Upper motor neuron system hypotonia refers to hypotonia that occurs as a consequence of disorders of the neurons that directly or indirectly make contact with the lower motor neurons. Upper motor neuron system hypotonia occurs with diseases of the brain, cerebellum, brainstem, and spinal cord (Figure 104.1 [1,2,3,5]).

Figure 104.1.— Schematic representation of the upper motor neuron system and lower motor unit structures demonstrating possible sites of anatomical involvement that lead to hypotonia. Generalized hypotonia may be due to an upper motor neuron lesion, a lower motor neuron lesion, or a combination of upper and lower motor neuron lesions. Upper motor neuron lesions may occur at the (1) brain, (2) brainstem, (3) rostral cervical spinal cord, or (5) cerebellum. Combined upper and lower motor lesions may occur with damage to lower motor neurons of the arms and the upper motor neurons of the legs in the lower cervical spinal cord (4). Lower motor neuron lesions may occur at the (6) alpha motor neuron, (7) nerve, (8) presynaptic myoneural junction, (9) postsynaptic myoneural junction, or (10) muscle. Brainstem structures that influence muscle tone (red nucleus, reticular formation, and lateral vestibular nucleus) are not shown in this figure. LMN: lower or alpha motor neuron; MNJ: myoneural junction.

Hypotonia due to upper motor neuron system lesions may be associated with decreased dynamic tone or increased dynamic tone. Upper motor neuron system dysfunction is often associated with clinical findings that reflect the involvement of neighboring neurological and nonneurological structures. Upper motor neuron system hypotonia is not associated with any specific electromyographic, nerve conduction velocity, repetitive stimulation test, or muscle biopsy abnormalities unless there is simultaneous involvement of the alpha motor neurons and the muscle fibers (motor-sensory unit). Simultaneous involvement of the upper motor neuron system and motor-sensory unit occur in Farber disease (brain, alpha motor neurons, and nerves), GM1 gangliosidosis (brain and alpha motor neurons), and hypoxic encephalopathy (brain, alpha motor neurons, and muscle), and in neuroaxonal dystrophy (brain and peripheral nerve), congenital muscular dystrophy (brain and muscle), and congenital myotonic dystrophy (brain and muscle).