What are the causes of pure sensory neuropathy?

Causes of Pure Sensory Neuropathy

Pure sensory neuropathy results from a limited spectrum of etiologies that primarily affect dorsal root ganglia (sensory neuronopathy/ganglionopathy) or selectively damage sensory nerve fibers, with the most common causes being paraneoplastic syndromes, Sjögren syndrome, vitamin B6 toxicity, hereditary conditions, and idiopathic disease.

Key Distinguishing Features

Pure sensory neuropathy presents with distinctive clinical characteristics that differentiate it from typical length-dependent polyneuropathies:

• Non-length-dependent distribution: Symptoms often begin in the arms or occur asymmetrically 1, 2
• Predominant sensory ataxia: Severe proprioceptive loss with preserved motor function 1, 3
• Subacute onset: Particularly in acquired forms 2
• Female predominance: Occurs more commonly in women 1

Primary Etiologies

Autoimmune/Inflammatory Causes

Sjögren syndrome is a critical diagnosis to consider, as it frequently causes pure sensory neuropathy through lymphocytic infiltration of dorsal root ganglia 3. Key features include:

• Chronic asymmetrical sensory deficit
• Initial hand involvement with predominant vibratory and joint position sense loss
• Association with Adie's pupil syndrome or trigeminal sensory neuropathy
• The neuropathy may precede sicca symptoms in 80% of cases 3
• Ro antibodies positive in most cases 3

Paraneoplastic Syndromes

Paraneoplastic sensory neuronopathy represents an oncologic emergency requiring immediate investigation 1, 2. This should be suspected particularly in patients with:

• Subacute onset
• Severe sensory ataxia
• Anti-Hu or other paraneoplastic antibodies

Toxic Causes

Pyridoxine (vitamin B6) toxicity causes a distinctive pure sensory neuropathy 4:

• Doses as low as 200 mg/day can cause neuropathy
• Duration before symptoms is inversely proportional to daily intake
• Presents as pure sensory central-peripheral distal axonopathy
• Improvement follows discontinuation in all cases with adequate follow-up 4

Metabolic/Nutritional

While the diabetes guidelines 5, 6, 5 extensively discuss diabetic peripheral neuropathy, diabetes typically causes sensorimotor neuropathy, not pure sensory neuropathy. However, when evaluating any neuropathy including pure sensory presentations, screen for:

• Vitamin B12 deficiency: Test serum B12 with metabolites (methylmalonic acid and homocysteine), as 44% of B12-deficient neuropathy patients have normal B12 levels but abnormal metabolites 7, 8
• Impaired glucose tolerance: Present in 25-36% of patients with idiopathic sensory neuropathy 7
• Hypothyroidism 5, 6
• Renal disease 5, 6

Monoclonal Gammopathies

Serum protein immunofixation electrophoresis (IFE) detects monoclonal proteins in 10% of patients with otherwise unexplained neuropathy 7, 8. IFE is superior to standard serum protein electrophoresis, detecting 30% of IgM gammopathies missed by routine testing 7.

Hereditary Causes

• Hereditary sensory and autonomic neuropathies
• Hexosaminidase A deficiency (GM2 gangliosidosis) 9
• These typically present with slowly progressive symptoms and additional neurological features 2

Infectious

• HIV infection 5, 6
• Other chronic infections

Idiopathic

Nearly 50% of pure sensory neuropathies remain idiopathic after comprehensive evaluation 1. These patients typically have:

• Stable or slowly progressive course in 75% of cases
• Poor functional prognosis due to intractable pain and ataxia
• Resistance to most treatments 1

Recommended Diagnostic Algorithm

Initial screening tests with highest yield 7, 8:

1. Blood glucose or glucose tolerance testing
2. Serum B12 with metabolites (methylmalonic acid ± homocysteine)
3. Serum protein immunofixation electrophoresis

Second-tier investigations for pure sensory presentations 6, 2, 3:

• Sjögren antibodies (anti-Ro/SSA, anti-La/SSB) with Schirmer test and salivary gland biopsy
• Paraneoplastic antibody panel (anti-Hu, anti-CV2, others)
• Detailed medication history (chemotherapy agents, pyridoxine supplementation)
• Thyroid function tests
• Complete blood count and metabolic panel
• Ganglioside antibodies if MGUS present

Neurophysiological testing: Electrophysiology shows simultaneous impairment of central and peripheral evoked cortical potentials, suggesting neuronal cell body lesions 3. This distinguishes sensory neuronopathy from distal axonopathy.

Critical Clinical Pitfalls

• Do not dismiss the diagnosis of Sjögren syndrome based on absence of sicca symptoms—the neuropathy precedes sicca complaints in most cases 3
• Do not overlook pyridoxine supplementation—patients may not volunteer this information, and "low-dose" supplements (200 mg) can cause neuropathy 4
• Do not assume diabetes causes pure sensory neuropathy—diabetic neuropathy is typically sensorimotor 5, 6, 8
• Do not use standard serum protein electrophoresis alone—immunofixation is required to detect small monoclonal proteins 7