What are the causes of truncal ataxia?

Causes of Truncal Ataxia

Truncal ataxia is primarily caused by pathology affecting the cerebellar vermis, with various etiologies including structural, inflammatory, toxic, genetic, and vascular conditions. 1, 2

Pathophysiological Basis

Truncal ataxia specifically refers to instability of the trunk and is characterized by:

• Difficulty sitting or standing without support
• Backward falls due to postural instability
• Wide-based gait with swaying

The cerebellar vermis, located in the midline of the cerebellum, is the primary structure responsible for trunk and postural stability. Damage to this area results in the characteristic truncal ataxia presentation. 2

Major Causes of Truncal Ataxia

1. Structural Lesions

• Posterior fossa tumors affecting the vermis
• Chiari malformations with compression of cerebellar structures
• Congenital malformations including:
  • Cerebellar hypoplasia or agenesis
  • Joubert syndrome
  • Dandy-Walker malformation 2

2. Inflammatory/Immune-Mediated Conditions

• Acute cerebellitis (viral or post-infectious)
• Multiple sclerosis with cerebellar plaques
• Acute disseminated encephalomyelitis (ADEM)
• Paraneoplastic cerebellar degeneration 2, 3

3. Vascular Causes

• Cerebellar stroke (especially affecting the territory of the superior cerebellar artery)
• Vertebrobasilar insufficiency
• Cerebellar hemorrhage 2, 4

4. Genetic/Hereditary Disorders

• Spinocerebellar ataxias (particularly those affecting the vermis)
• Friedreich's ataxia (can present with prominent truncal ataxia)
• Ataxia-telangiectasia
• Very late-onset Friedreich's ataxia (can mimic multiple system atrophy cerebellar type) 2, 5

5. Toxic/Metabolic Causes

• Alcohol-related cerebellar degeneration (particularly affects the anterior vermis)
• Anticonvulsant toxicity (phenytoin, carbamazepine)
• Vitamin deficiencies:
  • Vitamin E deficiency
  • Thiamine deficiency (Wernicke's encephalopathy)
  • Vitamin B12 deficiency 2, 6

6. Other Causes

• Multiple system atrophy (cerebellar type) - often presents with prominent truncal ataxia
• Drug-induced ataxia (certain chemotherapeutic agents, sedatives)
• Post-infectious cerebellar ataxia 2, 4

Clinical Pearls and Diagnostic Approach

1. Distinguish from other ataxias:

   • Truncal ataxia (vermian pathology) vs. limb ataxia (cerebellar hemispheric pathology)
   • Sensory ataxia (positive Romberg test) vs. cerebellar ataxia (negative Romberg test) 2

2. Key clinical features to assess:

   • Presence of nystagmus (may be absent in some central causes of vertigo with truncal ataxia)
   • Associated neurological signs (dysarthria, dysmetria, dysdiadochokinesia)
   • Time course (acute, subacute, chronic, progressive) 4

3. Imaging considerations:

   • MRI is superior to CT for visualizing the posterior fossa
   • Non-contrast brain MRI is the preferred initial imaging modality
   • Conventional neuroimaging may be normal in early disease 2

Important Caveats

• Truncal ataxia without nystagmus can occur in central causes of vertigo, making diagnosis challenging. In these cases, standard oculomotor assessments like HINTS may not identify the central cause. 4

• Pure cerebellar ataxia is rarely observed; most ataxia syndromes involve extracerebellar neurological pathology as well. 2

• The diagnosis of ataxia often requires a combination of clinical history, neurological examination, imaging, laboratory tests, and sometimes genetic testing. 2, 6

• Patterns of parenchymal injury can suggest specific subtypes of ataxia, but there is significant overlap of imaging features between various etiologies. 2