Spinocerebellar ataxia (spy-NO-sare-ah-BELL-ar ah-TAX-ee-ah)(SCA) is one of a group of inherited disorders in which individuals experience degeneration of the cerebellum, leading to uncoordinated and clumsy movements that get worse over time. The word ataxia literally means without order and refers to the symptom of uncoordinated (or unordered) movement.
When it comes to movement, a vital part of the body is the cerebellum (sare-uh-BELL-um), a portion of the brain located at the back of the head. It helps the body make smooth and coordinated movements. A group of 28 different types of disorders, together called spinocerebellar ataxias or spinal cerebellar ataxias (SCA), interfere with that control and lead to uncoordinated movement, known as ataxia. The 28 types of SCA are distinguished by the age of onset, the genes * involved, the symptoms in addition to ataxia, the degree of severity of symptoms, the progression of the disease, and the prognosis. The types of SCA are classified either by the chronological order of gene discovery or by the names of families in which they were first discovered (such as Machado-Joseph disease), or by the chronological order of gene discovery (such as SCA 3).
The SCAs are rare diseases. Accurate estimates of their incidence were unavailable as of 2016; however, it is thought that 150,000 people in the United States have SCA. Type 3 is the most common (23% of all SCAs). Other more common SCAs are Type 1 (16%), Type 2 (18%), Type 6 (17%), and Type 7 (2%–5%). Some types occur more often in certain ethnic groups, such as SCA 3 in persons of Portuguese and German descent, SCA 10 in people of Mexican descent, SCA 13 in individuals of French descent, and SCA 14 in people of Japanese descent.
People who experience ataxias usually begin having symptoms when they are between 30 and 50 years old, and the symptoms worsen over 10 to 20 years. The onset exceptions are SCA 2 and SCA 7, which start in childhood.
Spinocerebellar ataxias are inherited disorders. They arise because of a mutation * that disrupts the construction of DNA * , which in turn affects the production of proteins that are essential to properly running bodily functions. With SCAs, the problem begins in the building blocks of DNA.
To make a protein, the DNA unzips, leaving its bases exposed, and an enzyme (called RNA polymerase) starts making a new complementary strand. The new complementary strand is called messenger RNA, or mRNA. From there, another structure, called a ribosome, latches onto and runs along the newly formed mRNA. The ribosome is a matchmaker of sorts. It reads the bases on the mRNA three at a time and then matches that three-base set, called a codon (COE-don), with a particular amino acid, which is another type of chemical compound that serves as a building block of proteins. All of the proteins in the human body are made of 20 amino acids, and these different amino acids correspond to specific codons. By reading one codon after another on the mRNA, the ribosome can link together an exact chain of amino acids, which becomes a particular type of protein that has a specific job to do in the body.
A person with an inherited spinocerebellar ataxia has a mutation that can throw off the entire DNA-RNA-amino-acid-protein pathway. The mutation causes numerous repeated three-base sequences. These repeats, called trinucleotide repeats, result in the production of an abnormal protein (such as ataxin in SCA 1) that may lead to dysfunction or degeneration of parts of the nervous system * and other organs. Even healthy people have a certain number of trinucleotide repeats, and this normal condition does not cause SCA. When too many repeats exist, however, an individual can develop SCA. With a moderate number of repeats, a person has the disease but may or may not manifest the symptoms.
Individuals with a moderate or large number of repeats will transmit SCA by autosomal dominant inheritance, which means that a child need inherit an abnormal gene from only one parent to get the disease. In other words, every child born to an affected parent has a 50 percent chance of inheriting SCA. The number of repeats increases with each generation (which is called anticipation), and symptoms start earlier with an increasing number of repeats. Therefore, a child may develop symptoms at an earlier age than his or her parents or even the grandparents. In some individuals, SCA is not inherited. Instead, a mutation occurs for the first time in the egg or sperm and causes SCA. Such a first-time mutation is called a de novo mutation.
A neurologist * or a movement disorders specialist usually makes the diagnosis after taking a thorough medical and family history and conducting a physical exam. Frequently, the medical professional will also order blood tests to rule out other conditions such as vitamin or thyroid gland * deficiency, and may order magnetic resonance imaging * (MRI) of the brain. The MRI will show whether the patient has a shrunken cerebellum, and will also exclude other causes of ataxia such as multiple sclerosis * , ischemic strokes * , or tumors * .
The only way to differentiate the specific type of SCA a patient has is to look for genetic mutations in DNA, which is usually obtained from blood. Medical professionals can also run tests on adults (not children) who do not have SCA but have a strong family history of SCA. Doing so determines the likelihood that these individuals will develop symptoms later in life. Such tests are conducted only after individuals have undergone genetic and psychological counseling so they can understand the personal, professional, and health implications of a positive test. Testing cannot, however, pinpoint when symptoms will start if the person being tested presently has no symptoms.
