Alexander disease is an extremely rare, autosomal dominant leukodystrophy, which is a neurological condition caused by anomalies in the myelin which protects nerve fibers in the brain with progressive and fatal, neurological disorders affecting the midbrain and cerebellum of the central nervous system. Initially, it was detected most often during infancy or early childhood, but as better diagnostic tools have become available has been found to occur with similar frequency at all stages of life. Alexander disease has historically been included among the leukodystrophies–diseases of the white matter of the brain. These diseases affect the fatty material (myelin) that forms an insulating wrapping (sheath) around certain nerve fibers (axons). Myelin enables the efficient transmission of nerve impulses and provides the “whitish” appearance of the so-called white matter of the brain. There is a marked deficit in myelin formation in most early-onset patients with Alexander disease, and sometimes in later-onset patients, particularly in the front (frontal lobes) of the brain’s two hemispheres (cerebrum). However, white matter defects are sometimes not observed in later onset individuals. Instead, the unifying feature among all Alexander disease patients is the presence of abnormal protein aggregates known as “Rosenthal fibers” throughout certain regions of the brain and spinal cord (central nervous system [CNS]). These aggregates occur inside astrocytes, a common cell type in the CNS that helps maintain a normal CNS environment. Accordingly, it is more appropriate to consider Alexander’s disease a disease of astrocytes (an astrogliopathy) than a white matter disease (leukodystrophy). Alexander’s disease is named after the physician who first described the condition in 1949 (WS Alexander).
Another Name
- dysmyelogenic leukodystrophy
- dysmyelogenic leukodystrophy-megalopae
- fibrinoid degeneration of astrocytes
- fibrinoid leukodystrophy
- hyaline panneuropathy
- leukodystrophy with Rosenthal fibers
- megalencephaly with hyaline inclusion
- megalencephaly with hyaline pain neuropathy
Causes
About 95% of Alexander’s disease cases are caused by mutations in a gene called GFAP for a structural protein called glial fibrillary acidic protein that is found exclusively in astrocytes in the CNS. The cause of the other 5% of cases is not known.
The GFAP mutations are dominant. Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary to cause a particular disease. Thus, Alexander patients have one mutant copy and one normal copy of the GFAP gene. The abnormal gene can be inherited from either parent or can be the result of a new mutation (change in the DNA of the gene). Most Alexander patients have a new mutation, indicating that neither of their parents has the mutation, but the mutation arose at some point during the development of sperm or ova or an embryo. As the disease becomes better diagnosed, familial cases, in which the disease is passed from one generation to the next, are being increasingly recognized. The risk of transmitting the disorder from an affected parent to an offspring is 50 percent for each pregnancy. The risk is the same for males and females.
How the GFAP mutations produce Alexander’s disease is not known. The Rosenthal fibers, which contain GFAP, accumulate throughout the surfaces of the brain (cerebral cortex), in the white matter of the brain, and the lower regions of the brain (brainstem), and the spinal cord, and primarily appear under the innermost of the protective membranes (meninges) surrounding the brain and spinal cord (pia mater); under the lining of the fluid-filled cavities (ventricles) of the brain (subependymal regions); and around blood vessels (perivascular regions). Studies in mice indicate that the mutations act by producing a new, toxic effect, rather than by interfering with the normal function of GFAP. This toxic effect may be due to the presence of the Rosenthal fibers, or to the very large, abnormal amounts of GFAP that accumulate in Alexander astrocytes, or both. Astrocytes perform many critical functions in the CNS, and several of these are affected by the GFAP mutations, but the importance of these changes to the disease is not yet known.
It is caused by mutations in the gene for glial fibrillary acidic protein (GFAP) that maps to chromosome 17q21. It is inherited in an autosomal dominant manner, such that the child of a parent with the disease has a 50% chance of inheriting the condition if the parent is heterozygotic. However, most cases arise de novo as the result of sporadic mutations.[rx]
Alexander disease belongs to leukodystrophies, a group of diseases that affect the growth or development of the myelin sheath. The destruction of white matter in the brain is accompanied by the formation of fibrous, eosinophilic deposits known as Rosenthal fibers. Rosenthal fibers appear not to be present in healthy people but occur in specific diseases, like some forms of cancer, Alzheimer’s, Parkinson’s, Huntington’s, and ALS.[rx][rx][rx] The Rosenthal fibers found in Alexander disease do not share the distribution or concentration of other diseases and disorders.[rx]
Diagnosis
For many years a brain biopsy to determine the presence of Rosenthal fibers was required for the diagnosis of Alexander disease. However, even this procedure can be ambiguous, because Rosenthal fibers are also found in certain other disorders, such as tumors of astrocytes. More recently, MRI criteria have been developed that have a high degree of accuracy for diagnosing typical Type I (early onset) disease. These criteria have been less useful for some Type II cases, which have little or no white matter deficits in the brain, although abnormalities in the brainstem, cerebellum and spinal cord can suggest the diagnosis. Accordingly, when making a diagnosis of Alexander disease, more common diseases that have similar symptoms for which tests are available should first be ruled out. These include adrenoleukodystrophy, Canavan’s disease, glutaric aciduria, Krabbe leukodystrophy, Leigh syndrome, metachromatic leukodystrophy, Pelizaeus-Merzbacher, and Tay-Sachs disease. A definitive diagnosis of Alexander’s disease rests on the identification of a GFAP mutation in the patient’s DNA, which can be obtained from a blood sample or a swab of the inside of the cheek. DNA analysis is provided by several commercial and research laboratories. However, since no GFAP mutation has been found in about 5% of known cases, a negative result does not completely rule out the disease. Presently, Alexander patients without a GFAP mutation can be definitively diagnosed only at autopsy by the presence of disseminated, large numbers of Rosenthal fibers.
