Muscle wasting. Evident in axonal but absent in demyelinating neuropathies. Oedema of immobile limbs may mask wasting. The 1st dorsal interosseus muscle in the upper limbs and extensor digitorum brevis in the lower limbs are muscles that commonly first show wasting in the neuropathies, but examine all muscle groups. Look for fasciculations -irregular twitches of groups of muscle fibres due to diseased anterior horn cells, these may be induced by exercise or muscle percussion.
Muscle weakness. The degree of weakness is 'scored' using the MRC (Medical Research Council) scale.
Score 0 - No contraction Score 1 - Flicker
Score 2 - Active movement/gravity eliminated
Score 3 - Active movement against gravity
Score 4 - Active movement against gravity and resistance
Weakness is proportional to the number of affected motor neurones. It develops suddenly or slowly and is generally symmetrical, usually starting distally in the lower limbs and spreading to upper limbs in a similar manner before ascending into proximal muscle groups. This pattern of progression is supposedly due to the 'dying back' of axons towards their nerve cells - the longest being the most vulnerable. Some neuropathies, e.g. Guillain Barré, chronic inflammatory demyelinating polyneuropathy, may affect proximal muscle groups first.
In severe neuropathies, truncal and respiratory muscle involvement occurs. Respiratory muscle weakness may result in death.
The tendon reflex depends on:
- stretch of the muscle spindle (1),
- activation of spindle afferent fibres (2),
- monosynaptic projections to the alpha motoneurons (3)
The gamma motoneuron fibres, projecting to the spindle (4) 'modulate' activity in the reflex loop. Reflexes commonly tested: Deltoid - C5,6 - Circumflex nerve Biceps - C5,6 - Musculocutaneous nerve Supinator - C6,7 - Radial nerve
C6,7,8 - Radial nerve L2,3,4 - Femoral nerve S 1,2 - Sciatic nerve
The tendon reflexes are lost when any component of the reflex response is affected by disease. Reflexes are lost early in peripheral neuropathies when power and muscle bulk appear normal. Distal reflexes are generally lost before proximal ones.
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