The condition

The different types

More than forty genetically distinct spinocerebellar ataxias have been described. Here is what the number means, and why getting a specific one is worth the wait.

More than forty types have been described. A handful are common; most are rare. The number after “SCA” tells you which gene.

What the number refers to

The number identifies which gene carries the change — not severity, and not order of importance. The numbering is broadly historical: it reflects the order in which the types were described, which is why the low numbers are also the ones most often encountered.

Why the specific type matters

It changes several things that are practically useful. Types differ in what they affect besides coordination, in typical age of onset, in pace of progression, and in which research studies someone might be eligible for. SCA7 involves the retina, which means an eye examination belongs in the picture. SCA6 tends to stay more confined to the cerebellum. A trial recruiting for SCA3 will not enrol someone with SCA6.

It also matters for the wider family, since it makes accurate genetic counselling possible for relatives who want it.

Types most often encountered

A short reference, not a complete list, and not a way to work out which type someone has. Only genetic testing does that.

SCA1

Gene: ATXN1
Repeat: CAG
Inheritance: Autosomal dominant

Alongside the ataxia, signs pointing to nerve pathways outside the cerebellum are common, and progression is often faster than in SCA6.

One of the more common dominant ataxias worldwide.

SCA2

Gene: ATXN2
Repeat: CAG
Inheritance: Autosomal dominant

Notably slow eye movements are a recognised feature, and some people have prominent tremor or neuropathy.

Common worldwide, and particularly frequent in parts of Cuba where founder effects concentrate it.

SCA3

also called Machado-Joseph disease

Gene: ATXN3
Repeat: CAG
Inheritance: Autosomal dominant

The picture is broad and can include eye movement changes, muscle twitching, stiffness and neuropathy as well as ataxia.

The most common SCA genotype worldwide.

SCA6

Gene: CACNA1A
Repeat: CAG
Inheritance: Autosomal dominant

Tends to stay largely confined to the cerebellum, with later onset and slower progression than SCA1, 2 or 3.

Common among the dominant ataxias, especially in later-onset presentations.

SCA7

Gene: ATXN7
Repeat: CAG
Inheritance: Autosomal dominant

Distinctive because it also affects the retina, so vision loss can accompany or precede the ataxia. This is the one SCA where an eye examination is part of the picture.

Less common than SCA1, 2, 3 or 6.

SCA27B

Gene: FGF14
Repeat: GAA
Inheritance: Autosomal dominant

Often begins later in life, and symptoms can come in episodes early on before becoming constant. Because it was only described recently, many people carrying it were previously told their ataxia had no identified cause.

Since its description it has been found to account for a substantial share of previously unexplained late-onset ataxia in several populations.

Types are listed by number, which carries no ranking. Because SCA27B was described relatively recently, older reference material may not mention it at all.

Inheritance, and the number everyone misreads

Autosomal dominant means one altered copy of the gene is enough to cause the condition. A parent who has it has, on average, a one-in-two chance of passing the altered copy to each child. It is the same one-in-two chance for every pregnancy — it does not even out across a family, and an unaffected older sibling tells you nothing about the next.

How it passes down

Parent with SCA

Parent without

Child 1

Child 2

Child 3

Child 4

2 of 4 inherited it this time. Try running it again.

Each child independently has a one-in-two chance. It does not even out across a family, and an unaffected older sibling tells you nothing about the next one. That is the whole reason the number is so easy to misread.

If no type has been identified

A significant number of people with clearly inherited ataxia never get a specific genetic label. This is a gap in what science has discovered, not a gap in the care someone is receiving, and it does not change what supportive treatment is available.

It is also a moving target. SCA27B was only described recently and turned out to account for a substantial share of previously unexplained late-onset ataxia — meaning a number of people who had been told there was no answer now have one. If testing was done years ago, it can be worth asking a neurologist whether newer testing would add anything.

Sources for this page

GeneReviews — University of Washington / NCBI Bookshelf
MedlinePlus Genetics — US National Library of Medicine
Genetic and Rare Diseases Information Center (GARD) — NIH / NCATS

What is available now

Supportive care, and an honest account of why there is no drug list.