Motor System

General

There are five patterns of muscular weakness:

1. Upper motor neurone (UMN): increased tone, increased reflexes, pyramidal pattern of weakness (weak extensors in the arm, weak flexors in the leg).

2. Lower motor neurone (LMN): wasting, fasciculation, decreased tone and reduced or absent reflexes.

3. Muscle disease: wasting, decreased tone, impaired or absent reflexes.

4. Neuromuscular junction: fatigable weakness, normal or decreased tone, normal reflexes.

5. Functional weakness: normal tone, normal reflexes without wasting with erratic power.

The level of the nervous system affected can be determined by the distribution and pattern of the weakness and by associated findings (Table 15.1).

Table 15.1

Approach to weakness *

^*Consider distribution and whether upper or lower motor neurone or muscular.

Examples of brainstem signs (all contralateral to the upper motor neurone weakness): third, fourth and sixth palsies, seventh lower motor neurone loss, nystagmus and dysarthria.

Hemisphere signs: aphasia, visual field defects, inattention or neglect, higher function deficits.

Mixed UMN and LMN lesions: motor neurone disease (with normal sensation), or combined cervical myelopathy and radiculopathy and lumbar radiculopathy (with sensory abnormalities).

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