Bitemporal hemianopia: optic chiasm compression
Loss of the outer half of each eye's visual field (the view it takes in), called bitemporal hemianopia, points to the optic chiasm, where the optic nerves meet.
Remember
Compression of the optic chiasm, classically by a pituitary tumour, damages the crossing fibres, which carry the outer fields, so the outer halves of both fields are lost.
Memory aid
a pair of horse blinkers, which cut off the sides of the view
Picture horse blinkers to recall that the sides of the view are lost in both eyes.
The retina is where the image forms, and that image is reversed. So the nasal retinal fibres, those from the half of each retina nearest the nose, see the temporal field, the outer part towards the temple. These fibres cross, or decussate, to the opposite side at the optic chiasm, the X-shaped crossing point where the two optic nerves meet just above the pituitary gland. The temporal retinal fibres stay on their own side.
A mass pressing on the middle of the chiasm squeezes the crossing fibres from both eyes. A pituitary tumour is the most common cause, because the gland sits directly below the chiasm. Both temporal fields go together, which is called bitemporal hemianopia (hemianopia means loss of half of the visual field, and bitemporal means the temporal half of both eyes). Patients may bump into things at their sides, and visual acuity (the sharpness of vision) can stay normal.
The pattern of loss also tells you where the damage lies. In front of the chiasm, damage to one optic nerve affects one eye. Behind it, damage to an optic tract (the pathway behind the chiasm) gives a homonymous defect, in which the same side of the field is lost in both eyes. A pair of horse blinkers, which cut off the sides of the view, is a useful picture of bitemporal hemianopia.
Key facts
- Site
- the optic chiasm, where the two optic nerves meet just above the pituitary gland.
- Crossing fibres
- nasal retinal fibres cross at the chiasm and carry the temporal (outer) visual field of each eye.
- Uncrossed fibres
- temporal retinal fibres stay on their own side and run at the edges of the chiasm, so central compression spares them and the inner halves of the fields are kept.
- Classic cause
- a pituitary tumour pressing on the chiasm from below, with the pituitary gland lying directly below the chiasm.
- Typical story
- gradual loss of the edges of vision in both eyes, with acuity that can stay normal.
- In front of the chiasm
- damage to an optic nerve affects one eye.
- Behind the chiasm
- damage in an optic tract, for example, gives a homonymous hemianopia, with the same side of the field lost in both eyes.
Exam traps
- Temporal retinal fibres are not the ones that cross. They carry the inner field, so damage to them would give a binasal defect (loss of the inner halves of both fields), not loss of the outer halves.
- Loss of vision in one eye only is not classically produced by chiasm compression, because it suggests a problem in front of the chiasm, such as an optic nerve, while pressure on the chiasm from a pituitary tumour involves both eyes.
- Optic tract damage does not remove both outer fields, because the tract lies behind the chiasm and damage there removes the same half of the field in both eyes, a homonymous pattern.
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