Brain Laterality Case File

Brain Laterality Case File

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

CASE 41

A 15-year-old, right-handed boy presents to the neurosurgery office for a preoperative evaluation. The patient describes a lifetime history of complex partial seizures. Initially, these seizures were controllable with medication, but over the last several years, they have become refractory to medical treatment. His seizures have increased to over six a day despite maximal dosage of medication and the implantation of a vagus nerve stimulator. He had been doing well at home and in school despite the seizures; however, the large doses of medication and increase in seizure frequency have begun to affect his cognitive and psychological development. He was diagnosed with medial temporal lobe epilepsy.

• What is the significance of the patient being right-handed?
• How will the surgeons decide which cerebral hemisphere is dominant?

ANSWERS TO CASE 41: BRAIN LATERALITY

Summary: A 15-year-old right-handed boy undergoes epilepsy surgery for refractory tonic-clonic seizures. Through a combination of imaging and EEGs, he is diagnosed with medial temporal lobe epilepsy.

• Significance of the patient being right handed: The majority of both right-handed (>95%) and left-handed (70%) individuals are left brain dominant. Fifteen percent of left-handed individuals are equally dominant in both hemispheres. This is important to know because it gives clues as to the possible function of the brain tissue that is involved in the epileptic activity, and for surgical planning.
• Determination of cerebral hemisphere dominance: One method of determining if the patient is right or left brain dominant is the Wada test. In the Wada test, the patient receives an anesthetic injection into either the right or the left carotid artery. This isolates the function of cerebral hemisphere not receiving the injection, allowing for evaluation of function. Preoperative or intraoperative mapping is also utilized.

CLINICAL CORRELATION

For this patient, the epileptic focus is removed surgically and the patient recovers without complication. In consultation with the neurologist, the postoperative epilepsy medication is modified. Six months postoperation, the seizure frequency has decreased dramatically and the patient has been able to significantly decrease his antiseizure medication. This has led to an improvement in school function and overall quality of life.

Through unknown mechanisms, complex partial seizures originate from a specific area of the brain, then spread to surrounding regions. This focus of abnormal neuronal activity can be surgically removed. Seizure-free rates after surgery range from 75% to 80%. This is not the case for seizures that begin diffusely throughout the cortex, as there would be too much morbidity associated with the removal of the afflicted brain tissue. An option for generalized epilepsy would be to sever the connections between the two hemispheres of the brain, known as a corpus callosotomy. Up to 63% of seizures decrease with this intervention. Both of these surgical treatments for epilepsy accentuate the lateralized functions of the two cerebral hemispheres. Determination of candidacy for epilepsy surgery: first, the patient must fail medical management. It is important to determine if the seizure focus is diffuse or focal. Focal lesions may amend themselves to surgical removal. One must also consider the location of the abnormal firing. If it is located in a very difficult location, or associated with essential functions such as movement or language, surgical resection may carry unwanted morbidity. Anteromedial temporal resection for mesial temporal lobe epilepsy has shown particularly good results from epilepsy surgery.

APPROACH TO BRAIN LATERALITY

Objectives

1. To understand the differences in function between the two cerebral hemispheres.
2. Predict the manifestations of injury or disease depending upon laterality.
3. Conceptualize how each brain hemisphere perceives stimuli.

Definitions

Ipsilateral: Affecting the same side.

Contralateral: Affecting the opposite side.

Corpus callosum: The cerebral commissure of white matter connecting the left and right hemispheres.

Prosopagnosia: An inability to recognize a familiar person by looking at his or her face.

Anosognosia: The denial of obvious illness or disability.

Anosodiaphoria: An indifference or jocularity to a grave weakness or debilitation, distinct from denial.

Misoplegia: A hatred of limbs.

Nonbelonging: The feeling that an affected limb does not belong to the individual.

Auditory agnosia: An inability to interpret sounds.

Amusia: An inability to perceive impaired music perception.

DISCUSSION

The concept that the brain’s two hemispheres have distinct yet integrated roles was first described in the late 1800s with Paul Broca’s and the lateralized functions of speech. From there, physicians and scientists have continued to characterize differences in function between the right and left cerebral hemispheres. Generally speaking, the nondominant hemisphere has been described as the “artistic” side responsible for holistic function, whereas the dominant side is the more “scientific” half, dealing with analytic attention to detail. The dominant hemisphere separates complex stimuli into its discreet parts (when seeing a forest, focuses on the trees) whereas the nondominant hemisphere synthesizes the diverse parts into a whole (when seeing trees focuses on the forest). Both are required, and the communication between the cerebral hemispheres through the corpus callosum allows for seamless coordination of the separate functions. The lateralized functions of the brain can be divided into motor, language, music, visual/spatial perception, and executive capacity.

Descending motor information from one side of the cerebrum controls the opposite side of the body. Likewise, the majority of ascending sensory information crosses and maps out to the contralateral side of the brain. The coordination and memory of complex motor skills (eg, playing an instrument, dancing) appears to be controlled by the nondominant hemisphere.

Language, like mathematics, is a sequential activity. Language is lateralized to the left hemisphere in most individuals; however, 15% of left-handed individuals exhibit language control in both hemispheres. The cerebral hemisphere in control of language is what determines “dominance.” The dominant hemisphere controls the nonemotional aspects of language. This includes speech, writing, reading, comprehension, naming, verbal memory, vocabulary, concept formation, and language structure (spelling, grammar, syntax). The nondominant hemisphere controls organization, prosody, abstract language, interpretation and expression of mood, and the subtleties of which include intonation, connotation, body language, and facial expression. In a conversation, the dominant hemisphere concerns itself with the content of speech while the nondominant focuses on the general mood and affect of the interaction.

