What causes sensory ataxia?

Causes of Sensory Ataxia

Sensory ataxia is primarily caused by dysfunction of the proprioceptive pathways, which can occur at various levels including the dorsal columns of the spinal cord, dorsal root ganglia, peripheral sensory nerves, or sensory pathways to the cerebellum. 1

Pathophysiological Mechanism

Sensory ataxia results from impaired proprioception (position sense) and loss of normal position sense coordination. Unlike cerebellar ataxia, sensory ataxia is characterized by:

• Worsening with eye closure (positive Romberg test)
• Loss of coordination due to inability to sense limb position
• Preservation of cerebellar functions (unless there's concurrent cerebellar involvement)

Major Causes of Sensory Ataxia

1. Spinal Cord Disorders

• Compressive myelopathy - affecting dorsal columns
• Vitamin B12 deficiency - causing subacute combined degeneration
• Copper deficiency - mimicking B12 deficiency
• Tabes dorsalis (neurosyphilis) - affecting dorsal columns 1

2. Peripheral Nervous System Disorders

• Sensory neuropathies affecting large fibers:
  • Diabetic neuropathy
  • Toxic neuropathies (chemotherapy-induced, especially from bortezomib, thalidomide) 1
  • Immune-mediated neuropathies
  • Paraneoplastic sensory neuronopathies
  • Chronic idiopathic ataxic neuropathy 2

3. Dorsal Root Ganglion Disorders

• Sensory neuronopathies/ganglionopathies:
  • Paraneoplastic syndromes
  • Sjögren's syndrome
  • Autoimmune disorders
  • Vitamin B6 (pyridoxine) toxicity 2

4. Other Causes

• Monoclonal gammopathies and paraproteinemias 2
• Inflammatory/autoimmune disorders affecting sensory pathways
• Hereditary sensory neuropathies

Clinical Presentation and Diagnostic Features

Patients with sensory ataxia typically present with:

• Wide-based, unsteady gait that worsens in the dark or with eyes closed
• Loss of proprioception and vibratory sense
• Positive Romberg sign (inability to maintain balance with eyes closed)
• Pseudoathetosis (involuntary writhing movements when eyes are closed) 3
• Sensory loss in a stocking-and-glove distribution (in peripheral neuropathies) 1
• Reduced or absent deep tendon reflexes proportional to sensory loss 1

Diagnostic Approach

1. Distinguish sensory from cerebellar ataxia:

   • Sensory ataxia worsens with eye closure
   • Look for associated sensory deficits (proprioception, vibration)
   • Check for Romberg sign (positive in sensory ataxia)

2. Localize the lesion:

   • Central (spinal cord): Check for hyperreflexia, spasticity, and sensory level
   • Peripheral (neuropathy): Look for distal sensory loss, hyporeflexia, and weakness 1

3. Imaging studies:

   • MRI of cervical and thoracic spine without and with contrast is recommended for suspected dorsal column involvement 1
   • MRI of the brain may be needed to rule out other causes

4. Laboratory and neurophysiological testing:

   • Nerve conduction studies (typically showing low amplitude sensory action potentials) 1
   • Vitamin B12, copper levels
   • Immunological tests (for autoimmune causes)
   • Serum protein electrophoresis (for paraproteinemias) 2

Treatment Considerations

Treatment depends on identifying and addressing the underlying cause:

• Vitamin replacement for deficiencies
• Immunotherapy for autoimmune causes
• Dose modification or discontinuation of offending medications in toxic neuropathies
• Symptomatic management of neuropathic pain and sensory symptoms

Common Pitfalls

• Mistaking sensory ataxia for cerebellar ataxia, leading to incorrect diagnostic workup
• Failing to check for proprioceptive deficits in patients presenting with ataxia
• Not performing a thorough examination with eyes open and closed
• Overlooking potentially treatable causes (vitamin deficiencies, medication toxicity)
• Focusing only on brain imaging when spinal cord or peripheral nerve pathology may be responsible

Remember that sensory ataxia is a clinical sign rather than a diagnosis itself, and identifying the underlying cause is essential for appropriate management.