Spinocerebellar Ataxias
The spinocerebellar ataxias (SCAs) are a collection of rare, progressive, genetically inherited neurodegenerative disorders that primarily affect the cerebellum and its connections. Approximately 50 distinct SCAs have been identified and numbered based on the order of their discovery. These diseases typically emerge in mid-adulthood and the majority are characterized by autosomal dominant inheritance patterns. The estimated prevalence of SCAs in population-based studies is approximately 2.7 per 100,000 individuals (range 0–5.6 per 100,000), though this varies significantly by geographic region and ethnicity.
Bottom Line
- ~50 genetically distinct SCAs have been identified; the most prevalent are SCA3 (Machado-Joseph), followed by SCA2 and SCA6
- Polyglutamine (CAG) repeat expansions account for the most common SCAs (types 1, 2, 3, 6, 7, 17) with characteristic anticipation across generations
- SCA27b (FGF14) is a recently described late-onset ataxia caused by GAA repeats in intron 1 of FGF14 — it may be the most common form of adult-onset hereditary ataxia
- No FDA-approved disease-modifying therapies exist for SCAs; riluzole and troriluzole have shown variable results in clinical trials
- Next-generation sequencing has revolutionized genetic testing, enabling identification of novel variants; 30% of patients tested with conventional PCR remain undiagnosed
- Emerging therapies: Gene therapy (AAV vectors), antisense oligonucleotides (ASOs), and CRISPR-Cas9 are under active investigation
Genetics & Classification
Repeat Expansion SCAs
The most prevalent SCAs are linked to trinucleotide repeat expansion sequences, predominantly CAG repeats encoding polyglutamine tracts. These expansions cause abnormal elongation of polyglutamine tracts, disrupting protein folding and function. Anticipation — worsening symptoms with earlier onset in subsequent generations — is a hallmark of polyglutamine SCAs, caused by expansion of CAG repeats during transmission.
| SCA Type | Gene | Repeat Type | Key Clinical Features |
|---|---|---|---|
| SCA1 | ATXN1 | CAG (polyQ) | Ataxia, pyramidal signs, bulbar dysfunction, peripheral neuropathy; faster progression |
| SCA2 | ATXN2 | CAG (polyQ) | Ataxia, slow saccades (hallmark), hyporeflexia, peripheral neuropathy, parkinsonism in some; 2nd most common globally |
| SCA3 (Machado-Joseph) | ATXN3 | CAG (polyQ) | Most prevalent SCA worldwide; ataxia, pyramidal/extrapyramidal signs, progressive external ophthalmoplegia, peripheral neuropathy, bulging eyes, dystonia |
| SCA6 | CACNA1A | CAG (polyQ) | Pure cerebellar ataxia, downbeat nystagmus, late onset (>50), very slow progression; allelic to EA2 and FHM1 |
| SCA7 | ATXN7 | CAG (polyQ) | Ataxia with progressive retinal degeneration (cone-rod dystrophy) — unique among SCAs; visual loss may precede or follow ataxia |
| SCA8 | ATXN8/ATXN8OS | CTG/CAG | Slowly progressive cerebellar ataxia, sensory neuropathy; reduced penetrance makes interpretation challenging |
| SCA10 | ATXN10 | ATTCT pentanucleotide | Cerebellar ataxia with epilepsy; predominantly in Latin American populations |
| SCA12 | PPP2R2B | CAG (5′ UTR) | Action tremor preceding ataxia, dementia; more common in Indian populations |
| SCA17 | TBP | CAG (polyQ) | Ataxia, cognitive decline/dementia, psychiatric features, chorea; phenotypic overlap with Huntington disease (HDL4) |
| SCA27b | FGF14 | GAA intronic | Late-onset (>40), slowly progressive; downbeat nystagmus (55–80%), gaze-evoked nystagmus, vertigo, diplopia; may be the most common hereditary ataxia |
| SCA36 | NOP56 | GGCCTG hexanucleotide | Ataxia with motor neuron disease features; originally described in Japan (Asidan) |
Non-Repeat SCAs (Conventional Mutations)
Many SCAs result from missense variants, nonsense variants, insertions, or deletions rather than repeat expansions:
| SCA Type | Gene | Distinguishing Features |
