Hypotonia



Hypotonia


Lawrence W. Brown



INTRODUCTION

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.


DIFFERENTIAL DIAGNOSIS LIST


In Infants Central Hypotonia



  • Infection (e.g., sepsis, meningitis, encephalitis)


  • Genetic abnormalities (e.g., Down syndrome, Prader-Willi syndrome, fragile X)


  • Metabolic disease (e.g., leukodystrophy, glycogen storage disease)


  • Trauma (e.g., birth trauma, nonaccidental trauma)


  • Hypoxic ischemic encephalopathy (e.g., decreased perinatal perfusion)


  • Vascular (e.g., stroke, intracranial hemorrhage)


  • Anatomic (e.g., congenital brain malformations)


  • Drug/toxin (e.g., narcotics)


  • Other causes (e.g., benign congenital hypotonia, hypothyroidism)


Peripheral Hypotonia



  • Spinal muscular atrophy (SMA)


  • Nerve injury (e.g., brachial plexus injury)


  • Myasthenia gravis



  • Medications/toxins (e.g., botulinum toxin, hypermagnesemia)


  • Congenital myopathies


  • Congenital myotonic dystrophy


In Children Central Hypotonia



  • Infection (e.g., sepsis, encephalitis)


  • Metabolic disease (e.g., adrenoleukodystrophy, glycogen storage disease)


  • Neoplasm (e.g., brain tumor, infiltrative neoplasm)


  • Vascular (e.g., stroke)


  • Toxins/electrolyte abnormalities (e.g., ingestion)


Peripheral Hypotonia



  • Spinal muscular atrophy


  • Infection (e.g., myositis)


  • Trauma (e.g., nerve injury)


  • Neuropathy (e.g., Charcot-Marie-Tooth disease)


  • Guillain-Barré syndrome


  • Myasthenia gravis


  • Muscular dystrophy


  • Dermatomyositis


  • Tick paralysis


DIFFERENTIAL DIAGNOSIS DISCUSSION

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.









TABLE 44-1 Features of Central Versus Peripheral Hypotonia
































Central Hypotonia


Peripheral Hypotonia


History


Encephalopathy


Joint contractures



Hypoxic ischemic injury


Normal mental status



Seizures


No seizures


Examination


No weakness


Muscle weakness



Dysmorphic features


Sensory deficits



Normal or brisk reflexes


Diminished or absent reflexes



INFANTS


Central Hypotonia


Etiology and Clinical Features

Central hypotonia is caused by problems in the central nervous system (i.e., the brain or upper motor neurons in spinal cord). Tone abnormalities are global, involving the whole body, and often accompanied by encephalopathy, or altered mental status. Seizures and sleep/wake cycle disturbances also may suggest a central problem. Babies with central hypotonia have low resting tone but usually have preserved strength when they do make effort with their limbs. The numerous causes of central hypotonia may be grouped into larger categories.



  • Infection. Consider sepsis, meningitis, encephalitis, and toxoplasmosis, other, rubella, cytomegalovirus, herpes simplex (TORCH) infections.


  • Genetic abnormalities. A number of genetic syndromes include hypotonia as a presenting feature in the neonatal period, even before dysmorphic features or other affected organs might be apparent. There are dozens of definable syndromes, and more are being identified with the availability of genome-wide microarrays that can show novel or familial disease-causing copy number variants.

Important and common syndromes include:

Sep 14, 2016 | Posted by in PEDIATRICS | Comments Off on Hypotonia

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