396 Neuroanatomy: Draw It to Know It
Visual Field Defi cits (Cont.)
Case IV
Patient presents with diffi culty seeing the right half of
the world, which is bilateral (both eyes aff ected). Exam
reveals a right homonymous hemianopia: right visual
fi eld blindness in both eyes.
Our diagnosis is a lesion of the left postchiasmatic
pathway. Disruption of fi bers from both the nasal right
hemiretina and temporal left hemiretina produces a
right homonymous hemianopia. Th ese retinal fi bers
fi rst bundle posterior to the optic chiasm within the
optic tract; however, a lesion anywhere along the post-
chiasmatic pathway — in the left optic tract, left lateral
geniculate nucleus, left optic radiations, or left visual
cortex — will produce the described right homonymous
hemianopia.Case V
Patient presents with diffi culty seeing the right half of
the world, which is bilateral (both eyes aff ected). Exam
reveals a right homonymous inferior quadrantanopia.
Our diagnosis is a left superior optic radiation lesion.
Th e lateral geniculate nucleus projects through supe-
rior and inferior optic radiations to the visual cortex.
Th ese radiations maintain the same superior–inferior
retinotopic organization found in the retina; therefore,
a superior optic radiation lesion produces an inferior
fi eld defect. Th e hemianopia lateralizes to the right
side, which means that the lesion is postchiasmatic on
the left.Case VI
Patient presents with diffi culty seeing the right half of
the world, which is bilateral (both eyes aff ected). Exam
reveals a right homonymous superior quadrantanopia.
Our diagnosis is a left inferior optic radiation lesion.
As described in the previous case, projections from the
lateral geniculate nucleus to the visual cortex maintain
the same superior–inferior retinotopic organization as
found in the retina; therefore, an inferior radiation lesion
produces a superior fi eld defect. Also, as described previ-
ously, the hemianopia lateralizes to the right side, so the
lesion is postchiasmatic on the left.Case VII (Advanced )
Patient presents with diffi culty seeing the right half of
the world, which is bilateral (both eyes aff ected). Exam
reveals a right homonymous hemianopia with preserved
central vision, called macular sparing.
Our diagnosis is a left occipital lobe lesion. Our cur-
rent patient has a right homonymous hemianopia with
preserved central vision through sparing of left macular
cortical representation. According to the vascular model
for macular sparing , the posterior cerebral artery sup-
plies the occipital cortex except for the occipital pole,
which the middle cerebral artery supplies. Th erefore, in
the setting of posterior cerebral artery infarction, the
middle cerebral artery maintains perfusion of the occipi-
tal pole, which spares macular cortical representation.
Additionally, or alternatively, macular sparing is
argued to occur from redundant macular representation
in both occipital cortices in much the same way that
auditory information is redundantly localized to both
transverse temporal g yri (of Heschl).^13