Bitemporal hemianopia · Grey Matter

Bitemporal hemianopia is the loss of the outer half of the visual field in both eyes, the typical sign of something pressing on the middle of the optic chiasm, most often a tumour of the pituitary gland growing upward.

Bitemporal hemianopia. Bitemporal hemianopia is the loss of the outer half of the visual field in both eyes, the typical sign of something pressing on the middle of the optic chiasm, most often a tumour of the pituitary gland growing upward.

At the optic chiasm, fibres from the nasal half of each retina cross to the other side, and the nasal retina sees the temporal (outer) half of the field because the eye's optics invert the image. A mass pressing on the centre of the chiasm from below damages precisely these crossing fibres, so the right eye loses its right half-field and the left eye its left half-field, and the person keeps a central band of vision. The pituitary gland sits just below the chiasm in the sella turcica, which is why a pituitary adenoma is the commonest cause; craniopharyngiomas and meningiomas can do the same.

It often starts in the upper outer quarters. The lower crossing fibres, which carry the upper field, meet a tumour from below first.

It can go unnoticed for a long time, because each eye's lost half is covered by the other eye's intact half, and it may first appear as bumping into things at the sides or trouble with driving.

It is one of the few visual defects that comes with hormonal signs, since the same tumour may overproduce or suppress pituitary hormones.

Questions: Why does the nasal half of the retina see the outer half of the world? The cornea and lens act like a camera lens and project an inverted, reversed image onto the retina. Light from an object far out to the side enters the pupil at an angle and lands on the opposite part of the retina, which is the half nearest the nose. The nasal retina therefore serves the temporal field, and since its fibres are the ones that cross at the chiasm, damage at the crossing removes the outer field of each eye. Why does a pituitary tumour take away the outer half of vision in both eyes? The pituitary sits just below the optic chiasm, and a tumour growing upward out of the sella turcica presses on the chiasm's centre from below. The fibres crossing in that centre come from the nasal half of each retina, which, because the lens inverts the image, sees the outer (temporal) half of the visual field. Damaging them removes the right half-field of the right eye and the left half-field of the left eye, leaving a central band, the pattern called bitemporal hemianopia.