394 Neuroanatomy: Draw It to Know It
Case II
Visual Field Defi cits
Here, we will learn the diff erent visual fi eld defi cits.
First, let’s set up our axial diagram of the visual path-
ways. Label the left and right visual fi elds, then draw
the eyes, the optic chiasm, the lateral geniculate nuclei,
and the primary visual cortices. Next add the path-
ways. On the right-hand side of the page, for each case,
we will draw a pair of eyes with their left and right visual
fi elds.Case I
Patient presents with loss of vision in the left eye. Exam
reveals absent vision in the left eye, only. Th e right eye is
normal.
Our diagnosis is a left optic nerve lesion. Th e unilater-
ality of the injury localizes the defi cit to a prechiasmatic
lesion, either in the optic nerve or in the retina, itself. We
learn how to distinguish retinal and optic nerve lesions
later. Note that a relative aff erent pupillary defect would
almost certainly accompany this optic nerve injury (see
Drawing 23-6).Patient presents with right eye loss of vision. Exam
reveals left visual fi eld loss in the right eye, only. Th e left
eye is normal.
Our diagnosis is a right lateral optic chiasm lesion.
Again, the defi cit is limited to one eye; therefore, the
injury is anterior to the optic tract. Th e defi cit is in the
visual fi eld contralateral to the aff ected eye, which means
that the temporal retinal fi bers are selectively aff ected.
Note that this pattern of injury could also occur along
the optic nerve, but it is in the optic chiasm that the nasal
and temporal retinal fi bers split apart: this case high-
lights the separation of the nasal and temporal retinal
fi bers within the optic chiasm.Case III
Patient presents with bilateral loss of vision. Exam reveals
bitemporal hemianopia.
Our diagnosis is an optic chiasm lesion. Th e optic
chiasm contains crossing fi bers from the bilateral nasal
hemiretinae. Th e nasal hemiretinae receive the temporal
visual fi elds, which means that the left nasal retina per-
ceives the left visual fi eld and the right nasal retina per-
ceives the right visual fi eld. Bitemporal hemianopia is an
important exam fi nding because it oft en suggests the
presence of a sellar mass, such as a pituitary adenoma or
craniopharyngioma. Oft en it is stated that this lesion
produces a constriction of vision and a sensation of
“wearing horse blinders”; however, more accurately,
bitemporal hemianopia produces a loss of binocular
fusion, which results in “hemifi eld slide.”^12