|---|---|---|
| SCA5 | SPTBN2 | Pure cerebellar syndrome, very slow progression, described in Lincoln family descendants |
| SCA11 | TTBK2 | Mild, slowly progressive pure cerebellar ataxia |
| SCA13 | KCNC3 | Childhood-onset cerebellar ataxia with intellectual disability; or adult-onset pure cerebellar |
| SCA14 | PRKCG | Slowly progressive, axial myoclonus in some |
| SCA15/16 | ITPR1 | Very slowly progressive pure cerebellar ataxia, head tremor |
| SCA23 | PDYN | Late-onset, sensory neuropathy |
| SCA28 | AFG3L2 | Young adult onset, slow saccades, ptosis (mitochondrial protease dysfunction) |
| SCA35 | TGM6 | Pure cerebellar, described in Chinese families |
| SCA40 | CCDC88C | Pure cerebellar ataxia with spasticity |
| SCA48 | STUB1 | Cerebellar ataxia with cognitive-affective features; also linked to recessive ataxia (SCAR16) |
Related Autosomal Dominant Ataxias
| Disorder | Gene | Key Features |
|---|---|---|
| DRPLA (Dentatorubral-pallidoluysian atrophy) | ATN1 (CAG) | Ataxia, chorea, myoclonus, epilepsy, dementia; more common in Japan; strong anticipation |
| CAPOS | ATP1A3 | Cerebellar ataxia, areflexia, pes cavus, optic atrophy, sensorineural hearing loss |
SCA27b — A Newly Recognized Common Ataxia
Described by Pellerin et al. (2023, NEJM), SCA27b is caused by a deep intronic GAA repeat expansion in the FGF14 gene. It presents as a late-onset, slowly progressive cerebellar syndrome characterized by downbeat nystagmus (55–80%), gaze-evoked nystagmus, episodic vertigo, and diplopia. Preliminary studies suggest SCA27b may be more common than all other SCAs combined in some populations, though confirmation requires further epidemiologic data. Standard PCR panels may not detect it — specific repeat expansion testing or long-read sequencing is required.
Clinical Features
Despite their genetic heterogeneity, SCAs share core symptoms of progressive cerebellar dysfunction:
- Gait ataxia — wide-based, unsteady gait; difficulty with tandem walking; progressive need for assistive devices
- Limb incoordination — dysmetria on finger-to-nose and heel-to-shin testing, dysdiadochokinesia
- Dysarthria — scanning speech with irregular pauses, prolonged phonemes, distorted vowels
- Ocular dysfunction — nystagmus (gaze-evoked, downbeat), saccadic dysmetria, slow saccades (SCA2), impaired smooth pursuit, square-wave jerks
- Intention tremor — low-frequency tremor during goal-directed movements
- Cerebellar cognitive affective syndrome (Schmahmann syndrome) — executive dysfunction, impaired visuospatial processing, personality changes, language difficulties
Beyond cerebellar signs, individual SCAs may present with additional features that help narrow the differential:
| Extra-Cerebellar Feature | Associated SCA Types |
|---|---|
| Peripheral neuropathy | SCA1, SCA2, SCA3, SCA4, SCA25, SCA36 |
| Pyramidal signs (spasticity, hyperreflexia) | SCA1, SCA3, SCA7 |
| Parkinsonism / extrapyramidal signs | SCA2, SCA3, SCA17 |
| Dystonia | SCA3, SCA17, DRPLA |
| Cognitive decline / dementia | SCA12, SCA17, SCA48, DRPLA |
| Retinal degeneration / visual loss | SCA7 (pathognomonic) |
| Slow saccades | SCA2 (hallmark), SCA1, SCA3, SCA7, SCA28 |
| Downbeat nystagmus | SCA6, SCA27b |
| Epilepsy | SCA10, SCA17, DRPLA |
| Chorea | SCA17, DRPLA |
| Myoclonus | SCA14, DRPLA |
Diagnosis
Clinical Assessment
The diagnostic process begins with a comprehensive clinical evaluation including detailed family history (three-generation pedigree), age of onset, rate of progression, and associated symptoms. Clinical rating scales are used to quantify severity:
| Scale | Components | Use |
|---|---|---|
| SARA (Scale for the Assessment and Rating of Ataxia) | 8 items: gait, stance, sitting, speech, finger chase, nose-finger, alternating hand movements, heel-shin | Most widely used; high inter-rater and test-retest reliability |
