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.
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,
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 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.