This pattern is a bitemporal hemianopia, and it places the lesion at or near the optic chiasm. Bitemporal means the missing part is the temporal half, the outer half toward the ear, in both eyes. The chiasm sits just above the pituitary gland, so a pituitary tumor is a classic cause. Headaches with normal central vision fit that picture, because the center of the field is spared while the outer halves go.
Here is the wiring. Fibers from the nasal half of each retina carry the temporal half of the field. Those nasal fibers cross at the chiasm. A mass pressing in the middle of the chiasm squeezes the crossing fibers first. Losing both sets of crossing fibers wipes out the temporal field of each eye and leaves the nasal halves intact. Compare that with damage behind the chiasm. There the loss is homonymous, meaning the same right or left half is missing in both eyes. Stroke or brain tumor is the usual cause. Both patterns respect the vertical midline, which is the clue that the trouble is neurologic.
Learn the two names together. Bitemporal means chiasm; homonymous means behind the chiasm. A brand new defect of either kind is urgent, because the cause may threaten life as well as sight. Tell the ophthalmologist before the patient leaves the office, and do not wait for the next visit. Recognizing these patterns and reporting them promptly is a key duty in the COA content area of visual field testing.
Incorrect Answers
- A. Retinal disease in two eyes at once would rarely produce mirror-image temporal loss, and central vision would usually drop below 20/20.
- B. A lesion in one optic radiation gives a homonymous defect, so the same side of the field is lost in both eyes.
- D. An occipital lobe lesion on the left takes out the right half of both fields, not the outer half of each.
Testing Pearls
- Bitemporal hemianopia equals a lesion in or near the optic chiasm.
- Homonymous hemianopia equals damage to the optic tract or optic radiations.
- A missing quarter of the field is a quadrantanopia, often from the optic radiations.
- Nasal retinal fibers cross at the chiasm; temporal retinal fibers stay on their own side.
- Report new half-field or quarter-field loss the same day; the cause can be life threatening.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 11, Principles and Techniques of Perimetry.
