Excerpt from Frontal Lobe Syndromes

Please click here to view the full topic text: Frontal Lobe Syndromes

Background

The frontal lobe is the largest lobe in the brain, yet it is often not specifically evaluated in routine neurologic examinations. This may in part be due to the attention to detail and rigorous testing strategies required to probe frontal lobe functions. As successful completion of any cognitive task considered a frontal lobe function requires multiple brain regions both within and outside the frontal lobe, some authors prefer the term frontal systems disease. In any case, dysfunctions of the frontal lobe can give rise to relatively specific clinical syndromes. When a patient's history suggests frontal lobe dysfunction, detailed neurobehavioral evaluation is necessary.

Traditional classification systems divide the frontal lobes into the precentral cortex (the strip immediately anterior to the central or Sylvian fissure), prefrontal cortex (extending from the frontal poles to the precentral cortex and including the frontal operculum, dorsolateral, and superior mesial regions), and orbitofrontal cortex (including the orbitobasal or ventromedial and the inferior mesial regions). Each of these areas has widespread connectivity.

Given the unique connectivity between the frontal regions and deeper brain structures, lesions of these areas or their connections generate relatively distinctive clinical behaviors.

• The dorsolateral frontal cortex is concerned with planning, strategy formation, and executive function. Patients with dorsolateral frontal lesions tend to have apathy, personality changes, abulia, and lack of ability to plan or to sequence. These patients have poor working memory for verbal information (if the left hemisphere is predominantly affected) or spatial information (if the right hemisphere bears the lesion brunt).
• The frontal operculum contains the center for expression of language. Patients with left frontal operculum lesions may demonstrate Broca aphasia and defective verb retrieval, whereas patients with exclusively right opercular lesions tend to develop expressive aprosodia.
• The orbitofrontal cortex is concerned with response inhibition. Patients with orbitofrontal lesions tend to have difficulty with disinhibition, emotional lability, and memory disorders. Patients with such acquired sociopathy, or pseudopsychopathic disorder, are said to have an orbital personality. Personality changes from orbital damage include impulsiveness, puerility, a jocular attitude, sexual disinhibition, and complete lack of concern for others.
• Patients with superior mesial lesions typically develop akinetic mutism.
• Patients with inferior mesial (basal forebrain) lesions tend to manifest anterograde and retrograde amnesia and confabulation. 

Broca aphasia from a lesion in areas 44 and 45 on the left hemisphere leads to nonfluent speech, agrammatism, paraphasias, anomia, and poor repetition. Lesions anterior, superior, and deep (but sparing) to the Broca area produce abnormal syntax and grammar but repetition and automatic language are preserved. This disorder is known as transcortical motor aphasia and uninhibited echolalia is common. Memory disturbances only develop with lesion extension into the septal nucleus of the basal forebrain. Appreciation of verbal humor is most impaired in right frontal polar pathology.

Pathophysiology

A detailed discussion of the pathophysiology of frontal lobe dysfunction is beyond the scope of this review and the reader is referred to 2 excellent reviews by Mesulam (2002) and Bonelli and Cummings (2007).^{1, 2} As Mesulam has discussed, one way to think about the role of the frontal lobe is that it is the brain's way of modifying and interposing constraints on basic reflexive behaviors. For example, taking food when one is hungry is reflexive. Nonetheless, most adults can inhibit this behavior until the context is appropriate. Most hungry diners waiting in line at a restaurant do not usually help themselves to food from the plates of diners who have already been served, but some patients with frontal lobe dysfunction can't inhibit this response.

Unlike most animals, a human's mental state is preoccupied a great deal with what has happened in the past or what may happen in the future. Parts of the frontal lobe are essential for this type of "time travel." Indeed, good judgment requires evaluating the possible consequences of a variety of future activities and selecting the one with the most good consequences and the fewest bad consequences. Accordingly, poor judgment and inappropriately weighting the value of past experiences often occur with frontal lobe dysfunction.

Working memory involves a complex circuit that involves many brain regions, including the dorsolateral frontal cortex, thalamus, and parts of the temporal and parietal cortices. Working memory is defined as memory for a limited amount of information (such as a telephone number) that needs to be kept in consciousness for a few seconds (until the number is dialed) and then may be lost forever. Most patients are able to hold 6 or 7 digits in working memory. Patients with impairments of working memory may have decreased capacity in working memory (eg, shortened digit span) or difficulty manipulating information in working memory (eg, impaired reverse digit span test).

Frequency

United States

Data are not available for the epidemiology of frontal lobe dysfunction as a clinical syndrome, but data are available concerning the incidence and prevalence of the major causes of syndromes of frontal lobe dysfunction. For specifics on these data, please refer to the following linked eMedicine articles. Common causes (see also Causes) include the following:

• Mental retardation
• Traumatic brain injury (see Management and Staging of Traumatic Brain Injury and Traumatic Brain Injury: Definition, Epidemiology, Pathophysiology
• Brain tumors (see Metastatic Disease to the Brain and EEG in Brain Tumors)
• Degenerative dementias including Alzheimer disease, dementia with Lewy bodies, Parkinsonian dementias, and frontotemporal dementias
• Cerebrovascular disease
• Psychiatric diseases such as schizophrenia and major depression
• In addition, any neurologic or psychiatric disease that can affect the frontal lobe (eg, multiple sclerosis, CNS lupus) may be associated with frontal lobe dysfunction. 
• Frontal lobe dysfunction is associated with blood alcohol level and occurs during acute intoxication with many recreational drugs.

Sex

Traumatic brain injury is much more common in men than women both in the United States and worldwide. 

Age

The relative likelihood of different causes of frontal lobe dysfunction is a function of patient age. In teenagers and young adults, the most common causes are mental retardation, traumatic brain injury, and drug intoxication. In middle-aged patients, brain tumors, cerebrovascular disease, infections such as HIV, multiple sclerosis, and early onset degenerative dementias are common. In late life, cerebrovascular disease and degenerative dementias are predominant causes of frontal lobe dysfunction. 

Please click here to view the full topic text: Frontal Lobe Syndromes