| ICARS (International Cooperative Ataxia Rating Scale) | 100-point scale: posture/gait, limb kinetic function, speech, ocular motor | Comprehensive but lengthy |
| BARS (Brief Ataxia Rating Scale) | Gait, kinetic functions (arms/legs), speech, eye movements | Shorter; suitable for routine clinical use |
| SCA-FI (SCA Functional Index) | Functional abilities assessment | Designed for SCAs and Friedreich ataxia |
Genetic Testing Strategy
Choosing Appropriate Genetic Testing
An estimated 30% of patients tested with conventional PCR-based repeat expansion panels remain undiagnosed. Next-generation sequencing (whole-exome sequencing) evaluates a broader gene set but may miss short tandem repeats (including the SCA27b GAA expansion). Long-read sequencing technologies (PacBio, Oxford Nanopore) can detect both repeat expansions and conventional mutations. Patients with negative initial testing should be considered for retesting in several years as new genes are identified.
Testing approach:
- Known family variant: Single-gene targeted testing
- Unknown family history or multiple candidates: SCA repeat expansion panel (SCA1, 2, 3, 6, 7, 8, 10, 12, 17, 36, 27b, DRPLA)
- Negative panel: Whole-exome sequencing or comprehensive ataxia gene panel
- Still negative: Consider whole-genome sequencing, long-read sequencing, or research testing
Neuroimaging
MRI is important for assessing patterns of atrophy:
- Pure cerebellar atrophy: SCA5, SCA6, SCA11, SCA15/16, SCA27b
- Cerebellar + brainstem (olivopontocerebellar) atrophy: SCA1, SCA2, SCA3, SCA7
- Hot cross bun sign in pons: SCA2 (also seen in MSA-C)
- Cervical spinal cord atrophy: SCA3
- Cerebral cortical involvement: SCA17, DRPLA
Treatment
Current Pharmacotherapy
No FDA-approved disease-modifying therapies exist for any SCA. Treatment is primarily supportive and symptomatic:
| Agent | Mechanism | Evidence |
|---|---|---|
| Riluzole (50 mg BID) | Modulates small-conductance calcium-activated potassium (SK) channels in cerebellum; glutamate antagonist | Mixed results across trials. Romano et al. 2015 (Lancet Neurol): 55 patients with SCA or Friedreich ataxia, riluzole improved SARA by −1.53 points vs placebo (p=0.044) at 12 months. Coarelli et al. 2022 (Lancet Neurol): ATRIL trial in SCA2, did not meet primary endpoint at 12 months |
| Troriluzole | Prodrug of riluzole with improved bioavailability | Phase 3 trial in multiple SCA types completed; final results pending |
| 4-Aminopyridine (5–20 mg/day) | Potassium channel blocker; restores Purkinje cell firing regularity | Evidence primarily in EA2 and downbeat nystagmus; may improve gait and nystagmus in some SCAs |
| Acetazolamide | Carbonic anhydrase inhibitor | May help episodic features; used for downbeat nystagmus in SCA6 and SCA27b |
Symptomatic Management
| Symptom | Management |
|---|---|
| Gait ataxia and falls | Physical therapy (balance/coordination exercises), assistive devices (cane → walker → wheelchair), home safety evaluation |
| Dysarthria | Speech therapy; augmentative communication devices in advanced stages |
| Dysphagia | Swallowing evaluation, dietary modifications, PEG placement if needed |
| Tremor | Propranolol, primidone, botulinum toxin; DBS in selected cases |
| Spasticity | Baclofen, tizanidine, stretching programs |
| Depression | SSRIs/SNRIs; psychotherapy; screen actively (common in SCAs) |
| Sleep disturbance | Sleep study if RBD suspected; melatonin, clonazepam for RBD |
| Nystagmus / oscillopsia | 4-aminopyridine, acetazolamide, baclofen |
| Cognitive decline | Neuropsychological assessment; occupational therapy; cognitive rehabilitation |
Neuromodulation
Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) have shown promising results in some studies, with improvements in balance, gait, and SARA scores. However, results are not uniform across studies, and larger trials are needed to establish effectiveness and optimal parameters.
