Ataxia

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Here is a comprehensive overview of Ataxia drawn from Bradley and Daroff's Neurology in Clinical Practice and Neuroanatomy through Clinical Cases, 3rd Edition.

Ataxia

Definition

Ataxia (from the Greek "lack of order") refers to the disordered contractions of agonist and antagonist muscles and the lack of normal coordination between movements at different joints. Even simple movements require smooth coordination of multiple joints; in ataxia, movements follow an irregular, wavering course consisting of continuous overshooting, overcorrecting, and overshooting again around the intended trajectory. The fundamental defects are:

Dysmetria - abnormal trajectories through space
Dysrhythmia - abnormal timing of movements

Types of Ataxia

1. Cerebellar Ataxia

Caused by dysfunction of the cerebellar circuitry. The cerebellum functions as a neuronal learning machine (Marr-Albus-Ito theory), integrating sensorimotor inputs to generate predictive models for smooth movement. Dysfunction leads to erroneous motor learning and improper motor predictions.

2. Truncal vs. Appendicular Ataxia

Feature	Truncal Ataxia	Appendicular Ataxia
Lesion site	Cerebellar vermis (medial)	Cerebellar hemispheres (lateral/intermediate)
Motor system affected	Medial motor system	Lateral motor system
Clinical presentation	Wide-based, unsteady "drunk-like" gait; difficulty sitting without support	Ataxia of extremity movements (dysmetria on finger-nose test)
Laterality	Bilateral deficit	Ipsilateral to the lesion

3. Sensory Ataxia

Results from disruption of the posterior column-medial lemniscal pathway, causing loss of joint position sense (proprioception). Key distinguishing features:

Impaired joint position sense on exam (unlike cerebellar ataxia)
Worsens with eyes closed (Romberg positive)
Improved significantly by visual feedback

4. Ataxia-Hemiparesis

A syndrome (often from lacunar infarcts) combining unilateral upper motor neuron signs with ataxia on the same side. Both deficits are contralateral to the lesion. Most commonly caused by lesions of the corona radiata, internal capsule, or pons.

Localization

Cerebellar hemispheric lesions → ipsilateral limb ataxia
Vermis lesions → truncal/gait ataxia (bilateral)
Cerebellar peduncle lesions → ipsilateral ataxia (even without cerebellar hemisphere involvement)
Frontopontine pathway lesions / hydrocephalus → gait abnormality resembling truncal ataxia ("false localization")

Signs and Symptoms

Early Symptoms

Difficulty running, walking on high heels, or on stairs without holding railings
Walking "as if drunk" (may be intermittent initially, triggered by even small amounts of alcohol)
Slurred speech (scanning dysarthria) - from occasional word pronunciation difficulty to persistent problems
Hand tremor and clumsiness, poor handwriting
Dizziness (vertiginous or non-vertiginous)
Diplopia (double vision), especially with quick head turns

Later Symptoms

Falls
Dysphagia (swallowing difficulty)
Blurred vision
Loss of hand dexterity for daily activities (dressing, using utensils)

Associated Neurological Signs (important diagnostic clues)

Peripheral neuropathy, parkinsonism, dystonia, tremor, sleep dysfunction, autonomic symptoms, seizures, hearing loss

Finger-Nose-Finger Test: (A) Normal individual shows straight movement path; (B) appendicular ataxia shows irregular, wavering path due to continuous dysmetria

Finger-Nose-Finger Test. (A) Normal smooth trajectory. (B) Appendicular ataxia showing irregular, overshooting, wavering trajectory.

Causes (by Time Course)

Onset	Typical Causes
Acute	Vascular (stroke, hemorrhage), infectious, toxic (alcohol, drugs, heavy metals)
Subacute	Immune-mediated (paraneoplastic, autoimmune), Wernicke's encephalopathy, vitamin B12/E deficiency
Chronic/Progressive	Genetic (hereditary), degenerative (MSA-C, ILOCA), prion disease
Episodic	Episodic ataxia types 1 & 2, metabolic causes

