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The Floppy Infant

Evaluation of Hypotonia

Dawn E. Peredo, MD^*
Mark C. Hannibal, MD, PhD

^* Developmental Pediatrics, Madigan Army Medical Center, Tacoma, Wash.; Division of Developmental Medicine, Department of Pediatrics, University of Washington School of Medicine, Seattle, Wash
Division of Genetic Medicine, Department of Pediatrics, University of Washington School of Medicine; Seattle Children's Hospital, Seattle, Wash

The first 300 words of the full text of this article appear below.

	Objectives

 
After completing this article, readers should be able to:

1. Characterize the distinguishing features of hypotonia and muscle weakness.
   
2. Describe the differences between central and peripheral causes of hypotonia.
   
3. Generate a differential diagnosis of hypotonia in infants.
   
4. Discuss the appropriate medical and genetic evaluation of hypotonia in infants.
   
5. Understand the need to suspect infant botulism in an infant younger than 6 months of age who has signs and symptoms such as constipation, listlessness, poor feeding, weak cry, a decreased gag reflex, and hypotonia.
   

	Introduction

 
The "floppy infant" represents a diagnostic challenge to general pediatricians. Infants can present with hypotonia that is due to central or peripheral nervous system abnormalities, myopathies, genetic disorders, endocrinopathies, metabolic diseases, and acute or chronic illness (Table 1). A systematic approach to a child who has hypotonia, paying attention to the history and clinical examination, is paramount in localizing the problem to a specific region of the nervous system.

Table 1. Differential Diagnosis of Neuromuscular Disorders Presenting in Newborns

Anterior Horn Cell Disorders Acute infantile spinal muscular atrophy Traumatic myelopathy Hypoxic-ischemic myelopathy Neurogenic arthrogryposis Infantile neuronal degeneration Congenital Motor or Sensory Neuropathies Hypomyelinating neuropathy Congenital hypomyelinating neuropathy Charcot-Marie-Tooth disease Dejerine-Sottas disease Hereditary sensory and autonomic neuropathy Neuromuscular Junction Disorders Transient acquired neonatal myasthenia Congenital myasthenia Magnesium toxicity Aminoglycoside toxicity Infantile botulism Congenital Myopathies Nemaline myopathy Central core disease Myotubular myopathy Congenital fiber type disproportion myopathy Multicore myopathy Muscular Dystrophies Dystrophinopathies Congenital muscular dystrophy with merosin deficiency Congenital muscular dystrophy without merosin deficiency Congenital muscular dystrophy with brain malformations or intellectual disability Walker-Warburg disease Muscle-eye-brain disease Fukuyama disease Congenital muscular dystrophy with cerebellar atrophy/hypoplasia Congenital muscular dystrophy with occipital argyria Early infantile facioscapulohumeral dystrophy Congenital myotonic dystrophy Metabolic and Multisystem Diseases Disorders of glycogen metabolism Acid maltase deficiency Severe neonatal phosphofructokinase deficiency Severe neonatal phosphorylase deficiency Debrancher deficiency Primary carnitine deficiency Peroxisomal disorders Neonatal adrenoleukodystrophy Cerebrohepatorenal syndrome (Zellweger) Disorders of creatine metabolism Mitochondrial myopathies Cytochrome-c oxidase deficiency

It is important to distinguish weakness from hypotonia. Hypotonia is described as reduced resistance to passive range of motion in joints; weakness is reduction in the maximum power that can be generated. A more useful definition of hypotonia is an impairment of the ability to sustain postural control and movement against gravity. Thus, floppy infants exhibit poor control of movement, delayed motor skills, and hypotonic motor movement patterns. The abnormal motor patterns include alterations in postural control, increased range of motion of joints, and abnormal stability and movement mechanics. Weak infants always have hypotonia, but hypotonia may exist without weakness.

Because dysfunction at any level of the nervous system can cause hypotonia, the differential diagnosis is extensive. Central causes, both acute and chronic, are more common than are peripheral disorders. Central conditions include hypoxic-ischemic encephalopathy, other encephalopathies, brain insult, intracranial . . . [Full Text of this Article]

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