Disconnection Syndromes Case File

Disconnection Syndromes Case File

EUGENE C.TOY, MD, RAHUL JANDIAL, MD, PhD, EVAN YALE SNYDER, MD, PhD, MARTIN T. PAUKERT, MD

CASE 46

A 32-year-old right-handed male has a history of atonic seizures since childhood. He presents to the neurosurgery clinic for a preoperative appointment. HR’s seizures have been managed unsuccessfully with various high doses of medication, and he continues to have multiple “drop attacks” each day. He is resigned to wearing a helmet at all times. He gives a history of multiple occasions when he was not wearing his helmet, suffered an atonic seizure, and severely injured his head and body. Currently, he requests a corpus callosotomy to attempt to decrease the frequency of his atonic seizures.

• What is the purpose of the corpus callosum?
• What complications may occur with corpus callosotomy?

ANSWERS TO CASE 46: DISCONNECTION SYNDROMES

Summary: A 32-year old right-handed male with medically refractory atomic seizures prepares for a corpus callosotomy.

• Purpose of corpus callosum: Communication between the right and left cerebral hemispheres.
• Complications with corpus callosotomy: The acute disconnection symptoms and signs. Immediately following a corpus callosotomy, patients may experience acute symptoms and signs from the lack of communication between the two hemispheres. For example, patients may suffer from mild akinesia or competitive movements between the two hands. In fact, some patients express surprise at the purposeful and independent actions of an “alien” left hand. On examination, bilateral Babinski responses indicate damage to the brain. Brain edema may occur secondary to retraction on the brain to reach the surgical site. In patients with speech and handedness stored in opposite hemispheres, complete callosotomy may lead to difficulties in the production of spontaneous speech.

CLINICAL CORRELATION

Corpus callosotomy is most effective for atonic seizures, tonic-clonic seizures, and tonic seizures. Response to the treatment is high, with up to an 80% decrease in seizure frequency after partial callosotomy. The abnormal focus origin of the seizure will remain; however, that abnormal electrical

activity will not be able to generalize to the other half of the brain. This leads to a decrease in not only the frequency, but also the severity of the seizures. In the long term, signs and symptoms of disconnection via corpus callosotomy are indistinguishable from individuals with intact circuitry, with the exception of certain memory problems and responses under special lateralizing testing situations. The lack of interhemispheric communication does not seem to affect everyday life. For example, hemispheric specialization and the inhibition of interhemispheric transfer can be demonstrated when a right-handed individual is given a familiar object in the left hand (graphesthesia) but cannot name the item. Apparently split brain patients are able to compensate for these deficits through tricks to trigger the opposite hemisphere. For example, as the right hemisphere may recognize individual words, a patient may say out loud the name of an object in order to facilitate retrieval with the left hand.

APPROACH TO DISCONNECTION SYNDROMES

Objectives

1. Be able to identify different types of disconnection signs and syndromes.
2. Know the tests which can be performed to isolate the function of either hemispheres of the brain.

Definitions

Akinesia: “Freezing” of body movements; difficulty to begin or maintain a body motion.

Agenesis of the corpus callosum: Congenital lack of interhemispheric connecting neurons.

Babinski response: Firm stroking of the outer, dorsal heel to the pad of the foot leads to extension upward and spreading of the toes. Normal in infants less than 2 years of age. In adults is a sign of brain or spinal cord injury.

Graphesthesia: Tactile ability to recognize letters written on the skin.

Verbal anosmia: Inability to name smells presented only to the right nostril, however, the left hand can use tactile information to find a corresponding object.

Double hemianopsia: Inability to indicate the side of a visual stimulus with the contralateral hand.

Unilateral anomia: Inability to name an object with purely tactile stimuli. The object can be named when placed in the right hand.

Unilateral agraphia: Inability to write with the left hand, whereas, the right hand writes well.

Unilateral apraxia: Inability to perform a verbal command with the left hand, but is easily performed by the right hand.

Unilateral constructional apraxia: Inability of the right hand to perform tasks that require the right hemisphere’s abilities of spatial cognition. Examples include copying geometric forms and completing mathematics that requires writing down procedural steps.

DISCUSSION

Cortical disconnection signs and syndromes present secondary to subcortical white matter lesions connecting two brain regions. These can be classified as either intrahemispheric or interhemispheric.

Intrahemispheric disconnection syndromes involve damage to deep white matter connections between structures within the same hemisphere. In the dominant hemisphere, this can include language. The arcuate fasciculus connects Wernicke and Broca speech areas. Disruption of this connection leads to retention of fluent speech (although dysphagic) and comprehension; however, patients have difficulty with repetition. This is referred to as a conduction aphasia, as the information received in Wernicke area cannot be transferred to Broca area. Injury to the white matter connecting the primary auditory cortex and the auditory association areas leads to a pure word deafness. The patient can “hear” (ie, the physical and neuronal mechanisms for conducing auditory stimuli from the ear to the primary auditory cortex are intact), and audiometry examinations will be normal. Yet the patient will not be able to perceive and process the information, making the patient functionally deaf. A patient with pure word deafness will exhibit impaired comprehension of speech, but will be able to produce speech and comprehend written language normally.

Another disorder of language involves the motor apparatus used to produce speech. Apraxia of the buccal and lingual areas can occur secondary to disconnection between the association motor cortices in the subcortical region. The patient’s signs include right brachiofacial weakness, and apraxia of tongue, lip, and left limb movements.

