[The role of electrodiagnostic studies in the diagnosis of hypotonia in infancy].

Increased muscle extensibility and passivity characterize infantile hypotonia. It may reveal a peripheral neuromuscular disease as well as a disorder of the central nervous system. Electrodiagnostic studies in newborn and young infants are useful to guide the indication of other complementary investigations. Signs of denervation on needle electromyography strongly suggest infantile spinal muscular atrophy. Electrodiagnostic findings can distinguish rare conditions mimicking spinal muscular atrophy that are obstetrical tetraplegia and severe congenital neuropathies. Nerve conduction velocities are severely slowed in hereditary sensorimotor neuropathies and neurodegenerative disorders. Myopathic changes on needle electromyography are associated with congenital muscular dystrophies and structural or metabolic congenital myopathies. In congenital myotonic dystrophy, myotonic discharges can be recorded in the infant as well as in his/her mother. Myopathic changes may also be detected in collagen disorders, in cases of muscular atrophy secondary to hypomotility or malnutrition, and in patients with congenital myasthenic syndrome. Repetitive nerve stimulations are required to characterize myasthenic syndromes. Finally, normal results of electrodiagnostic studies constitute a relevant information that moves diagnostic procedures to search for central nervous system disorders. Benign congenital hypotonia is a quite rare condition that is diagnosed retrospectively, when hypotonia is strictly isolated and recovers completely before 2 years of age.