Vitamin B 12 is a water-soluble vitamin found naturally in meat and food of animal origin. Its physiological functions include erythropoiesis, the synthesis and maintenance of myelin sheath and the synthesis of deoxyribonucleic acid. The current recommended dietary allowance for vitamin B 12 has been set by the Institute of Medicine at 2.4 μg per day for males and females aged 14 years and above, with a serum level of vitamin B 12 of 120–180 pmol/l indicative of deficiency [6]. The mechanisms that cause deficiency are malabsorption, malnutrition or genetic deficiency of methylmalonyl-CoA mutase. Based on this case and considering malnutrition as the etiological factor, vegans who are not on vitamin B 12 supplements or fortified foods containing the vitamin are found to be at a higher risk of developing deficiency, compared to vegetarians [7].

B 12 deficiency presents with hematological, gastrointestinal and neuropsychiatric manifestations. Neuropsychiatric manifestations commonly associated with deficiency include myelopathy, neuropathy, dementia, neuropsychiatric abnormalities and rarely show optic nerve atrophy. SACD though a rare cause of myelopathy, is the most frequent clinical manifestation of B 12 deficiency [4]. The clinical signs of SACD typically include a spastic paraparesis, extensor plantar response, and impaired perception of position and vibration, and may have associated peripheral neuropathy which explains this patient’s stocking type numbness and absence of ankle jerks. Neuropathological lesions in SACD have been found to be due to overproduction of the myelinolytic Tumor necrosis factor α (TNFα) and to the reduced synthesis of the two neurotrophic agents; the Epidermal growth factor (EGF) and interleukin-6 caused by vitamin B 12 deficiency [8]. Neuropathological studies, including T2 weighted Magnetic resonance imaging (MRI), show lesions classically involving the posterior and lateral columns, predominantly in the upper thoracic and midthoracic regions, resulting in concomitant upper limb and lower limb involvement [9], in contrast to this case which has isolated lower limb involvement with normal upper limbs.

Neuropsychiatric manifestations include decreased memory, personality change, psychosis, emotional lability, and rarely delirium or coma, and may be seen in patients without haematological manifestations, or low normal B 12 levels [5]. However this patient had no such psychiatric manifestations nor anaemia. Unusual neurological manifestations of the disease are cerebellar ataxia, leukoencephalopathy, orthostatic tremors, myoclonus, ophthalmoplegia, catatonia, vocal cord paralysis, a syringomyelia like distribution of motor and sensory deficits, and autonomic dysfunction [10] which were not seen in this case.

Anemia is a common and early symptom leading to the diagnosis of vitamin B 12 deficiency, while as neurological symptoms are often considered as late manifestations, and are typically preceded by anemia [5], unlike in this rare case of vitamin B 12 deficiency with neurological manifestations in the absence of anaemia. However, haematological manifestations were found to be associated with neuropsychiatric manifestations in only 28 % in a population study [5]. Approximately two-thirds of patients with vitamin B 12 deficiency are shown to have hematological abnormalities characterized by anemia, an elevated MCV, the presence of hypersegmented neutrophils and macrocytosis on peripheral blood smears [11]. A previous study has shown 10 % cases to have life threatening haematological manifestations with 5 % having symptomatic pancytopenia [12].

Serum B 12 level assessment is the mainstay of investigation of vitamin B 12 deficiency, whereas an increase in serum levels of methylmalonic acid and homocysteine also indicate the diagnosis [13]. Electrophysiological abnormalities include nerve conduction studies suggestive of a sensorimotor axonopathy [14], as seen in this case. MRI of the cervical and dorsal spine shows increased T2 weighted signal intensity in the posterior and lateral columns of the cervical and upper thoracic spinal cord [15].