Neurologists focus on identifying the location of the pathologic lesion as the first step in determining etiology. In the case of muscle weakness this would be:
- upper motor lesion - cerebral, posterior fossa, or spinal cord above the muscle groups affected
- spinal cord,
- lower motor- anterior horn cells, motor nerves, neuromuscular junction - pre or post synaptic muscle fibers
The first step in making a differential diagnosis is a good history. The important points are:
- Is the weakness progressive?
- Does the weakness change at different times in the day?
- Is the weakness greater or less after exertion, or with cold temperature?
- Is there any pain associated with the weakness? Are there any associated sensory changes?
- Are bowel and bladder function normal?
- Are there any complaints referable to the bulbar muscles double vision, change in voice, trouble chewing or swallowing, shortness of breath?
- Are there any cognitive changes associated with the weakness?
- Is there a family history of weakness?
- What was the birth history?
- Is the child intellectually normal/Are developmental milestones normal?
- Have any other organ systems been affected such as skin, kidney, joints?
The exam provides the key to localizing the pathologic lesion and guiding the choice of diagnostic tests. The motor exam has several important features:
Look for ptosis, facial weakness, complete eye movements. Symmetrical facial weakness can easily be missed. Symmetric or asymmetric weakness of the face, eye muscles, or eyelids commonly occurs in disorders of the muscle unit, neuromuscular junction, or motor nerves. Unilateral facial weakness affecting only the lower division would indicate an upper motor neuron lesion in the cortex or basal ganglia.
Positioning of extremities.
Upper motor neuron lesions are usually associated with a loss of adductor tone in the legs, causing external rotation. Acutely, changes in positioning of the arms are less obvious, but with time, the arm and wrist are flexed. Ask the patient to extend the arms, palms up, with eyes closed. Pronation and drift indicates an upper motor neuron lesion.
longstanding processes, both upper and lower motor, are associated with muscle atrophy and sometimes shortening of an extremity.
Muscular dystrophy is associated with enlargement of muscles (pseudohypertophy). Inflammatory muscle disease is suggested by tenderness of muscles to palpation.
Appearance of muscles.
Denervation of muscles (anterior horn cell) causes gross contractions called fasiculations, particularly in the tongue. Inflammation of muscles produces irritability of the muscles which is sometimes visible with gentle percussion- Marked contractions of muscles after percussion, myotonia, indicates a disease of the muscle membrane.
This can be assessed by gently moving the arms, wrists, and legs. Acute upper motor neuron lesions cause decreased tone. After a few days the tone increases in the affected extremity.
Quickly assess a few muscle groups on each side of the body, both proximal and distal muscles - shoulder elevation, neck flexion and extension, arm flexion, leg elevation, foot flexion. Proximal weakness is characteristic of muscle disease (muscle or neuromuscular junction), whereas distal weakness is suggestive of a neuropathy. Bilateral weakness of lower extremities, even if asymmetric, would suggest a cord lesion.
This is an essential diagnostic point. Upper motor neuron lesions are associated with the presence of reflexes in the affected parts of the body; often in upper motor neuron lesions the reflexes are increased on the affected side. The Babinski response is characteristic of an upper motor neuron lesion. Neuromuscular junction processes have normal reflexes. Anterior horn cell, neuropathies, and usually muscle disease cause absence of reflexes.
Gait: Watch a patient walk! This is often the easiest way to demonstrate a hemiparesis from an upper motor neuron lesion. Look for circumduction of the leg or diminished swing of the arm. A waddling or lordotic gait occurs with chronic muscle weakness
Rapid alternating movements are a sensitive indicator of an upper motor neuron lesions. This can be due to corticospinal involvement or cerebellar involvement.
Hammer along the length of the spine. Spinal cord compression is often associated with pain (entrapped roots).
Check rectal tone. This affected in spinal cord lesions.
A simple sensory exam is important, looking for loss of position or vibration sense, a subjective sense of decreased sensation to touch, or an objective level of loss of sensation. Sensory changes are seen in neuropathies and spinal cord processes. A level of loss of sensation is an important feature of disease affecting the spinal cord.
Once you have localized the disease process to a particular unit, you can make a reasonable differential diagnosis:
Stroke, vasculitis, infection, tumor, degenerative disease, seizure, hemisyndrome migraine, alternating hemiplegia of childhood
Tumor (spinal or root pain, bowel/bladder impairment, sensory level, Brown-Sequard) transverse myelitis (sensory level, bowel/ bladder impairment, sudden onset), infection, trauma
Anterior horn cell.
Spinal muscular atrophy (absent DTR's, fasiculations, infants), polio (asymmetric weakness, absent DTR's, pain, rapid onset, history of diarrhea, fever)
Guillian-Barre (rapidly progressive, ascending or descending, absent DTR's, some mild sensory complaints), peripheral nerve toxins, acute intermittent prophyria
Myasthenia (bulbar symptoms, ptosis, worse with exercise, normal DTR's, respiratory insufficiency, autonomic system), botulism (baby, absent eye movements, bulbar symptoms, normal DTR's, hypoventilation) , organophosphate or carbamate poisoning
Genetic causes: muscular dystrophy (family history, proximal weakness, pseudohypertrophy absent DTR's), myotonic dystrophy (family history, temporal balding, MR, cataracts, arrhythmia); myositis (tender muscles, myoglobinuria, sudden onset, other system involvement); metabolic causes (nontender, sometimes decreased DTR's), rhabdomyolysis, trichinellosis (consumption of undercooked pork or wild boar), familial periodic paralysis
Electolyte disturbances (underlying hypokalemia, hypophosphatemia, hypocalcemia, hyponatremia, hypernatremia), drug-related (check history: isoniazid, nitrofurantoin, zidovudine, Mg sulfate, chemo), converstion disorder.
If spinal cord disease is in the differential, an MRI of the cord is essential and should be done emergently. An LP is helpful in neuropathies and in spinal cord disease. Nerve conductions are essential for neuropathies and neuromuscular junction problems. EMG may be helpful with muscle disease, but usually a muscle biopsy is essential. Inflammatory muscle diseases and some muscular dystrophies have elevated muscle enzymes. Check CMP for electrolyte abnormalities. Check urine for myoglobin.
- Saguil A. Evaluation of the patient with muscle weakness. American Family Physician April 2005
- Jackson C.E. A clinical approach to muscle disease. Seminars in Neurology 2008
- Dimberg E.L. The Office Evaluation of Weakness. Seminars in Neurolgoy 2011