The brainstem houses the motor neurons of the cranial nerve, their fibers, and several of the nuclei and pathways that make direct and indirect contact with the brainstem and spinal cord motor neurons (Figure 121.1). Hypotonia due to a brainstem lesion affects the brainstem motor nuclei, their fibers, and the fibers of the upper motor neuron systems. Apnea is frequent. Cranial nerve abnormalities may occur due to involvement of the cranial nerve fibers as they travel in the brainstem. Somatosensory and auditory evoked responses and MRI of the brain help localize the pathology to the brainstem.

Figure 121.1.— Salient features of brainstem hypotonia. Arrow indicates site of injury (brainstem). GAZE PREFERENCE with symbol: indicates that gaze preference is not overcome by cold caloric; POST. F US: posterior fontanelle ultrasound; SSER: somatosensory evoked responses; BEAR: brain evoked auditory responses.

The brainstem disorders that present with hypotonia in the neonatal period are Cleland-Chiari malformation, pontocerebellar degeneration, and carbohydrate deficient glycoprotein syndromes. Pontocerebellar degeneration and carbohydrate deficient glycoprotein syndromes involve the brainstem and the cerebellum. These neonates may also have signs of cerebellar involvement.

Cleland-Chiari Malformation
This anomaly consists of downward herniation of the cerebellar vermis through the foramen magnum. Spina bifida is usually present. Cleland-Chiari malformation in the neonatal period manifests with hypotonia and findings related to the spina bifida. The spinal bifida is usually located in the lumbosacral region and produces paraparesis.