This subject causes everyone considerable difficulty. The following paragraphs summarize the areas of principal clinical importance in general medicine .
The dominant hemisphere
The concept of cerebral dominance arose with the observation that right-handed stroke (and other) patients with acquired language disorders had destructive lesions within the left hemisphere. Almost all right-handed people have language function in the left hemisphere; so do over 70% of those who are apparently left-handed. Destructive lesions within the left frontotemporoparietal region cause disorders of:
SPOKEN LANGUAGE-knOwn as aphasia or dysphasia WRITING-knOwn as agraphia
READING-knOwn as alexia (or acquired dyslexia) Developmental dyslexia describes children who have delayed and disorganized reading and writing ability with normal intelligence.
The non-dominant hemisphere
Disorders in right-handed patients with right hemisphere lesions are more difficult to define but comprise abnormalities of perception of internal and external space. Examples of this are losing the way in familiar surroundings, failing to put on clothing correctly (‘dressing apraxia’) or failure to draw simple shapes (‘constructional apraxia’).
APHASIA AND DYSARTHRIA
Aphasia (or dysphasia) is a loss or defect in language and is caused by left frontotemporoparietal lesions. Dysarthria is simply disordered articulation. Any lesion that produces paralysis, slowing or incoordination of the muscles of articulation or local discomfort will cause dysarthria. Examples are upper and lower motor lesions of the lower cranial nerves, cerebellar lesions, Parkinson’s disease and local lesions of the mouth, larynx, pharynx and tongue. Many aphasic patients are also somewhat dysarthric.
Some varieties of aphasia
Broca’s aphasia (expressive aphasia, anterior aphasia). A lesion in the left frontal lobe causes reduced fluency of speech with comprehension relatively preserved. The patient makes great efforts to initiate speech. Language is reduced to a few disjointed words and there is failure to construct sentences.
Patients who recover from this form of aphasia say that they knew what they wanted to say. but ‘could not get he words out’.
Wernicke’s aphasia (receptive aphasia, posterior aphasia) A left temporoparietal lesion leaves language that is fluent but the words themselves are incorrect. This varies from the insertion of a few incorrect or non-existent words into fluent speech (when it may be difficult to recognize aphasia) to a profuse outpouring of jargon, i.e. rubbish with wholly non-existent words. This may be so bizarre as to be confused with psychotic behaviour. Patients who have recovered from Wernicke’s aphasia say that when aphasic they found the speech of others like a wholly unintelligible foreign language, and though they knew they were speaking could neither stop themselves nor understand what they said.
Nominal aphasia (anomie aphasia or amnestic aphasia)
This describes difficulty naming familiar objects. When it occurs in a severe and isolated form it is caused by a left posterior temporal/inferior parietal lesion. Naming difficulty is, however, an early sign in all types of aphasia. Global aphasia (central aphasia) This is the expressive disturbance characteristic of Broca’s aphasia and the loss of comprehension of Wernicke’s. It is due to widespread damage to the areas concerned with speech and is the commonest form of aphasia after a severe left hemisphere infarct. Writing and reading are also affected.
CLINICAL FEATURES OF LOCAL
Focal lesions of the cerebral cortex cause symptoms and signs by three processes:
1 Destruction or suppression of function of cortical neurones and surrounding structures.
2 Synchronous discharge of neurones by irritative lesions which cause partial (focal) seizures that may become generalized seizures.
3 Displacement of the intracranial contents and surrounding cerebral oedema.
MEMORY AND ITS DISORDERS
Disorders of memory follow damage to the medial surface of the temporal lobe and its brain stem connections, including the hippocampi, fornices and mammillary bodies. Bilateral lesions are usually necessary to cause amnesia.
Effects of an irritative lesion of the cortex.
It is characteristic of all organic disorders of memory that more recent events are recalled poorly in contrast to the relative preservation of distant memories. Memory loss is a part of dementia of any cause and occurs in a wide variety of clinical situations.
Alcohol (Wernicke-Korsakoff syndrome)
Head injury (severe)
Posterior cerebral artery occlusion (bilateral)
Herpes simplex encephalitis
Chronic sedative and solvent abuse
Bilateral invasive tumours