Describe perimetry and the visual field defects of glaucoma.

Principles of Perimetry

• Perimetry measures the function that glaucoma destroys, and is the basis on which progression is judged and treatment intensified
• The normal visual field extends 60 degrees superiorly, 60 nasally, 75 inferiorly and 100 temporally, with the blind spot 15 degrees temporal to fixation
• Traquair’s "island OF vision in a sea OF darkness" — the field is conceived as a hill of decreasing sensitivity from a peak at fixation, with the blind spot as a bottomless pit. Perimetry maps this hill

The Sequence of Glaucomatous Field Loss

• 1.
• Generalised depression and increased short-term fluctuation — non-specific, and also caused by cataract, a small pupil and an uncorrected refractive error → 2.
• Paracentral scotoma in bjerrum’S area — an isolated defect 10 to 20 degrees from fixation, characteristically in the superonasal field, corresponding to inferotemporal rim loss.
• Often the earliest identifiable defect → 3.
• Nasal step of ROENNE — a step-like difference in sensitivity across the horizontal meridian nasally, arising because the superior and inferior arcuate nerve fibre bundles meet at the temporal raphe and are damaged unequally.
• Highly characteristic → 4.
• Arcuate (bjerrum) scotoma — the paracentral defects coalesce into an arc sweeping from the blind spot around fixation to the nasal horizontal meridian, following the arcuate nerve fibre bundles → 5.
• SEIDEL scotoma — an arcuate defect still joined to the blind spot, forming a comma-shaped extension → 6.
• Double arcuate (ring) scotoma — superior and inferior arcuate defects join, encircling fixation → 7.
• Peripheral nasal breakthrough and progressive constriction → 8.
• A central and a temporal island remain — and this is the state in which a patient may still read 6/6 while being legally blind and unable to walk safely.

Applied Aspects

• Check the reliability indices before reading the field; an unreliable test is misleading rather than merely useless
• Never act on a single abnormal field; repeat it, because of the learning effect and short-term fluctuation
• Use 10-2 in advanced disease; a 24-2 has too few central points to monitor a remaining island, and progression will be missed
• Correct the refraction and check for ptosis and a small pupil before testing; each produces a defect that will be attributed to glaucoma
• Watch PSD rather than MD in a patient with cataract; the cataract depresses everything and inflates MD while PSD reflects the true glaucomatous loss