Hypotonia is defined as abnormally low resting muscle tone. It primarily manifests as decreased resistance to passive movement and abnormalities in resting posture. Hypotonia most commonly presents in infancy (often called “floppy babies”), although low tone may present acutely with a variety of illnesses at any age.

The first task when confronted with a patient with possible hypotonia is to differentiate decreased tone from weakness or lethargy, although some infants present with both problems simultaneously. Likewise, hyporeflexia is a separate finding with a distinct differential diagnosis.

It is critically important to localize the process within the nervous system—central hypotonia refers to suprasegmental disorders affecting brain, brainstem, and cervical spinal junction; lower motor neuron diseases affect anterior horn cell, peripheral nerve, neuromuscular junction, and muscle. A careful history and physical examination, including a thorough neurological examination, frequently narrows the differential diagnosis enough to necessitate only confirmatory diagnostic tests, sparing the patient a shotgun approach including unnecessary and often painful, costly, or invasive procedures.

Most “floppy babies” in the newborn period do not have hypotonia of a neurological etiology but are weak and/or lethargic from other causes, such as infection or dehydration. Likewise, older children with focal “decreased tone” can actually be manifesting orthopedic problems or joint laxity. The first step in considering the differential diagnosis of hypotonia for any patient, therefore, is to consider the possibility of nonneurological illness.

Once careful history and physical examination confirm that the problem is truly hypotonia, the next step is to determine localization in the neuraxis (see later). Central hypotonia is caused by problems in the central nervous system; peripheral hypotonia is related to disturbance somewhere at the level of the spinal cord, nerve, neuromuscular junction, or muscle. There are myriad causes of hypotonia, and a full differential is often beyond the scope of the general pediatrician. Distinguishing central from peripheral hypotonia, however, is usually easily accomplished during the initial evaluation and can help guide appropriate evaluation and referral (see Table 44-1).

The differential diagnosis differs extensively between infants and children >1 year of age; therefore, they are divided accordingly here. Selected diagnoses are described here, but the full differential is extensive and beyond the scope of this book.