Axonal pattern polyneuropathy due to vitamin b12 deficiency: a case report

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Abstract

Case Presentation: A 10-year-old female patient, overweight, with a history of ankle pain and paresthesia that progressed in weeks to lower limb weakness, followed by loss of sphincter control and motor ataxia. The mother reported a diet poor in meat and vegetables and an association of symptom onset with flu-like syndrome and gastroenteritis. Examination revealed flag sign in hair, grade I muscular strength in lower limbs, grade 1 patellar reflex, grade 3 Achilles reflex with clonus, hyperalgesia in all four extremities, preserved thermal sensitivity but absent vibratory and kinesthetic-postural sensitivity in both lower limbs, dysmetria in the index finger to nose test, and rebound horizontal nystagmus, stiffness in the neck and absence of Brudzinski and Kernig signs. Cerebrospinal fluid analysis was normal. Electroneuromyography: axonal pattern polyneuropathy with marked impairment in the sensory component and moderate to severe impairment in the motor component in the lower limbs. Laboratory: pancytopenia associated with reduced vitamin B12 and increased homocysteine levels. Head MRI: Diffusion restriction in the radiate crowns of the brain suggesting an inflammatory or demyelinating etiology. Thoracic and lumbar spine MRI: T2 hyperintensity in the posterior and median central portion of all segments of the thoracic spinal cord to the conus medullaris at the lumbar level. IM vitamin B12 replacement was indicated at a dose of 1000 mcg/day for 7 days. The patient's lower limb strength improved, but vibratory sensitivity remained absent. She was discharged from the hospital to receive weekly IM vitamin B12 replacement for 2 months and outpatient follow-up.

Discussion: Polyneuropathy can occur due to myelin dysfunction, compromise of the vasa nervorum or axonopathy. The main cause of demyelinating disorders is a parainfectious autoimmune process. In vasa nervorum involvement, ischemia due to atherosclerosis, vasculitis or hypercoagulable states can be the main cause. Axonal pattern polyneuropathy is usually due to toxic-metabolic causes, like diabetes mellitus, chronic renal failure, adverse effects of chemotherapy and nutritional deficiencies, including vitamin B12 deficiency.

Final Comments: Low levels of vitamin B12 can lead to an accumulation of methylmalonic acid, which leads to degeneration of the lateral and posterior portions of the spinal cord. This may explain the patient's neurological findings.