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Título

WHEN OCCAM´S RAZOR ISN´T SHARP: ASSOCIATION BETWEEN PROGRESSIVE MUSCULAR ATROPHY, PIRIDOXIN NEUROTOXICITY AND LATE ONSET MYASTHENIA GRAVIS

RESUMO

Case Report. A 71 years old male, Swiss, started recurrent episodes of cramps on both feet in the last 3 years, progressing to the hands in the next several months. The symptoms progressed insidiously to distal weakness first in the right foot, then to the left one, within 12 months, causing stumbling when walking. He started complex B vitamins supplementation by a neurologist prescription. He brought an electromyography (EMG) with signs of acute and chronic axonal cervical and lumbosacral motor degeneration. There was also a sensory axonal polyneuropathy (SAP) in the four limbs. He took gabapentin for chronic low back pain due to L5 herniated vertebral disc. He was told that he had motor neuron disease spectrum (MNDE) after extensive complementary exams, so he asked for a second opinion. Positive neurological findings on examination at this moment were: nasal voice, Romberg sign, deltoid myofasciculations (https://www.youtube.com/shorts/KSv9yZS4y0Y), absent Achilles reflex, hypoesteshia on the lateral aspect of the right leg with ipsilateral weakness on hallux dorsiflexion. There were no signs of upper motor neuron (UMN) disease. Curiously, patient state fatigue during the physical examination. Due to muscular fatigability, screening dosage of nicotinic acetylcholine (AChR)-binding antibody was requested and showed a title of 0,56 mmol/L (positive when greater than 0,40). Piridoxin seric levels were high (154,8 µg/L, normal range between 2,8-74,6 µg/L), but lowered spontaneously to 68,6 µg/L when vitamin B6 supplementation was stopped. Patient started prednisone 40mg/dia and pyridostigmine 360mg/dia. Discussion. The association between myasthenia gravis (MG) and amyotrophic lateral sclerosis (ALS) is strong, but its association with the lower motor neuron (LMN) sporadic phenotype (Progressive Muscular Atrophy, PMA) is unknown. PMA is a controversial entity and represents 3% of all cases of MNDE. One third of patients with PMA develop UMN features within 2 years during disease course. The patient had not palpebral ptosis or diplopia, but fatigability and nasal speech pointed to MG and not a bulbar UMN sign. Sensory ataxia due to B6 toxicity was a bias in the context of interpretation of EMG, since sensory nerve action potentials (SNAP) are normal in MND. Final Comments. Neurologists should be aware since muscular weakness may be either the common final path symptom or an coexisting condition in both neuromuscular junction disease and motor neuron disease.