50. Lower Extremity Tone

What Is Being Tested?

Many parts of the motor exam can help distinguish between upper motor neuron and lower motor neuron lesions (see Chapters 2 and 6 in Neuroanatomy through Clinical Cases 3e). Recall that upper motor neurons project via the corticospinal tract to lower motor neurons located in the anterior horn of the spinal cord. Signs of lower motor neuron lesions (see Table 3.3 below) include weakness, atrophy, fasciculations, and hyporeflexia (reduced reflexes). (See the section “Reflexes” in Chapter 3 in in Neuroanatomy through Clinical Cases 3e.) Signs of upper motor neuron lesions include weakness, hyperreflexia (increased reflexes), and increased tone. The hyperreflexia and increased tone seen with corticospinal lesions is apparently caused by damage to pathways that travel in close association with the corticospinal tract rather than directly by damage to the corticospinal tract itself. Note that with acute upper motor neuron lesions there is often flaccid paralysis, with decreased tone and decreased reflexes. With time (hours to weeks), increased tone and hyperreflexia usually develop.

Increased tone can occur in upper motor neuron lesions but can also occur in basal ganglia dysfunction (see KCC 16.1 in Neuroanatomy through Clinical Cases 3e). In addition, slow or awkward fine finger movements or toe tapping in the absence of weakness can signify a subtle abnormality of the corticospinal pathways, but these findings can also occur in lesions of the cerebellum or basal ganglia.

Table 3.3  Signs of Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) Lesions

^a Mild atrophy may develop as a result of disuse.

^b With acute upper motor neuron lesions, reflexes and tone may be decreased.