When both the upper motor unit system and the motor-sensory unit are involved, signs of one or the other tend to predominate. In general, signs of upper motor neuron system dysfunction predominate early, while those of the motor-sensory unit predominate later.
Diseases of the upper motor neuron should be further localized to one of the following anatomical structures: brain, brainstem, cerebellum, or spinal cord. Although these structures may be involved simultaneously, one tends to predominate in most cases. Localization is pivotal in determining the most likely pathological process involved and the most likely etiology.

Brain lesions may produce generalized hypotonia with decreased or increased dynamic tone. The findings associated with cerebral hypotonia in neonates are seizures, lethargy, subcortical release phenomena, myoclonus, irritability, gaze preference, evidence of inappropriate antidiuretic hormone secretion, hypothermia, abnormal retinal findings, microcephaly, macrocephaly, facial dysmorphism, skull fracture, and abnormalities in EEG, visual evoked response, and brain imaging (Figure 105.1). The pathological processes that affect the brain are trauma, metabolic disorders, chromosomal and genetic abnormalities, and degenerative diseases.

Figure 105.1.— Cerebral hypotonia. Red arrow indicates the anatomical location of involvement. IADH: increased antidiuretic hormone secretion; Symbol by the side of GAZE PREFERENCE indicates that it may be overcome by cold caloric testing; SCRP: subcortical release phenomena; Purple cube: tests often abnormal in cerebral hypotonia; US: ultrasound; EEG: electroencephalogram; VER: visual evoked responses; CT: computerized tomography; MRI: magnetic resonance imaging.