Ataxia
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Ataxia
Definition
Ataxia refers to a disturbance in the smooth performance of voluntary motor acts - causing muscular incoordination or impaired balance. Movements are flawed in rate, range, timing, direction, and force. In the absence of cerebellar inhibitory and modulating influences, skilled movements originating in the cerebral motor cortex become inaccurate and poorly controlled. Ataxia may affect the limbs, the trunk, or gait.
- Localization in Clinical Neurology, 8e, p. 968
Pathophysiology
Cerebellar lesions cause defective timing of sequential contractions of agonist and antagonist muscles. Ataxia is regarded as the "cerebellar sign par excellence." The cardinal features of cerebellar dysfunction include:
| Feature | Description |
|---|---|
| Hypotonia | Decreased fusimotor activity; pendular stretch reflexes; more marked proximally in upper limbs; ipsilateral to lesion |
| Dysmetria | Overshooting or undershooting a target |
| Dyssynergia | Decomposition of movement; irregular, variable steps |
| Dysdiadochokinesia | Impaired rapid alternating movements |
| Intention (kinetic) tremor | Tremor increasing as the limb approaches a target |
| Nystagmus | Often gaze-evoked; downbeat nystagmus in EA-2 |
| Gait ataxia | Wide-based, lurching, staggering; accentuated by heel-to-toe walking |
- Localization in Clinical Neurology, 8e, p. 968
Types of Ataxia by Localization
1. Truncal Ataxia (Midline/Vermal)
Lesions confined to the cerebellar vermis affect the medial motor system. Patients have a wide-based, unsteady, "drunk-like" gait with little or no limb coordination defect. In severe cases the patient may be unable to sit unsupported. Causes include paraneoplastic syndromes, malnutrition in alcoholism, and structural mass lesions of the vermis.
2. Appendicular Ataxia (Hemispheric)
Lesions of the intermediate and lateral cerebellar hemispheres affect the lateral motor system, producing ataxia on movement of the extremities. A unilateral hemispheric lesion causes ipsilateral limb ataxia (cerebellar connections cross twice or stay ipsilateral, so the ataxia is on the same side as the lesion).
3. Sensory (Posterior Column) Ataxia
Loss of proprioception from large-diameter afferent fiber disruption - in peripheral neuropathies, dorsal root ganglionopathies, or dorsal column lesions. Features:
- Wide-based, cautious gait with visual dependence
- Feet thrust forward with slapping sound on heel strike
- Markedly worse in the dark or with eye closure (Romberg sign)
- Unlike cerebellar ataxia, eye closure causes profound imbalance
4. Spastic Ataxia
A combination of spasticity and ataxia produces a "bouncing" gait. Clonus during standing/walking creates a vicious cycle of ataxic movements and increasing unsteadiness. Seen in multiple sclerosis, Arnold-Chiari malformation, and hydrocephalus.
- Bradley and Daroff's Neurology in Clinical Practice, p. 968; Neuroanatomy through Clinical Cases 3e, p. 2300
Temporal Classification
Acute Ataxia
| Category | Examples |
|---|---|
| Idiopathic | Acute cerebellar ataxia (post-infectious) |
| Infections | Bacterial/viral meningitis, varicella |
| Metabolic | Hypoglycemia, hyponatremia, hyperammonemia, Wernicke encephalopathy, biotinidase deficiency |
| Toxins/Drugs | Lithium, phenytoin, alcohol |
| Structural | Cerebellar infarction, hemorrhage, neoplasm, hydrocephalus |
| Immune | Miller Fisher syndrome (anti-GQ1b), Bickerstaff brainstem encephalitis |
Episodic/Recurrent Ataxia
The episodic ataxias (EAs) are channelopathies:
- EA-1: mutation in KCNA1 (voltage-gated K+ channel); interictal myokymia; attacks seconds-to-minutes
- EA-2: mutation in CACNA1A (P/Q-type Ca2+ channel); interictal gaze-evoked or downbeat nystagmus; attacks minutes-to-hours; responsive to acetazolamide
- EA-3 through EA-8: rarer, various genetic loci
Other episodic causes: migraine with brainstem aura, epileptic postictal state, metabolic (hypoglycemia, hyperammonemia, maple syrup urine disease, Hartnup disease, porphyria, Refsum disease), dominant paroxysmal ataxia.
Chronic Progressive Ataxia
This broad group includes hereditary and acquired causes (see full classification below).
