IOP every 3 to 6 months.
• Optic nerve evaluation 3 to 18 months depending on control.
• Visual field progression of 2 dB decline in 6 months indicates worsening.

PATIENT EDUCATION

• POAG is a "silent thief of vision."
• Emphasize medication compliance to preserve vision.

PROGNOSIS

• Slow visual field loss with treatment.
• Vision loss and blindness may still occur despite therapy.
• Surgical treatment may better preserve macular ganglion cell layer structure.

COMPLICATIONS

• Blindness

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REFERENCES

1. WuDunn D, Takusagawa HL, Sit AJ, et al. OCT angiography for the diagnosis of glaucoma: a report by the American Academy of Ophthalmology. Ophthalmology. 2021;128(8):1222-1235.
2. Shin JW, Song MK, Sung KR. Longitudinal macular ganglion cell-inner plexiform layer measurements to detect glaucoma progression in high myopia. Am J Ophthalmol. 2021;223:9-20.

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CODES

• ICD10: H40.1190 Primary open-angle glaucoma, unspecified eye, stage unspecified

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CLINICAL PEARLS

• Painless, slowly progressive visual loss; central vision spared until late.
• Patients may lose vision even when treated appropriately.
• Topical/systemic steroids may increase IOP.