A defect that stops at the horizontal line points to the retinal nerve fiber layer or the optic nerve. The rule comes straight from anatomy. Nerve fibers run in a superior group and an inferior group. Neither group crosses the horizontal midline of the retina. So damage to those fibers makes a defect that stops at that line, exactly as seen here. Only the right eye is involved, which also fits an optic nerve problem in that eye.
Two midlines sort most field defects. A defect that respects the horizontal midline comes from the retinal nerve fiber layer or the optic nerve, and glaucoma is the common cause. A defect that respects the vertical midline comes from the optic chiasm or farther back in the brain. That is because fibers from the nasal half of each retina cross at the chiasm. After the crossing, each side of the brain handles one half of the world. A defect that respects neither midline is more likely to start in the retina itself. Words matter here: to respect a midline means the defect does not cross it. Say it that way when you chart, and the doctor will know what you saw.
Look at the shape before you look at the numbers. The outline of a defect tells you where in the pathway to look. Stops at the horizontal line means think optic nerve. Stops at the vertical line means think brain. Reading the shape of a field printout and knowing what it suggests is central to the COA content area of visual field testing.
Incorrect Answers
- A. A lesion at the chiasm gives a defect that respects the vertical midline, and it usually shows in both eyes at once.
- C. The optic radiations sit behind the chiasm, so their defects also stop at the vertical line and appear in both eyes.
- D. Damage in the occipital cortex produces a matching defect in the same half of both fields. It does not make a one-eye arc that stops at the horizontal line.
Testing Pearls
- Retinal nerve fibers never cross the horizontal midline, so nerve defects stop there.
- Defects that respect the vertical midline mean a problem at or behind the optic chiasm.
- Defects that respect neither midline may start in the retina.
- An arc-shaped defect above or below fixation is an arcuate scotoma, typical of glaucoma.
- Always name the eye tested; the right eye (OD) is charted first.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 11, Principles and Techniques of Perimetry.
