Neuronopathies

Neuronopathy means the cell body is affected rather than the myelin or axon as in peripheral neuropathy. This article is focused on the sensory neuronopathies where there is primary degeneration of the dorsal root ganglion and trigeminal ganglion sensory neurons.

Classification

The classification of neuropathic disorders is as follows

• Neuronopathies (pure sensory or pure motor or autonomic)
  • Sensory neuronopathies (ganglionopathies)
  • Motor neuronopathies (motor neuron disease)
  • Autonomic neuropathies
• Peripheral neuropathies (usually sensorimotor)
  • Myelinopathies
  • Axonopathies
    • Large- and small-fibre
    • Small-fibre

Clinical Features

The sensory neuronopathies classically present with early onset sensory ataxia, potentially secondary to proximal muscle spindle and joint denervation. Note, sensory ataxia can also be caused by large fibre neuropathy and pathology localised to the dorsal column.

If the small and medium sized nerves are also involved then the patient may experience neuropathic pain (positive sensory symptoms).

This leads to the other classic feature being that the negative symptoms (numbness) and positive symptoms often present with a non-length-dependent and multifocal pattern. This differentiates them from typical length dependent axonal sensory polyneuropathies. Misdiagnosis can occur if the examiner is testing sensation from distal to proximal and stops at the presumed sensory level, thereby missing the multifocal presentation.

Vestibular dysfunction can occur, such as in CANVAS syndrome .

Power is usually normal, but patients may appear weak due to lack of proprioceptive feedback.

The muscle stretch reflexes are often unelicitable.

Aetiology

• Immune mediated disease
• Paraneoplastic disease
• Viral infections
• Vitamin B6
• Intoxication
• Neurotoxic drugs
• Hereditary and degenerative disorders: hereditary sensory autonomic neuropathy (HSAN), Charcot-Marie-tooth disease type 2B (CMT2B), sensory ataxia neuropathy with dysarthria and ophthalmoparesis (SANDO), facial-onset sensory motor neuronopathy (FOSMN), and Cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS)
• Idiopathic

Investigations

Diagnostic algorithm from 2016 Continuum article