Emerging Therapies
Gene Therapy & Molecular Approaches
- Antisense oligonucleotides (ASOs): Designed to target and reduce expression of toxic mutant proteins (e.g., ATXN1 in SCA1, ATXN2 in SCA2, ATXN3 in SCA3). ASO silencing has reversed abnormal neurochemistry in SCA3 mouse models (McLoughlin et al. 2023, Ann Neurol)
- AAV gene therapy: Adeno-associated virus vectors under investigation for gene replacement or gene silencing in multiple SCA subtypes
- CRISPR-Cas9: Gene editing to eliminate expanded CAG repeats has been demonstrated in induced pluripotent stem cells from SCA3 patients (He et al. 2021; Ouyang et al. 2018)
- Protein clearance strategies: Autophagy enhancers and proteostasis regulators to clear aggregated mutant proteins
Genetic Counseling
Genetic counseling is essential for all patients with SCAs and their families:
- Autosomal dominant inheritance: each child of an affected parent has a 50% risk of inheriting the pathogenic variant
- Predictive (presymptomatic) testing is available for at-risk family members but should follow international guidelines (pre-test counseling, informed consent, psychological support)
- Anticipation in repeat expansion SCAs means offspring may have earlier onset and more severe disease
- Reproductive options include preimplantation genetic testing (PGT) for couples planning families
- Negative genetic test does not exclude the possibility of an as-yet-unidentified SCA — consider retesting as new genes are discovered
Prognosis
Most SCAs are progressive over decades. The rate of progression varies by genotype:
- Fastest progression: SCA1, SCA2, SCA7 (wheelchair-bound within 10–15 years of onset)
- Moderate progression: SCA3 (15–20 years to wheelchair)
- Slowest progression: SCA6, SCA27b, SCA5, SCA11 (may remain ambulatory for decades)
- Common causes of death: aspiration pneumonia, respiratory failure, and complications of immobility
- Average disease duration from onset to death: 10–30 years depending on SCA type
References
- Zesiewicz TA. Ataxia. Continuum (Minneap Minn). 2025;31(4, Movement Disorders):1093–1119.
- Klockgether T, Mariotti C, Paulson HL. Spinocerebellar ataxia. Nat Rev Dis Primers. 2019;5(1):24.
- Paulson HL, Shakkottai VG, Clark HB, Orr HT. Polyglutamine spinocerebellar ataxias — from genes to potential treatments. Nat Rev Neurosci. 2017;18(10):613–626.
- Pellerin D, Danzi MC, Wilke C, et al. Deep intronic FGF14 GAA repeat expansion in late-onset cerebellar ataxia. N Engl J Med. 2023;388(2):128–141.
- Wirth T, Clément G, Delvallée C, et al. Natural history and phenotypic spectrum of GAA-FGF14 sporadic late-onset cerebellar ataxia (SCA27B). Mov Disord. 2023;38(10):1950–1956.
- Romano S, Coarelli G, Marcotulli C, et al. Riluzole in patients with hereditary cerebellar ataxia: a randomised, double-blind, placebo-controlled trial. Lancet Neurol. 2015;14(10):985–991.
- Coarelli G, Heinzmann A, Ewenczyk C, et al. Safety and efficacy of riluzole in spinocerebellar ataxia type 2 in France (ATRIL). Lancet Neurol. 2022;21(3):225–233.
- McLoughlin HS, Gundry K, Rainwater O, et al. Antisense oligonucleotide silencing reverses abnormal neurochemistry in spinocerebellar ataxia 3 mice. Ann Neurol. 2023;94(4):658–671.
- Ruano L, Melo C, Silva MC, Coutinho P. The global epidemiology of hereditary ataxia and spastic paraplegia: a systematic review of prevalence studies. Neuroepidemiology. 2014;42(3):174–183.