Spinocerebellar Ataxia Types

Spinocerebellar ataxia (SCA) is a group of inherited conditions that cause slowly worsening problems with balance, coordination and speech. They happen when nerve cells in the cerebellum (the part of the brain that coordinates movement) and its connections in the brainstem and spinal cord gradually degenerate.

Ataxia itself simply means a loss of coordination, and it can have many causes, such as a stroke, a tumour, alcohol or a vitamin deficiency. Spinocerebellar ataxias are different because they are genetic: they are caused by changes (variants) in specific genes and are passed down through families.

More than 40 types of SCA have now been identified. They are named SCA1, SCA2, SCA3 and so on, in the order in which they were discovered. The number does not indicate how severe the type is.

Almost all SCAs are inherited in an autosomal dominant way. Only one parent needs to pass on the changed gene, and each child of an affected parent has a 50% chance of inheriting it. Men and women are affected equally.

The most common types worldwide are SCA3, SCA1, SCA2 and SCA6, followed by SCA7. Together, these make up around half or more of SCA cases, although how common each type is varies a lot between countries. In all five, the cause is an expanded stretch of repeated DNA (a "CAG repeat"). Longer repeats tend to cause earlier symptoms and faster progression.

SCA Type 1

In SCA1, symptoms usually start between the ages of 30 and 40, although they can begin earlier. The first signs are usually unsteady walking and poor coordination, followed by slurred speech and, later, difficulty swallowing. Weakness of the eye and eyelid muscles, muscle stiffness and nerve problems in the limbs are also common.

SCA1 tends to progress faster than the other common types. In adults, the illness usually lasts 10 to 30 years from onset. In rare cases where symptoms start in childhood (before about age 13), the disease is more severe and progresses much more quickly.

SCA Type 2

SCA2 often starts in the 30s, but onset ranges from childhood to old age. A distinctive feature is very slow eye movements. People with SCA2 may also have reduced reflexes, nerve problems in the limbs, tremor, and in some cases parkinsonism (Parkinson's-like symptoms) or problems with thinking and memory. SCA2 is especially common in Cuba.

SCA Type 3 (Machado-Joseph Disease)

SCA3, also called Machado-Joseph disease, is the most common SCA worldwide. Symptoms usually begin between 30 and 40. Along with ataxia, people may have double vision, a staring appearance, muscle stiffness, dystonia (sustained muscle contractions that cause twisting or repetitive movements), parkinsonism, nerve problems in the limbs, and sleep problems such as restless legs and daytime tiredness.

SCA Type 6

SCA6 usually starts later, often between 40 and 60, and progresses more slowly than SCA1, SCA2 and SCA3. It mainly causes a "pure" ataxia with unsteady walking, slurred speech and eye movement problems, with fewer other symptoms. Life expectancy is often close to normal.

SCA Type 7

SCA7 is less common. As well as ataxia, it causes progressive loss of vision due to damage to the retina, which sets it apart from the other common types.

Other types

Most of the remaining types are rare, and some have been described in only a single family. Some progress very slowly and may hardly affect daily life for many years.

Diagnosis

Because the types overlap so much, the exact type can usually only be confirmed with a genetic blood test. Testing for the most common repeat-expansion types is widely available, and newer methods such as gene panels and genome sequencing can find many rarer types. Even so, some families with a clear inherited ataxia still have no identified gene. A neurologist, ideally with ataxia expertise, and a genetic counsellor can help decide which tests are appropriate and what the results mean for other family members.

Outlook and support

There is currently no approved treatment that slows the progression of SCA, although several treatments are being tested in clinical trials. Care focuses on staying as independent as possible, through physiotherapy, speech and language therapy, occupational therapy, mobility aids such as walking sticks, walkers and wheelchairs, home adaptations, and medicines for specific symptoms.

For SCA1, SCA2 and SCA3, average survival after ataxia starts is around 18 to 22 years. For SCA6 it is much longer, around 35 years. In the later stages, the most common causes of death are chest infections and pneumonia related to swallowing difficulties, and breathing problems.