This degenerative disorder is of unknown origin but is believed to reflect a general abnormality or dysfunction of astrocytes (see T.ab!e...30-6 ). As a result, there is a global dysmyelination or demyelination of the CNS. Eosinophilic hyaline bodies, which are particularly numerous beneath the pial arachnoid membranes and around the cerebral blood vessels, are found throughout most of the brain. Located in the cytoplasm of the white matter, astrocytes may form characteristic fibers. Hyaline inclusions occur especially in the optic nerves, optic tracts, optic chiasm, cerebral peduncles, and spinal cord. Abnormal astrocytes are also observed in the basal ganglion, thalamus, and hypothalamus. Although the astrocytes become distended, there is no evidence of abnormal storage material within neurons, and no biochemical defect has been detected. It is also possible that the demyelination is a secondary event. The white matter is soft and retracted and shows a variable loss of myelin, which appears more marked and diffuse in infantile cases. Axon cylinders are preserved.
The most frequent form of Alexander's syndrome is the infantile form, which has an average onset at 6 months of age. However, onset may occur at any time from shortly before birth to as late as 2 years of age. The average duration of disease is 2 to 3 years, but it can vary from a few months to several years. Predominant psychomotor retardation exists initially, and progressive spasticity and seizures in the context of megaloencephaly, with or without frank hydrocephalus, develop. When hydrocephalus occurs, it may result from proliferation of the ependymal lining of the aqueduct by the propagation of astrocytes containing excessive amounts of eosinophilic cytoplasmic material. The peripheral nervous system is not involved.
In the juvenile form, which is much less common than the infantile form, onset usually occurs between 7 and 14 years of age, and the duration is approximately 8 years. Bulbar and pseudobulbar dysfunction predominates, with dysphagia, dysarthria, nystagmus, ptosis, full facial palsy, and tongue atrophy. y , y Generalized spasticity and weakness may also occur, but unlike the severe mental retardation characteristic of the infantile form, mentation tends to remain intact. y The adult form of Alexander's syndrome resembles multiple sclerosis clinically and is characterized by blurred vision, spasticity, nystagmus, dysarthria, and dysphagia.
CT changes include low attenuation in the deep cerebral white matter, most extensively in the frontal lobes and subependymal regions. There is inconsistent abnormal enhancement of the caudate nuclei, anterior columns of the fornix, optic radiations, and periventricular areas.
Since the types of Alexander's disease are phenotypically distinct, the differential diagnosis varies by age. If one encounters an infant with chronically developing megencephaly or macrocephaly with mild regression in psychomotor milestones in the absence of any other obvious cause, Alexander's syndrome is a highly probable diagnosis. Juvenile leukoencephalopathy must be considered in children and multiple sclerosis in adults.
No specific therapy is available for Alexander's syndrome. Much supportive care, however, is necessary, including good nutrition and generous use of antibiotics and antiepileptics. Despite these measures, the prognosis for infants and children with this disease at present is poor. Recent advances in positron emission tomography (PET) scanning and single-photon emission computed tomography (SPECT) indicate that there is an abnormal flow of spinal fluid through the blood-brain barrier. This may be an important point in fully understanding these disorders and may aid in developing interventions.
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