History and Physical
Alexander disease most often affects infants and children. Patients usually present with microcephaly, developmental delay, progressive quadriparesis, and seizures.
Traditionally, Alexander’s disease has been divided into four subtypes, which include neonatal, infantile, juvenile, and adult.[rx]
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Neonatal form: It is characterized by seizures, increased intracranial pressure, and severe motor dysfunction. Of note, unlike infantile, ataxia and hyperreflexia are not present. Death usually occurs within the first couple of weeks to years.
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Infantile form: The onset is before age 2, accounting for approximately 42 percent of all cases. This form is associated with seizures, ataxia, hyperreflexia, hydrocephalus, megalencephaly, feeding difficulties, and spasticity. It is possible to detect progressive psychomotor retardation with changes in developmental stages. Death typically occurs within weeks or several years of life.[rx]
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Juvenile form: Onset is usually slower than infantile form and is normocephalic. It is characterized by bulbar and pseudobulbar signs, such as speech difficulties and swallowing. Patients may also have spasticity and ataxia. A progressive worsening of intellectual faculties and epilepsy can be found. The survival is variable with death, which may occur in early adolescence or up to 20-30 years.
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Adult form: It is the least common form of the disease and has variable features. Some of these features include pseudobulbar and bulbar signs (dysphasia, dysarthria, dysphonia), progressive ataxia, quadriparesis, dysmorphic features, and dysautonomia.[rx] Other clinical features may include sleep disturbances (sleep apnea), postural alterations (scoliosis, kyphosis), palatal abnormalities, short neck, epilepsy, and diplopia. Survival is variable with death after a few years or even after a few decades of illness. Many cases are occasionally detected at autopsy, and death is often, in these circumstances, attributable to other intercurrent illnesses.
Alexander disease classification was revised in 2011 based on statistical analysis.[rx] Although this model system predicts trends, it does not allow the classification of an individual with complete certainty.
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Type 1: This type is typically more severe, with a median survival of 14 years. It is characterized by seizures, early-onset, macrocephaly, encephalopathy, motor delay, failure to thrive, and the typical imaging features (described below).[rx]
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Type 2: This type is usually less severe, with a median survival of 25 years. It is characterized by autonomic dysfunction, bulbar symptoms, eye movement abnormalities, lacking neurocognitive deficits, and the atypical neuroimaging features (described below).
Signs of Alexander’s disease can be demonstrated on CT and MRI, with MRI being the preferred imaging modality because of its sensitivity. Findings usually demonstrate cerebral white matter changes and swelling. The disease begins anteriorly and extends posteriorly. The subcortical U-fibers are generally spared early on in the course.
MRI – typical features: Demonstrates abnormal signal in the following locations (enhancement may be seen in the same areas):
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Symmetric bifrontal white matter
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Periventricular rim
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Caudate head and less commonly globus pallidus, thalamus, and brain stem
Of note, there is relative sparing of the temporal and occipital lobe white matter.
MRI criteria for diagnosis: Four out of five are required to make a diagnosis.[rx]
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White matter changes more pronounced in the front.
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Periventricular rim abnormalities
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Basal ganglia and thalami abnormalities
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Brainstem abnormalities
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Contest enhancement of one of the following structures (ventricular lining, frontal white matter, optic chiasm, basal ganglia, thalamus, fornix, dentate nucleus, brainstem)
MRI- atypical features: Demonstrates abnormal signal in the following locations:[rx]
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Upper cervical cord and medulla
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Pons, superior/middle cerebellar peduncle
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Ventricular garlands
Magnetic resonance spectroscopy: [rx]
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Increased choline in the basal ganglia
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Reduce N-acetylaspartylglutamate (NAAG) in the frontal white matter
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Elevated lactate in the frontal white matter
The diagnosis of Alexander’s disease is primarily based on imaging features and clinical presentation. The diagnosis is typically confirmed genetically because of the variability of this disease. When children with Alexander disease present with typical clinical symptoms, the diagnosis can be made if four out of the five MRI criteria are met. The MRI findings are also invaluable when genetic testing is equivocal. But in children with atypical imaging features, the imaging findings cannot exclude the diagnosis. Thus, genetic testing is necessary to confirm the diagnosis in children with atypical imaging findings. In this regard, tests for the most frequent mutations (exons 1,4,8) are available in the clinical setting, and it is possible to resort to the sequencing of the entire gene in the case of recurrent mutations. Genetic study is also practicable in the field of prenatal diagnosis.
Treatment
Treatment is symptomatic and supportive. Genetic counseling may be of benefit for patients and their families. A fetal diagnosis is an option for a couple who has had a previously affected child.
Treatment of Alexander’s disease is centered upon supportive care and utilizing a multidisciplinary team.[rx] Using these methods can improve the quality of life of the affected individuals. These methods are based on the treatment of manifestations and maintaining close surveillance. Some examples include:
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Controlling seizures with anti-seizure drugs
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Improving spasticity and hypertonia with physical therapy and baclofen
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Increasing mobility by utilizing properly fitted equipment such as ankle braces
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Treating obstructive hydrocephalus with a ventriculoperitoneal shunt
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Improving malnutrition with a feeding specialist
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Controlling reflux or vomiting with a proton pump inhibitor
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Treating urinary retention and incontinence with bladder training, catheterization, and various medications
Overall the treatment of Alexander’s disease remains supportive. Nevertheless, decreasing the expression of the GFAP gene can represent an interesting future therapeutic perspective.
References
Alagille Syndrome (ALGS)