Three main structures allow for communication between the distinct cerebral hemispheres: anterior commissure (olfactory and limbic), hippocampal commissure (limbic), corpus callosum (hemispheric cortical processing). Interhemispheric disconnection results from a lesion of the corpus callosum. The acute and chronic disconnection syndrome is detailed in the previous case questions. One example of a symptom that may be seen in the acute postoperative period is left-hand apraxia to verbal commands. This occurs secondary to disconnection of the right motor strip from the left language area. Unlike unilateral lesions, the acute disconnection syndrome does not include symptoms such as hemineglect and aphasia. In fact, within months most patients appear normal, both in initial social interactions and on routine neurological examinations. Patients do appear to suffer from subtle memory, language, and personality deficits, yet split brain patients provide an important example of brain plasticity. For example, initially left hand apraxia to verbal commands is a manifestation of the right hemisphere’s poor language comprehension. As there is a contralateral increase in language comprehension and ipsilateral increase in motor control, the left hand apraxia improves.

Agenesis of the corpus callosum, when accompanied by other brain defects, carries severe symptoms (seizures, mental retardation, hydrocephalus, spasticity). There are, however, some individuals with isolated agenesis of the corpus callosum who have normal intelligence, whose deficits are only elicited upon exercises requiring matching of visual patterns. Many of the minimally affected patients with callosal agenesis do not manifest the disconnection signs, compared to an older individual with a corpus callosotomy.

Multiple tests can be performed to isolate the function of either hemisphere. For example, when a split brain patient sees a chimeric portrait, one image is sent to the left side of the brain and a distinct image to the right. When asked to point to the whole picture, the patient will usually choose the image sent to the right hemisphere (facial recognition); however, when required to verbally choose the picture, the patient will say the image sent to the left hemisphere (speech dominance). Object location is also not affected, as the superior colliculus has bilateral input for spatial localization. Stereognostic input from one side is only perceived by the contralateral hemisphere. For a patient with an interhemispheric disconnection syndrome, an object held in the left hand can only be perceived by the right hemisphere and cannot be named. When different words are presented in either ear, the ipsilateral pathway from the left ear is suppressed by the dominant contralateral right ear, and the patient will repeat the word presented in the right ear.

COMPREHENSION QUESTIONS

[46.1] A 13-year-old girl has severe, intractable generalized epilepsy. She has daily tonic-clonic seizures that are unresponsive to treatment with therapeutic dosages of many different antiepileptic drugs. In an effort to control her seizures, her family is considering surgery to help manage her epilepsy. What structure, that normally allows for interhemispheric cortical communication, can be ablated to help control seizures in extreme cases like this?

A. Anterior commissure

B. Corpus callosum

C. Hippocampal commissure

D. Posterior commissure

[46.2] A 62-year-old man is brought into the physician’s office by his family because of the abrupt onset of strange behavior. According to his wife, one day prior, he simply stopped responding to her speaking to him. On examination, he responds to sounds by turning toward them, can read and understand perfectly, and can generate comprehensible speech without any problem. The only deficit seems to be in understanding spoken speech. What term best describes the deficit this patient is experiencing?

A. Wernicke aphasia

B. Global aphasia

C. Pure word deafness

D. Cortical deafness

[46.3] A 57-year-old man presents to the clinic with some difficulties speaking, particularly when it comes to reading aloud and repeating words. The physician performs an MRI, which shows a lesion in the area of the arcuate fasciculus. A lesion in this area could result in what speech-related deficit?

A. Productive aphasia

B. Receptive aphasia

C. Transcortical aphasia

D. Conductive aphasia

Answers

[46.1] B. The corpus callosum is a bundle of white matter that in the normal brain serves as the major communication between the cerebral hemispheres and is involved in interhemispheric cortical processing. It can be severed to help control severe epilepsy, with fairly good results in patients with incapacitating seizures. The anterior commissure and hippocampal commissure are involved in the limbic system, and the posterior commissure is involved in the pupillary light reflex.

[46.2] C. This patient is experiencing what is known as pure word deafness, which is caused by an insult to the white matter that connects the primary auditory cortex located in the transverse temporal gyrus of Heschl, to Wernicke area, located on the posterior aspect of the superior temporal gyrus. This insult inhibits information about heard speech from arriving at the comprehension center in Wernicke area, making the patient functionally deaf when it comes to speech. They can hear, however, and will turn to face noises, and they comprehend written words, because the connections between the visual cortex and Wernicke area are intact. In cortical deafness, lesions to the bilateral auditory cortex make patients deaf, although lower brain structures respond to sound, and with aphasia patients have difficulty generating speech.

[46.3] D. The arcuate fasciculus connects Wernicke and Broca areas, and a lesion to it results in what is known as a conduction aphasia. The primary manifestations of this are difficulties in repeating words and in reading aloud, as the comprehension of the words in Wernicke area cannot be communicated to Broca area for generation.

NEUROSCIENCE PEARLS ❖ Cortical disconnection signs and syndromes present secondary to subcortical white matter lesions connecting two brain regions. ❖ Interhemispheric disconnection results from a lesion of the corpus callosum. ❖ Patients with a corpus callosotomy appear normal in initial social interactions and only specific neurological examinations can elicit the disconnection signs.

REFERENCES

Bear MF, Connors B, Paradiso M, eds. Neuroscience: Exploring the Brain. 3rd ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006. 

Kandel E, Schwartz J, Jessell T, eds. Principles of Neural Science. 5th ed. New York, NY: McGraw-Hill; 2000. 

Zigmond MJ, Squire LR, Bloom FE, Landis SC, Roberts JL, eds. Fundamental Neuroscience. 2nd ed. San Diego, CA: Academic Press; 1999.

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