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.