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General Information ⬇

Definition

POAG occurring in patients without IOP elevation.

Symptoms ⬆ ⬇

See 9.1, PRIMARY OPEN-ANGLE GLAUCOMA.

Signs ⬆ ⬇

Critical

See Signs in 9.1, PRIMARY OPEN-ANGLE GLAUCOMA, except IOP is consistently below 22 mm Hg. There is a greater likelihood of optic disc hemorrhages. Visual field defects are denser, more localized, and closer to fixation. A dense nasal paracentral defect is typical.

Differential Diagnosis ⬆ ⬇

NOTE:

If optic nerve changes and atrophy are unrelated to IOP, it is imperative to investigate potential etiologies of an optic neuropathy other than glaucoma.

  • POAG: IOP may be underestimated secondary to large diurnal fluctuations or thin corneas. See 9.1, PRIMARY OPEN-ANGLE GLAUCOMA.
  • Shock-related optic neuropathy from previous episode of systemic hypotension (e.g., acute blood loss, myocardial infarction, coronary artery bypass surgery, arrhythmia). Visual field loss should not progress.
  • Intermittent IOP elevation (e.g., angle closure glaucoma, glaucomatocyclitic crisis).
  • Previous glaucomatous insult with severe IOP elevation that has subsequently resolved. Nonprogressive (e.g., traumatic glaucoma, steroid-induced glaucoma).
  • Nonglaucomatous optic neuropathy and others. See DIFFERENTIAL DIAGNOSIS in 9.1, PRIMARY OPEN-ANGLE GLAUCOMA.

Etiology ⬆ ⬇

Controversial. Most investigators believe that IOP plays an important role in low-tension POAG. Other IOP-independent proposed etiologies include vascular dysregulation (e.g., systemic or nocturnal hypotension, vasospasm, or loss of autoregulation), microischemic disease, accelerated apoptosis, and autoimmune disease.

Reference(s) ⬆ ⬇

Comparison of glaucomatous progression between untreated patients with normal-tension glaucoma and patients with therapeutically reduced intraocular pressures. Collaborative Normal-Tension Glaucoma Study Group. Am J Ophthalmol. 1998;126(4):487-497.

Work Up ⬆ ⬇

Workup

See Workup in 9.1, PRIMARY OPEN-ANGLE GLAUCOMA. Also consider:

  1. History: Evidence of vasospasm (history of migraine or Raynaud phenomenon)? History of hypotensive crisis (recent surgery), anemia, or heart disease? Prior corticosteroid use by any route? Prior ocular trauma or uveitis? Has the vision loss been acute or chronic? GCA symptoms? Additional cardiovascular risk factors such as elevated cholesterol, hypertension, and systemic hypotension (including nocturnal “dippers” in the early morning hours)?
  2. Check color plates to rule out optic neuropathy.
  3. Check gonioscopy to rule out angle closure, angle recession, or PAS.
  4. Consider obtaining a diurnal curve of IOP measurements to help confirm the diagnosis.
  5. Consider carotid Dopplers to evaluate ocular blood flow. Check blood pressure (consider 24-hour automated blood pressure home monitor).
  6. Consider CT or MRI to rule out compressive lesions of the optic nerve or chiasm especially in cases of decreased visual acuity, color plates, or visual fields suggestive of nonglaucomatous process.

Treatment ⬆ ⬇

  1. The Collaborative Normal Tension Glaucoma Study (CNTGS) established treatment guidelines for this entity. IOP lowering by at least 30% reduced the 5-year risk of visual field progression from 35% to 12%, thus target IOPs are at least 30% lower than the level at which progressive damage was occurring. Therapies are those for POAG. See 9.1, PRIMARY OPEN-ANGLE GLAUCOMA, for a more in-depth discussion of these therapies. There is evidence that initial therapy with brimonidine 0.2% b.i.d. may be superior to timolol 0.5% b.i.d. in preventing visual field progression in low-tension POAG.
  2. Ischemia to the optic nerve head may play a role in the pathogenesis of low-tension POAG. Modification of cardiovascular risk factors is appropriate in managing general health but has not proven beneficial in managing glaucoma. Refer to an internist for control of blood pressure, cholesterol, and optimal management of other comorbid conditions to maximize optic nerve perfusion. If possible, avoid use of antihypertensive drugs at bedtime and use preferentially in the morning.
  3. Presence of disc hemorrhages is more common in low-tension glaucoma and is suggestive of progressive disease, warranting more aggressive treatment.

Follow Up ⬆

See 9.1, PRIMARY OPEN-ANGLE GLAUCOMA.