Agnosia, Apraxia, and Related Disorders of Higher Cortical Function

FIGURE 10.1 The Rey-Osterrieth complex figure for evaluating constructional ability.

In dressing apraxia, the patient loses the ability to don clothing correctly. Dressing requires bimanual cooperation to solve a complex spatial problem. There is loss of the ability to manipulate the clothing in space and to understand its three-dimensional relationships. Patients with hemineglect may fail to dress one side of the body. A useful test for dressing apraxia is to turn one sleeve of the hospital gown or robe inside out, and then ask the patient to put it on. Patients with dressing apraxia are often baffled. Dressing apraxia can be particularly disabling, as the patient struggles for a long period of time each morning simply to get dressed. Constructional apraxia would be very disabling for a patient who was an artist or craftsman. Dressing apraxia often occurs in conjunction with constructional apraxia.

DISCONNECTION SYNDROMES

Disconnection syndromes are disorders in which the fiber tracts that interconnect primary cortical areas are disrupted, with preservation of the cortical areas of origin. Neurologic dysfunction occurs not because of destruction of cortex but because of defects in intrahemispheric or interhemispheric communication. In 1874, Wernicke was the first to suggest that such a pathoanatomic mechanism might exist when he described conduction aphasia in his MD thesis, written at the age of 26. Dejerine added alexia without agraphia in 1892. In his 1965 paper, Disconnection syndromes in animals and man, which became the manifesto of behavioral neurology, Geschwind expanded and popularized the concept, describing several new examples. Other disconnection syndromes include ideomotor apraxia, sympathetic apraxia, pure word deafness, conduction aphasia, and the transcortical aphasias. The modality-specific agnosias may be disconnection syndromes in which the primary sensory area for a given modality is disconnected from the language and memory areas of the brain that are responsible for recognition and naming. Disconnection syndromes may result from any process that disrupts subcortical white matter, including infarction, hemorrhage, neoplasm, and trauma. There have been reports of patients with double disconnection syndromes.

The disconnection concept has been expanded to include disorders as diverse as schizophrenia, autism, and dyslexia, where disconnecting “lesions” remain inferred rather than demonstrable. Studies of connections in the living human brain in normal subjects and patients with neurologic and psychiatric disorders using techniques such as diffusion tensor imaging, tractography, and electrophysiology are expanding the concepts of disconnection syndromes. Contemporary models invoke a network of multiple specialized cortical areas, grouped into territories and connected through parallel, bidirectional pathways. Concepts are expanding beyond white matter disconnections and cortical deficits to include white matter hyperconnectivity and cortical hyperfunction. Dysfunction may range from the loss of a specialized cortical region, for example, prosopagnosia from lesions of face-specialized cortex, to positive symptoms, for example, face hallucinations related to the hyperexcitability and spontaneous activation of face-specialized cortex. A combination of frontofrontal hyperconnectivity and frontal disconnection from other brain regions has been postulated in autism.

The syndrome of alexia without agraphia (pure alexia, pure word blindness, agnosic alexia, central alexia, visual verbal agnosia) was elegantly described by Dejerine. These patients have a left occipital lobe lesion, usually an infarction, which extends anteriorly to involve the splenium of the corpus callosum or the adjacent white matter. They usually have a right homonymous hemianopia because of the occipital lobe lesion. Although the right occipital lobe and left visual field are intact, fibers from the right occipital lobe are disconnected from the language centers in the left parietal lobe because of disruption of commissural fibers in the splenium. The patients are unable to read because the visual information from the right occipital lobe cannot be transferred to the region of the opposite angular gyrus. They are typically better able to read letters than words, and individual letters better than letter strings. Preservation of number reading may occur. Because the angular gyrus is itself intact, patients are able to write without difficulty but are unable to read what they may have just written. Rarely, alexia without agraphia occurs without an accompanying hemianopia.

In pure word deafness (auditory verbal agnosia, isolated speech deafness), patients are unable to understand speech but other language modalities are unimpaired. Spontaneous speech, reading, and writing are preserved in the face of a severe auditory comprehension deficit. Hearing is intact and nonlanguage auditory processing (e.g., for music) is undisturbed. Responsible pathology is typically bitemporal or dominant temporal, causing disconnection of Wernicke’s area from the primary auditory cortex. In pure word blindness, the patient cannot read, but other language functions are intact. The lesion disconnects the visual cortex from the language centers. There is conjecture that the dysphagia in Wallenberg’s lateral medullary syndrome may be due to a disconnection between premotor neurons related to swallowing and the bulbar nuclei responsible for execution.

In callosal disconnection syndromes, there is evidence of interhemispheric disconnection causing deficits in corpus callosum function that resemble those seen in split-brain patients. Patients with anterior callosal lesions may have unilateral tactile anomia, unilateral agraphia, unilateral apraxia, difficulty in copying drawings, dyscalculia, abnormalities of somesthetic transfer, and the alien hand phenomenon. Posterior callosal lesions may cause left tactile anomia, left visual anomia, and agraphia of the left hand. A patient with infarction of the total length of the corpus callosum had unilateral verbal anosmia, hemialexia, unilateral ideomotor apraxia, unilateral agraphia, unilateral tactile anomia, unilateral constructional apraxia, lack of somesthetic transfer, and dissociative phenomena. Callosal apraxia refers to impaired ability to pantomime to command, imitate, or use actual objects with the left hand, with spared ability to perform these tasks with the right hand, due to a callosal lesion. Evidence of callosal disconnection has been reported in infarction, hemorrhage, Marchiafava-Bignami disease, multiple sclerosis, and Alzheimer’s disease.

Attentional Deficits

Only gold members can continue reading. Log In or Register to continue