The Diagnostic Approach to the Hypotonic and Weak Infant

Hypotonia in infancy is a common clinical scenario that is encountered by a pediatric neurologist. This presentation can be nonspecific without other findings, often making the diagnosis challenging. A systematic diagnostic approach is important to localization, beginning with distinguishing between central and peripheral causes of hypotonia. This chapter describes how to apply the clinical evaluation and ancillary testing including electrodiagnostic, biochemical, genetic, neuroimaging, and histopathological studies to distinguish between central and peripheral hypotonia, then differentiate between neuropathic and myopathic disorders, and then finally arrive at an underlying etiology. This chapter describes the most common disorders associated with hypotonia in infancy including spinal muscular atrophy (SMA), neuropathy, and various forms of myopathic disorders such as congenital muscular dystrophies (CMDs) and congenital myopathies. In addition, we describe the management and treatment approach for infants with neuropathic and myopathic disorders. The availability of three FDA-approved therapies for SMA is described, making an early diagnosis of this and other disorders a high priority.