The sites of pathology in neonates with hypotonia and decreased dynamic tone are the brain, brainstem, cerebellum, rostral spinal cord, anterior horn cell, nerve, presynaptic-myoneural junction, postsynaptic-myoneural junction, muscle, or a combination of these sites (Figure 99.1). The distinction among them can sometimes be made based on associated neurological findings: seizures, weakness of facial muscles, increased facial dynamic tone, parasympathetic pupil abnormalities, lack of bowel movements, and anal sphincter weakness.

Figure 99.1.— Schematic representation of the possible sites of neuromuscular involvement in neonates with hypotonia and decreased or normal dynamic tone: (1) brain; (2) brainstem; (3) rostral spinal cord; (4) lower cervical spinal cord; (5) cerebellum; (6) alpha motor neuron; (7) nerve; (8) presynaptic-myoneural junction; (9) postsynaptic-myoneural junction; (10) muscle. Green broken line implies myelin of Scwann cell origin. Orange broken line implies myelin of oligodendrocite origin.

Neonates with hypotonia and normal dynamic tone are likely to have an upper motor neuron system dysfunction but certain diseases of the motor-sensory unit, such as myasthenia gravis and some myopathies, have normal dynamic tone.