While the nondominant hemisphere is often thought of as the “artistic” side, the dominant hemisphere controls the rhythm and language of music. The nondominant hemisphere contributes to musical understanding, and temporal lesions may cause auditory agnosia or amusia.

The dominant and nondominant hemispheres “see” the external world differently. This difference can be demonstrated in facial recognition. When the dominant hemisphere perceives a face, it sees the hairline, the mouth, the skin pigment, but cannot place all of the different pieces into one complete picture, that of a face. The nondominant hemisphere, on the other hand, compiles this information, determining not only that this is a face, but also that this is a familiar or unfamiliar face, and can consider the mood expressed by the face.

The nondominant hemisphere is largely responsible for the perception of space, which is why it is central to the creation and appreciation of art. It is responsible for conceptualizing the “whole picture,” including spatial relations, pattern recognition, geometrical shapes and forms, and esthetics. These elements perceived by the nondominant hemisphere are all concurrently present, in comparison to the work of the dominant hemisphere, which deals with sequential tasks. In fact, spatial memory is stored in the nondominant hemisphere. The concept of personal space is controlled by the left hemisphere, whereas extrapersonal space resides in the nondominant hemisphere. For example, a lesion to the nondominant parietal lobe may lead to anosognosia. Patients deny concurrent debilitations, for example, believing there is nothing wrong with a paralyzed half of their body. Patients may even deny a paralyzed limb belongs to them. Anosognosia carries significant morbidity as the patients are at risk of injury caused by the lack of recognition of their limitations.

Higher executive functions are also lateralized. The dominant hemisphere is responsible for abstract and rational thinking, analytical reasoning, initiative, attention, and linear thought processes. It also has the power of introspection and a sense of self. This provides the ability to recognize and adhere to social norms and behavior, allowing for integration into society. The nondominant hemisphere is more intuitive, giving a general gestalt to an interaction, situation, or perception. It is also responsible for the ability to day dream, and to have complex and rich dream imagery of sleep.

COMPREHENSION QUESTIONS

[41.1] A 72-year-old man comes into the emergency department complaining of weakness of his left arm. He states that the weakness began a few days ago, and is getting progressively worse. On examination, you note 3/5 strength in the left distal upper extremity, 4/5 strength in the left proximal upper extremity, and you also note some left-sided nasolabial fold flattening and asymmetric facial movements. On further questioning, the man reports that he fell getting out of the shower several weeks ago. Based on your suspicions, you perform a CT of his head, which shows a chronic subdural hematoma. Given the nature of his symptoms, which part of his brain do you expect to be affected?

A. Left precentral gyrus

B. Left postcentral gyrus

C. Right precentral gyrus

D. Right postcentral gyrus

[41.2] A 67-year-old left-handed man is brought into the clinic by his family because they say he has been talking strangely for several days. They say he used to be a very animated speaker, with lots of gestures and changes in his voice, but for the past several days has been speaking in a near complete monotone with few gestures and few facial expressions. He does seem to understand body language and inflection, however. Based on this description of symptoms, where would the physician expect to find a neurologic lesion?

A. Pars triangularis and opercularis of the left inferior frontal gyrus

B. Pars triangularis and opercularis of the right inferior frontal gyrus

C. Posterior part of the left superior temporal gyrus

D. Posterior part of the right superior temporal gyrus

[41.3] A 42-year-old right-handed architect comes into the clinic because he has been having a lot of trouble doing his job recently. In particular, he seems to be having a lot of trouble with “the big picture” of his projects. He can design individual parts or a project, but can’t seem to get them to go together in the right way like he used to be able to do. If there is a brain lesion responsible for these symptoms, where would the physician expect to find it?

A. Right frontal lobe

B. Left frontal lobe

C. Right parietal lobe

D. Left parietal lobe

Answers

[41.1] C. The most likely region to be affected is the right precentral gyrus. This man shows signs and symptoms of motor system dysfunction, and a lesion affecting his primary motor cortex could account for this. The primary motor cortex is located in the precentral gyrus and paracentral lobule on the contralateral side of the body. Since his dysfunction is left sided, involving the upper extremity and face, we would expect the lesion to be located on the opposite side of the body.

[41.2] B. The most likely location for the lesion is the pars triangularis and opercularis of the right inferior frontal gyrus. This man presents with what appears to be a productive aprosody: he can understand intonation and body language, but not produce it. Prosody is produced in the nondominant hemisphere, in areas analogous to the speech areas in the dominant hemisphere. Since this is a productive aprosody, we would expect the lesion to be located where Broca’s area is, only in the nondominant hemisphere. Even though this man in left handed, it is still likely that he is left-hemisphere dominant (70% of left-handed people are left dominant), so this lesion should affect his right side.

[41.3] C. The most likely location of the lesion is the right parietal lobe. This man is having trouble with his visuospatial perception, and with artistic/aesthetic design, both of which are nondominant hemisphere processes that are localized to the parietal or parietal-occipital areas of the brain. Since he is right handed, it is very likely that he is lefthemisphere dominant, so we would expect his lesion to be on the right side of his brain.

NEUROSCIENCE PEARLS ❖ The majority of the population is right-brain dominant, as determined by handedness and the location of language control. ❖ The dominant cerebral hemisphere is analytic, versus the more holistic nondominant hemisphere. ❖ The corpus callosum facilitates communication and coordination between the two distinct hemispheres

REFERENCES

Engel J Jr, Wiebe S, French J, et al. Practice parameter: temporal lobe and localized neocortical resections for epilepsy. Epilepsia. 2003 Jun;44(6):741-751.

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