Motor System: General

Published on 09/04/2015 by admin

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Last modified 22/04/2025

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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).

Functional weakness should be considered when:

• the weakness is not in a distribution that can be understood on an anatomical basis

• the movements are very variable and power is erratic

• there is a difference between the apparent power of moving a limb voluntarily and when power is being tested

• as long as there are no changes in tone or reflexes.

GRADING POWER

Power, when tested, is graded conventionally using the Medical Research Council (MRC) scale. This is usually amended to divide grade 4 into 4 +, 4 and 4 −.

5 = normal power

4 + = submaximal movement against resistance

4 = moderate movement against resistance

4 − = slight movement against resistance

3 = moves against gravity but not resistance

2 = moves with gravity eliminated

0 = no movement

Power should be graded according to the maximum power attained, no matter how briefly this is maintained.

WHAT TO DO

Look at the position of the patient overall.

• Look especially for a hemiplegic positioning, flexion of elbow and wrist with extension of knee and ankle.

Look for wasting.

• Compare the right side with the left side.

Look for fasciculation.

• Fasciculations are fine subcutaneous movements that represent contractions of a motor unit.

COMMON MISTAKES

• Fibrillations are spontaneous discharges from a single muscle fibre and are found on electromyography (EMG). They cannot be seen by the naked eye. Confusingly, fasciculations in the tongue have sometimes incorrectly been called fibrillations.

Test for tone.

Test muscle groups in asystematic way to test power.

Test reflexes.

Testing muscles of respiration and trunk muscles can be very important in specific situations. These are described in Chapter 25.

General comments

Always:

• Describe what to do in simple terms.

• Demonstrate the movements you require.

• Test simple movements across single joints.

• Fix or hold the joint to isolate the movement you want to test.

• Allow the patient to move the joint through the full range before testing power. When testing power, look at or feel the muscle contract.

• Compare the strength of the right side with the left side.

• Do not be afraid to repeat power tests so as to be certain of your findings.

• Think about what you are finding as you do the examination. It can be useful to ‘summarise’ what you find in your head as you do the examination. This makes it easier when you come to write your findings down in the notes (or report them to an examiner!).

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