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.
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.