To describe a rare presentation of muscle spasms and fasciculations associated with severe vitamin B12 deficiency and highlights the importance of early recognition to prevent irreversible neurologic injury.

Vitamin B12 deficiency typically presents with myeloneuropathy, classically presenting with paresthesia, gait disturbance, and sensory ataxia. Fasciculations are uncommon and may mimic motor neuron disease such as amyotrophic lateral sclerosis (ALS), leading to potential misdiagnosis. Early recognition and treatment are crucial, as neurological recovery depends on timely vitamin B12 replacement.

Case report

A 74-year-old woman presented with dizziness, gait imbalance, and 15-pound unintentional weight loss over one month. She reported muscle spasms in lower extremities, and a sensation of “not feeling the ground” when walking. Neurologic examination revealed normal cognition, global muscle atrophy, fasciculations in bilateral triceps, gastrocnemius, deltoid muscles, positive Romberg sign with decreased light touch, pinprick, and temperature sensation in both legs. Laboratory evaluation showed severe vitamin B12 deficiency (<159 pg/mL), elevated methylmalonic acid (0.69 µmol/L), positive parietal cell antibody (44.5 units), low zinc, and normocytic anemia. MRI of the brain and spine was unremarkable. Electromyography demonstrated chronic denervation and reinnervation with neurogenic recruitment in the right upper and bilateral lower extremities but no active denervation to support ALS. The patient was diagnosed with pernicious anemia based on labs, causing muscle spasm and fasciculations. Parenteral vitamin B12 replacement led to gradual improvement and resolution of fasciculations and muscle cramps along with some improvement in sensory symptoms.  

Severe Vitamin B12 deficiency can lead to defective myelin synthesis and axonal degeneration and rarely can present with both upper and lower motor neuron signs. Recognition of this reversible cause is critical as prompt vitamin B12 replacement can lead to significant or complete neurological recovery, especially if initiated before irreversible damage occurs.