Prenatal (prior to birth) tests, such as chorionic villus sampling * and amniocentesis * , can determine whether the fetus has inherited SCA. Based on the results, parents can decide whether they want to continue with the pregnancy or proceed to abortion on medical grounds. Such genetic testing can also look for SCA in artificially implanted embryos.
As of 2016, no treatment was available for SCA. Over time, patients may need walkers or wheelchairs. Some patients may also require speech therapy. Therapists can help with impaired swallowing, and nutritionists can instruct patients about the types of foods they can eat. Physical and occupational therapists * can help patients learn how to modify their home and work environment to make life as easy as possible.
Patients with milder forms of SCAs can anticipate a normal life expectancy. However, SCAs that start in infancy lead to severe disability and death in adulthood. Most people die from respiratory failure or lung complications such as pneumonia * .
See also Genetic Diseases: Overview
LeDoux, Mark S. Movement Disorders: Genetics and Models, 2nd ed. San Diego, CA: Academic, 2014.
Genetics Home Reference. “Spinocerebellar Ataxia Type 1.” U.S. National Library of Medicine. http://ghr.nlm.nih.gov/condition/spinocerebellar-ataxia-type-1 (accessed July 11, 2016).
National Ataxia Foundation. 2600 Fernbrook Ln., Suite 119, Minneapolis, MN 55447. Telephone: 763-553-0020. Website: http://www.ataxia.org (accessed July 11, 2016).
National Institute of Neurological Disorders and Stroke. P.O. Box 5801, Bethesda, MD 20824. Toll-free: 800-352-9424. Website: http://www.ninds.nih.gov/index.htm (accessed July 11, 2016).
* genes (JEENS) are the functional units of heredity that are composed of deoxyribonucleic acid (DNA) and that help determine a person's body structure and physical characteristics. Inherited from a person's parents, genes are segments of chromosomes found in the nuclei of the body's cells.
* mutation (myoo-TAY-shun) is a change in an organism's gene or genes.
* DNA, or deoxyribonucleic acid (dee-OX-see-ry-bo-nyoo-klay-ik AH-sid), is the specialized chemical substance that contains the genetic code necessary to build and maintain the structures and functions of living organisms.
* nervous system is a network of specialized tissue made of nerve cells, or neurons, that process messages to and from different parts of the human body.
* nystagmus (nis-TAG-mus) are rapid, involuntary movements of the eyes.
* brain stem is the part of the brain that connects to the spinal cord. The brain stem controls the basic functions of life, such as breathing and blood pressure.
* autonomic nervous system is a branch of the peripheral nervous system that controls various involuntary body activities, such as body temperature, metabolism, heart rate, blood pressure, breathing, and digestion. The autonomic nervous system has two parts—the sympathetic and parasympathetic branches.
* retina (RET-i-nuh) is the tissue that forms the inner surface of the back of the eyeballs. It receives the light that enters the eye and transmits it through the optic nerves to the brain to produce visual images.
* vertigo (VER-ti-go) is the feeling that either the environment or one's own body is revolving or spinning even though they are not.
* convulsions (kon-VUL-shuns), also called seizures, are involuntary muscle contractions caused by electrical discharges within the brain and are usually accompanied by changes in consciousness.
* neurologist (new-RHAL-eh-jist) is a physician who specializes in diagnosing and treating diseases of the nervous system.
* thyroid gland (THY-roid) is located in the lower part of the front of the neck. The thyroid produces hormones that regulate the body's metabolism (me-TABo-LIZ-um), the processes the body uses to produce energy, to grow, and to maintain body tissues.
* magnetic resonance imaging (or MRI) uses magnetic waves instead of x-rays to scan the body and produce detailed pictures of the body's structures.
* multiple sclerosis (skluh-RO-sis), or MS, is an inflammatory disease of the nervous system that disrupts communication between the brain and other parts of the body. MS can result in paralysis, loss of vision, and other symptoms.
* ischemic (is-KEM-ik) strokes are events that occur when a blood vessel bringing oxygen and nutrients to the brain becomes clogged by a blood clot or other particle. As a result, nerve cells in the affected area of the brain cannot function properly.
* tumors (TOO-morz) are abnormal growths of body tissue that have no known cause or physiologic purpose. Tumors may or may not be cancerous.
* chorionic villus sampling (KORee-on-ik VIL-lus) is a test in which a small tube is inserted through the cervix and a small piece of the placenta supporting the fetus is removed for genetic testing.
* amniocentesis (am-nee-o-sen-TEE-sis) is a test in which a long, thin needle is inserted in the mother's uterus to obtain a sample of the amniotic fluid from the sac that surrounds the fetus. The fetal cells in the fluid are then examined for genetic defects.
* physical and occupational therapists are professionals who are trained to treat injured people by means of activities designed to help them recover or relearn specific functions or movements and restore their abilities to perform the tasks of daily living.
* pneumonia (nu-MO-nyah) is inflammation of the lungs.