Electromyography: Beyond positive diagnosis of myastenic syndrome

Abstract

Objectives Electromyography (EMG) plays a major role to confirm the presence of neuromuscular transmission disorders. Furthermore, it helps to recognize the presynaptic or postsynaptic nature of the disorder. Indeed, specialized neurophysiological tests and characteristic electrophysiological findings are widely useful for etiological diagnosis [1] , [2] . Results Here we report two cases, a 36 years-old woman and 37 years-old men who presented to our department for facial muscle weakness. The female presented for drooping of both eyelids with diplopia, which appears while reading. Neurological examination showed bilateral asymmetric ptosis and complete ophtalmoplegia. Pupillary light reflex was normal as well as the rest of the neurological examination. The neostigmine test was positive. Repetitive nerve stimulation (RNS) testat 3 Hz stimulation, showed a significant decremental response in both area and amplitude with repetitive compound muscle action potentials (R-CMAP). Muscle biopsy eliminated centro-nuclear myopathy. We then considered the diagnosis of postsynaptic congenital myasthenic syndrome. The man presented with a ten days history of facial muscle weakness, dysphonia and swallowing disorders without diurnal fluctuation. Neurological examination noted symmetric proximal limb weakness. Clinical picture was concordant with acute inflammatory demyelinating polyradiculoneuropathy. Lumbar puncture did not evidence albumino-cytologic dissociation. Electrodiagnostic studies demonstrated a reduction in the initial CMAP amplitude, a decremental response to slow stimulation rates (3 Hz), followed by incremental response after a tetanic stimulation consistant with Lambert Eaton Syndrome. Conclusion In general, full anamnestic data and careful clinical examination are sufficient to make the diagnosis of myastenic syndrome. Nevertheless, the lack of diurnal fluctuation and a clear pattern of muscle distribution can sometimes lead to misdiagnosing this disorder [2] , [3] . Moreover, myastenic syndrome with predominant facial muscle involvement share overlapping symptoms with other muscle disease like mitochondrial and centro-nuclear myopathy. These findings delineate the contribution of EMG to differential and etiologic diagnosis [1] .