Many metabolic disorders can produce hypotonia. Electrolyte abnormalities, inborn errors of metabolism, and hepatic and renal encephalopathy usually produce transient hypotonia that resolves when the primary disorder improves. They are diagnosed by the appropriate laboratory investigations. Hypotonia with decreased dynamic tone usually occurs initially. After several weeks, if permanent damage results from brain swelling, hypotonia with increased dynamic tone appears. The two most frequent metabolic disorders that produce hypotonia are hypoxic-ischemic encephalopathy and hypothyroidism.

Hypoxic-ischemic encephalopathy
Hypoxic-ischemic encephalopathy is probably the most frequent cause of hypotonia during the neonatal period. Mild hypoxic encephalopathy produces a transient generalized hypotonia with decreased dynamic tone that lasts for a few days. Severe hypoxic ischemic encephalopathy initially produces hypotonia with decreased dynamic tone followed several weeks later by hypotonia with increased dynamic tone. A hypotonic neonate with increased dynamic tone at birth did not suffer asphyxia during labor. Hypoxic-ischemic encephalopathy often involves the brain and the brainstem. Very severe hypoxic-ischemic encephalopathy (Sarnat Stage III) may involve the brain, brainstem (Figure 110.1), spinal cord, and muscle. Magnetic resonance imaging of the brain in neonates with hypotonia due to hypoxic-ischemic encephalopathy shows loss of gray-white matter interface, cortical necrosis, or status marmoratusof the basal ganglia and thalamus. More about... 40, 56, 66

Figure 110.1.— Cerebral and brainstem hypotonia. Top rectangle identifies neurological characteristics of combined cerebral and brainstem hypotonia. Arrows indicate the anatomical location of involvement. Green rectangle identifies frequent findings associated with combined cerebral and brainstem hypotonia. SIADH: increased antidiuretic hormone secretion; GAZE PREFERENCE with symbol: gaze preferenece not overcome by cold caloric testing; SCRP: subcortical release phenomena; Purple cube: tests often abnormal in combined cerebral and brainstem hypotonia. US: ultrasound; EEG: electroencephalogram; VER: visual evoked responses; CT: computerized tomography; MRI: magnetic resonance imaging.

Congenital hypothyroidism
Congenital hypothyroidism is an infrequent but important cause of generalized hypotonia. Symptoms of hypothyroidism may be difficult to detect at birth. Neonates with congenital hypothyroidism are usually post-term and heavy. Jaundice is prolonged, anterior and posterior fontanelle are large, skin is mottled, and umbilical hernia and hoarse cry may be present. The tongue is thick. Congenital hypothyroidism is diagnosed by determining serum thyroxine and thyroid stimulating hormone levels. Radiographs show delayed bone maturation in only 50% of cases and therefore are not a reliable method of excluding the possibility of hypothyroidism. Treatment consists of thyroid hormone supplementation.

Figure 110.1.— Congenital hypothyroidim. Radiograph of a 10 day old neonate (gestational age 40 weeks). Absence of distal femoral and proximal tibial epiphyses. Normally the distal femoral and proximal tibial epiphyses calcify at about 34 to 36 weeks of conceptional age. Retardation of bone maturation is present in about half of neonates with congenital hypothyroidism.