An infant presents with hypotonia and hyporeflexia. During his intrauterine period there was polyhydramnios and decreased fetal movements. Most probable diagnosis is -

Ans. is 'a' i.e., Spinal muscular atrophy o Out of the given options only spinal muscular atrophy is able to cause hypotonia and hyporeflexia during infancy. Rest of them either present late in childhood or do not give the symptoms. o Polyhydromnios and decreased fetal movements can occur in any neuromuscular disorder which present during intrauterine life. o Lets see each option one by one. Spinal muscular atrophy (SMA) SMA is a degenerative disease of motor neurons that begin in fetal life and continue to progress in infancy and childhood. o The cardinal features are: Hypotonia Generalized weakness Absent tendon stretch refluxes Involvement of tongue face and jaw muscle but sparing of extraocular muscle and sphincters. Congenital mytonia o Congenital myotonia or myotonia congenita is a chloride channel defect of skeletal muscle. o There is weakness and generalized hyperophy of muscles. o Its ruled out as Presents in early childhood (2-3 years age) Deep tendon reflexes are preserved. Congenital myaesthenia o Its a disease of neuromuscular junction similar to myasthenia gravis except that its not autoimmune but rather due to genetic mutations. o It presents in infancy and childhood with symptom similar to myasthenia gravis. o It can be ruled out as? o It does not present in the intrauterine life (so does not cause polyhydramnios). DTRs are preserved Muscular dystrophies o Muscular dystrophy refers to a group of hereditary progressive diseases. o It includes Duchenes muscular dystrophy Limb-girdle Congenital myopathies Becker Myotonic dystrophica o All of these except congenital myopathies are easily ruled out as they present in childhood age. some may rarely present in infancy but never in the intrauterine life. o Congenital myopathies are the most difficult one to rule out. They present with hypotonia in infancy. It may also present in intrauterine life. o Only reason I'm with 'Spinal muscular atrophy' is that congenital myopathy is a very rare disease.