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.
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.
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