Muscle Weakness

Muscle testing

Place the muscle in a position in which its primary action can be evaluated.

The grading of muscle power is usually denoted on a 0–5 scale:

Evaluation of Muscle Weakness

Muscle Weakness

Upper Motor Neuron (ie, Corticospinal Tract Lesions)

Lesions of the corticospinal pathways from the motor cortex to the anterior horn cell are most often incomplete*.

They affect particular groups of muscles and movements rather than individual muscles.

Weakness is importantly noted in the abductors (eg, Deltoid's) and extensors* (eg, Triceps) of the upper extremity.

Patients are UNABLE to perform individual fractionated movements (Fine Motor Movements) due to finger flexor movement.
Corticospinal lesions show distal greater than proximal weakness and are associated with:

Spasticity
Hemiplegic gait
Babinski’s sign
They demonstrate a fix on arm roll, drift of both arm and leg and the shortening response (Fig. 4.51–4.54).

Extrapyramidal Lesions

Dysregulation of tone so that the agonist muscle has to move the joints of the limb against the resistance of an antagonist muscle.
If the patient is asked to sustain finger movement, it is noted that each preprogrammed movement gets smaller and smaller.

Each letter becomes smaller and smaller and the end of the last name is almost a straight line (micrographia).
Initiation of movement is slow (bradykinesia).
The increased tone affects the whole limb and is associated with rigidity and generally decreased reflexes.
There are fewer spontaneous and associated movements.

Lower Motor Neuron (Anterior Horn Cell Disease)

Marked weakness and often associated with atrophy and fasciculations.
Viral illness such as polio or other neurotrophic viruses may affect individual anterior horn cells.
Weakness is asymmetrical and may involve individual small muscle groups such as the intrinsic hand muscles.

One or two lumbricales or one extensor muscle would be involved in this instance.
If the process affects only the anterior horn cells such as occurs with hereditary motor neuron disease (progressive spinal muscle atrophy), wasting is prominent in the end and tends to be symmetrical while reflexes are lost.
ALS in general is asymmetrical.
There is wasting of muscle in the presence of hyperactive reflexes.
A lesion of the ventral root affects those muscles whose sole or major supply is from that segment.
A common example of this process is seen with the prominent deltoid wasting of severe cervical spondylosis often with induced or spontaneous fasciculations (“spondylitic ALS”).
If the peripheral nerve is lesioned (e.g., sciatic), all muscles supplied by that nerve are affected.

Motor neuropathies that are dying back (metabolic, toxic and occasionally genetic) show maximal weakness peripherally and symmetrically in the arm and legs.
Autoimmune peripheral neuropathies may be asymmetrical and affect the upper extremities prior to the lower (GM1 neuropathy or multifocal neuropathy with conduction block).
Weakness resulting from muscular lesions In most acquired inflammatory myopathies the weakness is proximal.

The exception is inclusion body myositis (IBM) which affects forearm flexors with associated contracture of finger flexors. Hereditary IBM may specifically target the distal quadriceps.
The distal genetic myopathies affect the anterior or posterior compartments of the lower legs (Welander’s,

Miyoshi’s, Markesberry/Griggs/Udd) or the sternocleidomastoids, masseter and distal extremity (myotonic dystrophy type I).

Local injury or compartment syndromes may affect the specific muscles of that compartment.

Nerve/Nerve Root lesions

Neuromuscular Junction Weakness (eg, Myasthenia Gravis)

Specific muscle groups may be affected.
There is an ocular form alone and a primary bulbar form.
Characteristically, in generalized myasthenia, the degree of weakness varies with time, increases with use, but recovers with rest often after a short period of time.
Myasthenic weakness is most frequently seen in the eyelids, the external ocular muscles, the tongue (triple

furrowed tongue), the throat and larynx, the muscles of the back, shoulder and hand.

Cogan’s twitch sign demonstrates subtle weakness of the superior rectus and inferior oblique muscles.

If a patient with possible myasthenic weakness gets stronger with exercise, one should suspect Lambert–Eaton myasthenic syndrome from small cell cancer of the lung.
In general, with Lambert–Eaton syndrome ptosis is more common than extraocular muscle weakness, the legs are weaker than the arms and there is a “load in the pants” gait.
The autonomic nervous system is affected from the autoimmune attack and patients complain of a dry mouth and inability to swallow.

Adapted from: Robert J. Schwartzman, MD Neurologic Examination (2006) Massachusetts, USA: Blackwell Publishing.