Etiological Classification
Autosomal Dominant
- Spinocerebellar Ataxias (SCAs): >48 types identified (SCA1-48+), most due to CAG repeat expansions
- SCA7 has added retinal degeneration
- SCA2 has slow saccades
- SCA3/Machado-Joseph disease is the most common worldwide
- Dentatorubropallidoluysian atrophy (DRPLA)
- Episodic Ataxias (EA1-EA8)
- Familial hemiplegic migraine
Autosomal Recessive
| Disorder | Key Features |
|---|---|
| Friedreich Ataxia | Most common hereditary ataxia; GAA repeat in FXN (frataxin); onset <25 yrs; areflexia, cardiomyopathy, pes cavus |
| Ataxia-Telangiectasia | ATM mutation; cerebellar ataxia + oculocutaneous telangiectasia + immunodeficiency + cancer risk |
| Ataxia with oculomotor apraxia types 1 & 2 | Oculomotor apraxia, neuropathy |
| Ataxia with Vitamin E deficiency | Treatable - vitamin E supplementation |
| Abetalipoproteinemia (Bassen-Kornzweig) | Acanthocytosis, fat malabsorption, low serum lipids |
| Ataxia-oculomotor apraxia (Ramsay Hunt) | Myoclonus + ataxia |
| Marinesco-Sjögren syndrome | Cataract + mental retardation + ataxia |
| Cerebrotendinous xanthomatosis | Treatable - chenodeoxycholic acid |
| Wilson disease | Treatable - copper chelation |
| Mitochondrial ataxias (POLG, MERRF, MELAS) | Multisystem involvement |
X-linked
- Fragile X premutation syndrome (FXTAS)
- X-linked ataxia with lactic acidosis
Acquired/Non-Genetic
- Paraneoplastic cerebellar degeneration (anti-Yo, anti-Hu, anti-Ri antibodies)
- Autoimmune cerebellar ataxia (anti-GAD65, anti-CASPR2, anti-NMDAR)
- Gluten ataxia / celiac disease
- Hypothyroid ataxia
- Superficial CNS siderosis
- CANVAS (Cerebellar Ataxia, Neuropathy, and Vestibular Areflexia Syndrome)
- CNS Whipple disease
- Multiple System Atrophy - cerebellar type (MSA-C)
Fixed (Non-Progressive)
-
Cerebral palsy (ataxic type)
-
Congenital malformations: Dandy-Walker, Chiari, Joubert syndrome, rhombencephalosynapsis, cerebellar hypoplasia/agenesis
-
Localization in Clinical Neurology, 8e, pp. 969-972
Diagnostic Approach
The neurological examination, family history, and genetic testing are most helpful in reaching a specific diagnosis in hereditary ataxias; neuroimaging has a supportive role. Key imaging findings:
| Disorder | MRI Finding |
|---|---|
| MSA-C | "Hot cross bun" sign in pons; cerebellar/pontine atrophy |
| Friedreich ataxia | Cervical cord atrophy |
| Ataxia-telangiectasia | Cerebellar atrophy |
| Hereditary spastic paraplegia | Thin corpus callosum |
| Fragile X premutation (FXTAS) | T2 hyperintensity in middle cerebellar peduncles |
MRI is almost always obtained to rule out mass lesions (posterior fossa neoplasm, abscess, or hemorrhage) before attributing ataxia to a hereditary/degenerative cause.
- Bradley and Daroff's Neurology in Clinical Practice, p. 1102
Management
-
Acute ataxia: Treat the underlying cause (e.g., thiamine for Wernicke, antibiotics for meningitis, antitoxin).
-
EA-2: Acetazolamide (reduces attack frequency).
-
Friedreich ataxia: No disease-modifying therapy proven effective; cardiac monitoring essential. Omaveloxolone was FDA-approved in 2023 as the first treatment.
-
Vitamin E deficiency ataxia: High-dose vitamin E supplementation.
-
Cerebrotendinous xanthomatosis / Wilson disease: Specific metabolic treatments.
-
Autoimmune/paraneoplastic: Immunotherapy (steroids, IVIG, plasma exchange), treat underlying tumor.
-
Symptomatic gait ataxia: Weighted wrist cuffs to reduce kinetic tremor amplitude; mobility aids (cane, walker, wheelchair) to prevent falls; displacing the center of gravity forward improves gait in elderly patients with postural instability.
-
Bradley and Daroff's Neurology in Clinical Practice, p. 1078; Localization in Clinical Neurology 8e, p. 968
False Localization
Ataxia is often caused by lesions outside the cerebellum involving its input/output pathways. Lesions of the cerebellar peduncles or pons, hydrocephalus affecting frontopontine pathways, and prefrontal cortex lesions can all produce gait ataxia resembling a cerebellar syndrome.
- Neuroanatomy through Clinical Cases 3e, p. 2312
Recent evidence (2024-2025): A 2024 review on ataxia-telangiectasia (PMID 39622612) and a 2025 Continuum review on ataxia broadly (PMID 40748127) reflect updated understanding and classification - these are consistent with the textbook content above.
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