Key Acquired Causes

Toxic: Alcohol, medications (phenytoin, lithium, chemotherapy), heavy metals
Nutritional: Vitamin B1 (thiamine) deficiency - Wernicke's; Vitamin B12, Vitamin E deficiency
Vascular: Cerebellar stroke or hemorrhage
Immune/Paraneoplastic cerebellar degeneration (PCD): Most common paraneoplastic neurological syndrome (~24.3%); subacute onset over months; associated antibodies (anti-Yo, anti-Hu, etc.); associated with breast, ovarian, lung cancers
Structural: Tumors (vermis tumors → gait ataxia; hemispheric → limb ataxia), hydrocephalus

Key Genetic Causes

Autosomal recessive: Friedreich's ataxia (most common hereditary ataxia), ataxia-telangiectasia, POLG-ataxia, ataxia with vitamin E deficiency, abetalipoproteinemia
Autosomal dominant: Spinocerebellar ataxias (SCAs) - >40 types; SCA1, 2, 3, 6, 7 most common
Repeat expansion-associated: Must be screened before standard gene sequencing (conventional sequencing misses repeat expansions); includes SCA types and Friedreich's ataxia
X-linked: FXTAS (Fragile X-associated tremor/ataxia syndrome)
Other: CANVAS (cerebellar ataxia, neuropathy, vestibular areflexia syndrome)

Neurological Examination

Key tests:

Gait assessment - wide-based gait, tandem walking
Finger-nose-finger test - tests limb dysmetria
Heel-shin test
Rapid alternating movements - dysdiadochokinesia
Romberg test - if positive (worsens with eyes closed), suggests sensory ataxia
Eye movement examination - nystagmus, saccadic pursuit, gaze-evoked nystagmus
Speech - scanning dysarthria
SARA scale (Scale for the Assessment and Rating of Ataxia) - standardized rating

Also assess for mimickers: tremor, dystonia, myoclonus, parkinsonism, sensory neuropathy, muscle weakness, pyramidal signs.

Diagnostic Workup

Step-by-step approach (Bradley & Daroff):

Recognize signs/symptoms and associated neurological features
Determine chronicity (acute / subacute / chronic) and rate of progression
Exclude structural, nutritional, and immunological causes (MRI brain, vitamin levels, autoimmune antibody panel)
Screen for repeat expansion disorders (BEFORE gene sequencing): Friedreich's ataxia (GAA repeat in FXN), SCAs, FXTAS
Next-generation sequencing (WES/WGS) if repeat expansions excluded
Degenerative causes (MSA-C, ILOCA) considered when all else is negative, especially late onset

MRI Findings

MRI patterns in ataxia: cerebellar atrophy (A-D), FXTAS cerebellar peduncle hyperintensity (E), POLG olivary hyperintensity (F), MSA hot-cross-bun sign in pons (G), MSA putaminal rim (H), superficial siderosis (I)

Brain MRI patterns: (A-C) Cerebellar atrophy (prominent foliation/sulci) in ILOCA. (D) Axial FLAIR showing atrophy. (E) T2 hyperintensity in bilateral cerebellar peduncles - FXTAS. (F) T2 hyperintensity in inferior olivary nuclei - POLG-ataxia. (G) "Hot-cross-bun" sign in pons - MSA-cerebellar type. (H) Linear putaminal rim hyperintensity - MSA. (I) Brainstem hemosiderin on GRE - superficial siderosis.

Management

Pharmacological

Treat reversible causes aggressively (thiamine for Wernicke's, immunotherapy for paraneoplastic/autoimmune, remove offending toxins)
Symptomatic: riluzole and valproate have shown modest benefit in some cerebellar ataxias
Tremor component: weighted wrist cuffs reduce kinetic tremor amplitude (added inertia dampens oscillation)

Non-Pharmacological

Gait ataxia: mobility aids (cane, walker, wheelchair), fall prevention
Displacing center of gravity forward (raising heel height, lowering walker height) helps elderly patients with postural instability
Speech therapy for dysarthria
Occupational therapy for hand dexterity

Targeted Molecular Therapy

Emerging RNA-based and small molecule approaches for specific genetic ataxias (e.g., antisense oligonucleotides for SCA3)

Sources:

Bradley and Daroff's Neurology in Clinical Practice, Ch. 23 (Cerebellar Ataxia)
Neuroanatomy through Clinical Cases, 3rd Edition, Ch. 15 (Cerebellar localization)

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