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

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. Polyhydramnios and decreased fetal movements can occur in any neuromuscular disorder which present during intrauterine life. 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. The cardinal features are: Hypotonia Generalized weakness Absent tendon stretch reflexes Involvement of tongue face and jaw muscle but sparing of extraocular muscle and sphincters. Congenital myotonia Congenital myotonia or myotonia congenita is a chloride channel defect of skeletal muscle. There are weakness and generalized hypertrophy of muscles. Its ruled out as Presents in early childhood (2-3 years age) Deep tendon reflexes are preserved. Congenital myasthenia It's a disease of neuromuscular junction similar to myasthenia gravis except that its not autoimmune but rather due to genetic mutations. It presents in infancy and childhood with symptom similar to myasthenia gravis. It can be ruled out as- It does not present in the intrauterine life (so does not cause polyhydramnios). DTRs are preserved Muscular dystrophies Muscular dystrophy refers to a group of hereditary progressive diseases. It includes Duchene's muscular dystrophy         Limb-girdle       Congenital myopathies Becker   Myotonic dystrophic 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. Congenital myopathies are the most difficult one to rule out. They present with hypotonia in infancy. It may also present in intrauterine life. The only reason I'm with 'Spinal muscular atrophy' is that congenital myopathy is a very rare disease.