Dr. Chin of the Kennedy Krieger Institute have no relevant financial relationships to disclose.)
Dr. Johnston of Johns Hopkins University School of Medicine has no relevant financial relationships to disclose.)
Hypotonia in childhood may occur secondary to disorders affecting any point in the central and/or peripheral motor nervous system. Central hypotonia implies a localization above the level of the lower motor neuron. Hypoxic-ischemic encephalopathy is the predominant attributed etiology for congenital hypotonia, but the differential diagnosis is broad and encompasses over 500 identified genetic disorders. A logical, stepwise approach to diagnosis is essential. For most of these entities, there is no cure, and prognosis is variable. Treatment is individualized and may include supportive therapies (such as physical and occupational therapy or symptomatic treatment) or diagnosis-specific management.
• Hypotonia is reduced tension or resistance of passive range of motion and can occur with varying degrees of weakness.
• The first step in the evaluation of a child with hypotonia is localization to the central or peripheral nervous system (or both).
• Central hypotonia is more likely to be noted axially with normal strength and hyperactive to normal deep tendon reflexes.
• Other features common in central hypotonia include dysmorphic facies, macro or microcephaly, developmental delay (global, motor, speech, or cognitive), seizures, malformations of other organs, altered level of consciousness, abnormal eye movements, abnormal breathing pattern, or other signs of central nervous system dysfunction.
• History and physical exam provide crucial diagnostic clues, though neuroimaging, genetic testing, and other laboratory evaluations are also important parts of assessment.
Historical note and terminology
Literature since the 1960s describes cerebral influences on developmental tone and motor control. The monograph by Victor Dubowitz in the late 1960s entitled “The Floppy Infant” provided a practical approach to diagnosis and classification of children with hypotonia (Dubowitz 1969). He emphasized 2 main questions when confronted with a floppy baby or child:
(1) “Is this a paralyzed child with incidental hypotonia?” and
(2) “Is this a hypotonic child without significant muscle weakness?”
His categorization of the paralytic conditions of children with hypotonia with weakness with “incidental” hypotonia was found to be most often lower motor neuron diseases: proximal spinal muscular atrophies, congenital myopathies, and other neuromuscular disorders. The infants with nonparalytic conditions who had hypotonia without significant weakness included disorders of the central nervous system, connective tissue disorders, metabolic, nutritional, and endocrine disorders, acute illness, and essential (or benign) hypotonia. This subdivision, central versus peripheral hypotonia, remains clinically useful and represents the starting point for modern diagnostic algorithms (Lisi and Cohn 2011).
Currently, neonatal central hypotonia may be recognized as a part of a wider spectrum of neonatal encephalopathy, or “altered behavior in the newborn characteristic of a disturbance in central nervous functioning” (Inder and Volpe 2018). Most neonatal hypotonia is attributed to perinatal hypoxic-ischemic encephalopathy (Darras and Volpe 2018), though the remainder of cases continue to represent a wide spectrum of disorders. Increasingly sophisticated genetic, biochemical, and imaging studies are allowing increasingly specific etiologic diagnoses in these cases (Prasad and Prasad 2011; Sparks 2015; Ahmed et al 2016).
Over time, once an individual's etiology and clinical course become clear, additional functional (rather than etiologic) diagnostic labels may be applied. Individuals with persistently disordered movement or posture secondary to early nonprogressive injury or malformation of the developing brain fall under a diagnosis of cerebral palsy. Movement disorders in cerebral palsy are typically classified as spastic, dyskinetic, ataxic/hypotonic, or mixed; persistent central hypotonia (particularly axially) is common in dyskinetic, ataxic/hypotonic, and mixed subtypes. In individuals with less motor impairment, diagnoses of developmental coordination disorder (DSM-V) or specific developmental disorder of motor function (ICD-9) may be applied. Population-based prognostic data are lacking, and it is unclear when historical labels such as “benign congenital hypotonia” may be confidently applied.
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