Ataxia Spinocerebellar: SCA Facts and Information
Author: Thomas C. Weiss : Contact: Disabled World
Published: 2010-04-13 : (Rev. 2020-03-21)
Synopsis and Key Points:
Spinocerebellar ataxia (SCA) is a genetically inherited disorder characterized by abnormalities in brain functioning.
People affected by SCA develop a degenerative condition that affects their cerebellum, which is located behind their brainstem.
SCA is generally an adult-onset disorder, with the severity of progressive degeneration depending largely on the underlying defect.
Defining Ataxia Spinocerebellar
Spinocerebellar ataxia (SCA) is a form of genetically inherited disorder that is characterized by abnormalities in the person's brain functioning. The disorder represents a varied group of related disorders and is commonly inherited as a dominant trait, meaning that people who carry one of the various different gene mutations are not affected by it. It also means that people who carry the disorder present a fifty-percent chance of having a child who is affected by the disorder, despite the genetic background of their mate. People with SCA experience brain and spinal cord degeneration.
People affected by SCA develop a degenerative condition that affects their cerebellum, which is located behind their brainstem. The main function of a person's brainstem is to coordinate their body's ability to move. People with SCA experience a progressive atrophy, or muscle wasting. Their spine atrophies, potentially leading to spasticity.
SCA may be a physically devastating disorder involving the progressive loss of the affected person's ability to coordinate their movements, as well as the emotional complications that accompany such losses, and significant lifestyle changes. Adverse effects of the disorder can involve the person's hands, legs, and speech. There are currently eleven types of SCA known, with a number of different gene mutations that cause the disorder and accompanying names for each type. The numbers for each type span from one through twenty-five, with the exception of nine, and carry designations based upon the time at which they were identified and characterized. SCA is the same disorder as spinal cerebellar ataxia.
Causes of Ataxia Spinocerebellar
There are a number of gene mutations on different chromosomes that cause SCA.
The frequency of these genes within different populations varies greatly. Because of the various and different types, there are difficulties in estimating the incidence of a particular type within a particular population. Generally, the incidence is believed to be around one to five people in every one-hundred thousand. SCA does not differentiate between genders; as with nearly every autosomal dominant disorder, both females and males are equally likely to inherit a defective gene.
Symptoms of Ataxia Spinocerebellar
The most common type of SCA are SCA1-8.
Basic labeled diagram of the human brain structure.
The age of onset associated with these types of SCA is, on average, between the ages of twenty and thirty, with the exception of SCA6, which usually happens between the ages of forty and fifty. People with SCA8 usually experience symptoms in their late thirties. People with SCA2 usually experience slow eye movements and dementia. People with SCA8 commonly have an average lifespan, while people with SCA1 usually have active reflexes. People with SCA7 develop loss of vision. SCA3 is also referred to as, 'Machado-Joseph Disease.'
People with SCA types 1-3 and 7 may experience an earlier age of onset combined with increased severity of the disorder; something referred to as, 'anticipation,' as the defect is passed along from one generation to another. What this means is that children may be more severely affected at earlier ages than their parents. The size of the repeat of nucleotides in the affected genes is believed to correlate with the age and severity of onset in children. As the repeat size expands, the severity worsens, and the age of onset becomes earlier when compared with parents who are affected by the disorder. The repeat size does not; however, predict the exact age of onset, nor does it predict the specific symptoms that will develop in a particular individual.
The term, 'penetrance,' refers to the likelihood that people with a genetic defect will develop the disease. In SCA, the penetrance is high. There are; however, some rare cases involving people who have not developed symptoms. The reasons why are not known.
People with SCA initially develop coordination issues or, 'ataxia.' The development of poor movement coordination in people with SCA is manifested by abnormalities in eye or hand movements, difficulties with walking, as well as speech difficulties. SCA is generally an adult-onset disorder, with the severity of progressive degeneration depending largely on the underlying defect.
Ataxia many times happens when parts of the person's nervous system that control movement are damaged. People with ataxia experience failure of muscle control in their legs and arms. The result is a lack of coordination and balance, or disturbance in their gait. The term, 'ataxia,' is one that is mainly used to describe this set of symptoms, although it is sometimes also used to describe a family of disorders. Ataxia is not; however, a particular diagnosis.
Diagnosing Ataxia Spinocerebellar
Most of the disorders that result in ataxia cause the cells in the part of the person's brain called the cerebellum to atrophy, or degenerate. At times, the person's spine may also be affected. The phrases, 'cerebellar degeneration,' and, 'spinocerebellar degeneration,' are ones that describe changes which have taken place in an individual's nervous system. Neither of these terms constitutes a particular diagnosis. Spinocerebellar and cerebellar degeneration have a number of different causes. The person's age of onset and the resulting ataxia varies depending on the underlying cause of the degeneration.
Many people with ataxia have inherited it. These forms of ataxia are classified by chromosomal location and pattern of inheritance:
- Autosomal recessive - both parents pass on a copy of the faulty gene
- Autosomal dominant - the affected person inherits a normal gene from one parent and a faulty gene from the other parent
The more common forms of inherited ataxias include Machado-Joseph disease and Friedreich's ataxia. Spradic ataxias may also occur in families with no prior history of them. Ataxia may also be acquired. Stroke, tumors, alcoholism, multiple sclerosis, metabolic disorders, vitamin deficiencies, and peripheral neuropathy are among the conditions that can cause acquired ataxia.
A diagnosis of SCA is suspected if the person presents with the adult onset of the symptoms associated with the disorder.
An MRI scan has the potential to detect atrophy of the person's cerebellum, something that is a common finding in people with SCA. Clinical evaluation involves an extensive neurological examination. Genetic testing is an important part of achieving a diagnosis of SCA because the symptoms between different types of SCA are similar. A molecular genetic test in order to determine the gene that has the tinucleotide repeat expansion may assist in identification of additional carriers in the person's family. Once the genetic defect has been characterized, the person's family members may also be tested. Genetic testing is not; however, always one-hundred percent informative. There are rare cases of SCA diagnosed clinically that cannot be explained through any known genetic defects. Estimates place approximately fifty-to-sixty percent of the white population with a dominant familial form of SCA in the range of those whom DNA testing may provide a definitive diagnosis.
Treatment of Ataxia Spinocerebellar
Medical science has not discovered a cure for hereditary ataxias as of this time.
If the ataxia the person is experiencing is caused by another condition, the underlying condition is treated first. In one example, ataxia that is caused by a metabolic disorder might be treated with a controlled diet and medications. A vitamin deficiency is treated with vitamin therapy. There are a number of medications that can be administered to treat swallowing and gait disorders. Assistive technologies can help people in their activities of daily living, and physical therapy can help to strengthen their muscles.
People who present with symptoms and receive a diagnosis of SCA at a later time usually require a careful evaluation by a neurologist.
The forms of treatment they receive are based on lessening the symptoms they experience as they develop. The person will eventually require a full-time caretaker and nursing support in the later stages of the disease. Psychological counseling is often indicated, depending on the person, family, and their particular needs.
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