White Matter Commissures & Disconnection Syndromes

White Matter Commissures & Disconnection Syndromes

Alexia without Agraphia
Alien Hand Syndrome

Commissural fibers

Connect corresponding areas of the cerebrum

Notably the corpus callosum (which connects dorsal cortical areas).
The anterior commissure (which connects ventral areas).

Anterior Commissure & Corpus Callosum

The anterior commissure bundle connects the bilateral ventral posterior frontal lobes and also the bilateral anterior temporal lobes.
The corpus callosum fibers connect the bilateral hemispheres from the frontal pole to the occipital pole.

The frontal fibers are called the anterior forceps (aka forceps minor, frontal forceps).
The occipital fibers are called the posterior forceps (aka forceps major, occipital forceps).

Corpus callosum: Four regions.

The rostrum is the anterior-inferior portion of the corpus callosum.
The genu is the anterior-superior portion (the anterior bend).
The body is the length of the corpus callosum.
The splenium is the posterior portion.

Fiber Pathways through the Corpus Callosum

The prefrontal fibers run through the rostrum and genu.
The frontal fibers run through the anterior body.
The temporo-parietal fibers run through the posterior body.
The temporo-occipital fibers run through the splenium.

Relationship between the forceps and the Callosal Regions

The genu fibers correspond to the anterior forceps.
The splenial fibers correspond to the posterior forceps.
The body fibers correspond to the midportion of the callosum.
The rostral fibers run beneath the genu and are inferior to the plane of this axial section.

Additional Commissural Pathways

Note that additional smaller commissural fiber pathways exist, including:

The hippocampal commissure, which lies inferior to the splenium of the corpus callosum and connects the bilateral hippocampal formations.
The posterior, habenular, and supraoptic commissures.

Disconnection Syndromes: Alexia without Agraphia & Alien Hand syndrome

History/Function

The function of the commissural fibers has aroused great interest throughout the past several centuries, and although the commissural fibers have long been understood to provide interhemispheric communication, the full extent of their function is still not known.
Functional analysis of the corpus callosum suggests a role for it in both sensory integration and high-level cognitive processing.
Much of what is known about the commissural bundles, however, comes from callosal resection surgeries, which are still done today to prevent the transmission of epileptic activity between the cerebral hemispheres (ie, to stem the propagation of seizures), and also from commissural fiber disruption injuries, which can result in disconnection syndromes.
Here, we will address two major disconnection syndromes:

Pure alexia without agraphia

Alien hand syndrome.

Pure alexia without agraphia

In 1892, Dejerine memorably described the syndrome of pure alexia without agraphia, which is a syndrome in which patients are unable to read but are still able to write.

There is a posterior cerebral lesion that affects the medial left posterior occipital lobe and the related visual projection fibers, and indicate that the lesion also involves the right visual projection fibers after they have crossed midline within the splenium of the corpus callosum. From this lesion, the left visual center is directly cut off from the language center and the communication between the right visual center and the language center is disrupted from the injury to the corpus callosum. But the language center still connects with the motor areas, so writing is preserved.*

Left posterior cerebral artery infarction is a common cause of alexia without agraphia.
Note that a coincident right homonymous hemianopia often occurs in this syndrome due to the left posterior occipital injury, and also note that our diagram is an anatomic drawing (opposite orientation of a radiographic view).

Alien hand syndrome (aka alien limb sign)

A disconnection syndrome with three overlapping variants – we'll address their key manifestation, localization, and dominance:

The callosal variant manifests with intermanual conflict or so-called self-oppositional behavior.

It is most commonly caused by a lesion in the anterior corpus callosum and that the nondominant hand (ie, the left hand) is most commonly affected, meaning the left hand opposes the actions of the right.

The frontal variant manifests with involuntary grasping, groping, or manipulation of objects.

The lesion is most commonly in the dominant hemisphere and the dominant hand (ie, the right hand) is most commonly affected; most often due to a lesion to the medial frontal lobe.
The lesion is most commonly in the dominant hemisphere and the dominant hand (ie, the right hand) is most commonly affected.
Left anterior cerebral artery infarction is a notable cause of this variant of alien hand syndrome.

The sensory variant manifests with limb ataxia.

Nondominant hemispheric, parieto-occipital lobe lesions cause this iteration of alien hand syndrome (thus the left hand (the nondominant hand) is most commonly affected.
Corticobasal degeneration, a neurodegenerative cause of dementia, is a classic cause of this alien hand syndrome variant.

Demonstration of Alien Limb Sign

To show yourself what is meant by self-oppositional behavior, open a desk drawer with your right hand and then immediately slam it shut with your left.
To demonstrate what happens in the frontal variant, with one hand involuntarily grasp at an object.
To demonstrate the sensory variant of alien hand syndrome, let your hand dangle out away from your body and try to forget its existence.

Other key clinical aspects of the corpus callosum

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.

Metabolic disorders can also affect 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 complex deficiency.

CNS Demyelinating Disorders

Select Key Central Nervous System Demyelinating Disorders

White Matter Commissures & Disconnection Syndromes

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