Ataxia

Definition and Core Concepts

Ataxia (or dystaxia) refers to a profound lack of coordination of voluntary motor acts, severely impairing their smooth performance. The deficit may affect the rate, range, timing, direction, and force of a movement. While the term "ataxia" is most frequently used in clinical practice to refer to a primary cerebellar problem, other distinct forms such as sensory ataxia, optic ataxia, and frontal ataxia are also well-described. Therefore, it is clinically best to qualify the type of ataxia rather than using the word in isolation.

Clinical Subtypes of Ataxia

Depending on the underlying neuroanatomical lesion, ataxia can present in several distinct clinical forms:

• Cerebellar Ataxia: Defective timing of agonist and antagonist muscle contraction (asynergia) produces jerking, staggering, and inaccurate movements (decomposition of movement). It may manifest as an intention tremor, dysmetria (past-pointing), dysdiadochokinesia, ataxic dysarthria (sometimes known as scanning speech), excessive rebound phenomenon, macrographia, head tremor (titubation), gait ataxia, and abnormal eye movements (e.g., nystagmus, square-wave jerks, saccadic intrusions). There may be concurrent limb hypotonia. Cerebellar hemisphere lesions cause ipsilateral limb ataxia (hemiataxia), whereas midline cerebellar lesions involving the vermis produce selective truncal and gait ataxia.
• Sensory Ataxia: Results from impaired proprioception. This may be seen in diseases of the dorsal (posterior) columns of the spinal cord ("spinal ataxia"), sensory neuropathies, and neuronopathies affecting the dorsal root ganglia. Unlike cerebellar ataxia, sensory ataxia is markedly exacerbated by the removal of visual cues (demonstrated by a positive Romberg’s sign) and may lead to pseudoathetosis.
• Optic Ataxia: Characterized by misreaching for visually presented targets, with dysmetria. This is typically due to a parieto-occipital lesion and is a key component of Balint’s syndrome.
• Frontal Ataxia: Often clinically similar to, and sometimes indistinguishable from, cerebellar ataxia, but results from lesions of the contralateral frontal cortex or frontopontine fibers (frequently from tumors invading the frontal lobe or corpus callosum). These fibers run in the corticopontocerebellar tract, synapsing in the pons before passing through the middle cerebellar peduncle to the contralateral cerebellar hemisphere.

Triple ataxia refers to the rare concurrence of cerebellar, sensory, and optic types of ataxia, which may be associated with an alien limb phenomenon (sensory type).

Causes of Cerebellar Ataxia

There is a vast differential diagnosis for cerebellar ataxia, which can be broadly divided into inherited and acquired etiologies:

• Inherited Causes:
  • Autosomal recessive: Friedreich’s ataxia.
  • Autosomal dominant: Clinically known as ADCA types I, II, and III, now genetically reclassified as the spinocerebellar ataxias (SCAs), with types 1-25 and beyond described.
  • Episodic ataxias: Channelopathies involving potassium (type 1) and calcium (type 2) channels.
  • Other: Mitochondrial disorders, Huntington’s disease, Dentatorubropallidoluysian atrophy (DRPLA), and inherited prion diseases (especially Gerstmann-Sträussler-Scheinker (GSS) syndrome).
• Acquired Causes:
  • Cerebrovascular events: Infarction or hemorrhage (usually causing hemiataxia); postanoxic cerebellar ataxia.
  • Inflammatory/Demyelinating: Multiple sclerosis, Miller Fisher variant of Guillain-Barré syndrome, central pontine myelinolysis.
  • Inflammatory/Infectious: Cerebellitis associated with Epstein-Barr virus; encephalitis with Mycoplasma; HIV.
  • Neoplasia: Primary or metastatic tumors, and paraneoplastic syndromes.
  • Neurodegeneration: Multiple system atrophy cerebellar variant (MSA-C); prion diseases (Brownell-Oppenheimer variant of sporadic Creutzfeldt-Jakob disease, kuru); idiopathic late-onset cerebellar ataxia.
  • Drugs/Toxins: Alcohol abuse, anticonvulsants (e.g., phenytoin toxicity).
  • Metabolic/Nutritional: Vitamin E deficiency, thiamine deficiency (Wernicke’s encephalopathy), gluten ataxia, and hypothyroidism (though this remains debatable).

 

References

Klockgether T (ed.). Handbook of ataxia disorders. New York: Marcel Dekker, 2000

Wood NW, Harding AE. Ataxic disorders. In: Bradley WG, Daroff RB, Fenichel GM, Marsden CD (eds.). Neurology in clinical practice: principles of diagnosis and management (3rd edition). Boston: Butterworth-Heinemann, 2000 309-317

 

Cross References

Alien hand, Alien limb; Asynergia; Balint’s syndrome; Cerebellar syndromes; Dysarthria; Dysdiadochokinesia; Dysmetria; Head tremor; Hemiataxia; Hypotonia, Hypotonus; Macrographia; Nystagmus; Optic ataxia; Proprioception; Pseudoathetosis; Rebound phenomenon; Rombergism, Romberg’s sign; Saccadic intrusion, Saccadic pursuit; Scanning speech; Square-wave jerks; Tandem walking; Tremor