300 Neuroanatomy: Draw It to Know It
Commissures & Disconnections ( Advanced ) (Cont.)
Left posterior cerebral artery infarction is a common
cause of alexia without agraphia. Note that a coincident
right homonymous hemianopia oft en occurs in this syn-
drome due to the left posterior occipital injury, and also
note that our diagram is an anatomic drawing , in which
the left side of the brain is on the left -hand side of the
page; radiographic images have the opposite orientation
(see Drawing 1-2).^10
Another disconnection syndrome that receives atten-
tion from both neurologists and Hollywood, alike, is
called alien hand syndrome (aka alien limb sign). It
was cinematized most memorably in Stanley Kubrick’s
Dr. Strangelove. Th ree forms of alien hand syndrome
exist, each with a discrete anatomic localization; however,
patients commonly present with overlapping symptoms.
Across the top of the page, label variant, manifestation,
localization, and dominance. First, label the callosal vari-
ant, which manifests with intermanual confl ict or so-
called self-oppositional behavior. In this alien hand
syndrome variant, one hand is in direct confl ict with the
other, most commonly the left with the right. To show
yourself what is meant by self-oppositional behavior,
open a desk drawer with your right hand and then imme-
diately slam it shut with your left. Indicate that this vari-
ant is most commonly caused by a lesion in the anterior
corpus callosum and that the nondominant hand (ie, the
left hand) is most commonly aff ected, meaning the left
hand opposes the actions of the right.
Next, label the frontal variant, which manifests with
involuntary grasping , groping , or manipulation of
objects. To demonstrate what happens in this syndrome,
with one hand involuntarily grasp at an object. Indicate
that this variant is due to a lesion to the medial frontal
lobe. Th e lesion is most commonly in the dominant
hemisphere and the dominant hand (ie, the right hand)
is most commonly aff ected. Left anterior cerebral artery
infarction is a notable cause of this variant of alien hand
syndrome.^11 – 13
Finally, label the sensory variant, which manifests
with limb ataxia. To demonstrate the sensory variant of
alien hand syndrome, let your hand dangle out away from
your body and try to forget its existence. Patients with
the sensory variant are generally unaware of their limb or
even frankly deny that it’s their own. Indicate that pari-
eto-occipital lobe lesions cause this iteration of alien
hand syndrome and that the nondominant hemisphere
and hand are most commonly aff ected. Corticobasal
degeneration, a neurodegenerative cause of dementia,
can cause this alien hand syndrome variant.^11 – 13
Other important clinical aspects regarding the corpus
callosum are the frequency with which it fails to develop
and the wide range of diseases that aff ect it. Congenital
failure of the corpus callosum to fully develop is known
as callosal agenesis and it occurs either in isolation or as
part of a more complex syndrome, such as the Arnold-
Chiari, Dandy-Walker, or Aicardi syndromes. Acquired
forms of corpus callosum abnormalities occur from a
wide range of causes, including demyelinating disorders,
such as multiple sclerosis, and head trauma. Also, certain
tumors characteristically invade the corpus callosum,
such as glial tumors — most commonly glioblastoma
multiforme, lymphomas, and lipomas. Lastly, metabolic
disorders can also aff ect the corpus callosum, such as
Marchiafava-Bignami, which is a rare cause of necrotic
layering of the corpus callosum. Marchiafava-Bignami
was originally described from the autopsies of three
Italian men known to have been heavy red wine drinkers
but has subsequently been found in both alcoholics and
non-alcoholics, alike, and is ascribed to vitamin B com-
plex defi ciency.^3 , 14 – 18