Approach to the Patient with Acute Muscle Weakness

Holli A. Horak

Muscular weakness implies lack or diminution of muscle strength, which leads to an inability to perform the usual function of a given muscle or group of muscles. Muscle weakness should be differentiated from fatigue, which is the subjective perception of being “weak.” In other words, weakness is the objective evidence of lack of strength, and fatigue is a subjective symptom. After the existence of “true” weakness is established, an etiologic search should be conducted. Muscular weakness has diverse causes. This chapter emphasizes the diagnostic evaluation and differential diagnosis of the leading neurologic causes of acute weakness involving the peripheral nervous system (PNS).

I. EVALUATION

A. History.

Determine the onset, course, and distribution of weakness and any associ ated neurologic findings (such as cranial nerve involvement). Ask if there is a history of recent febrile illness, change of medications, or exposure to toxic agents. Ask the patient if he/she has had prior episodes of weakness or if there is a family history of muscle disease.

B. General physical examination.

Examine the skin for evidence of connective tissue disease or dermatomyositis-associated skin changes (Gottron’s papules, heliotrope rash). Evaluate the patient for thyroid enlargement, proptosis, or tachyarrhythmia to assess for any evidence of hyperthyroidism. Evaluate the respiratory system, including neuromus cular parameters such as cough, ability to count up to 30 during exhalation, or, more objectively, bedside forced vital capacity (FVC) and negative inspiratory force (NIF).

C. Neurologic examination.

The neurologic examination focusing on the PNS is highlighted.

1. Distribution of weakness.

a. Proximal symmetric muscle weakness is usually found among patients with primary muscle disease such as polymyositis or dermatomyositis (PM/DM) or occa sionally in patients with acute polyradiculoneuropathy, such as the Guillain-Barré’s syndrome (GBS).
b. Proximal asymmetric muscle weakness occurs among patients with nerve root trauma or acute brachial or lumbosacral plexopathies.
c. Distal symmetric muscle weakness is rarely acute but may be seen with GBS or a subacute onset of chronic inflammatory demyelinating polyneuropathy (CIDP).
d. Predominantly distal asymmetric muscle weakness occurs in patients with acute mononeuropathy such as foot-drop secondary to peroneal nerve palsy or wrist-drop resulting from radial nerve palsy. Mononeuritis multiplex (vasculitis of the PNS) manifests as a multifocal asymmetric peripheral weakness. Focal weakness also occurs with anterior horn cell involvement such as acute anterior poliomyelitis.
e. Acute diffuse muscle weakness is found among patients with GBS, myasthenia gravis (MG), periodic paralysis, or tick paralysis.

2. Muscle bulk.

Decreased muscle bulk is more often present in patients with chronic neuromuscular disease, such as muscular dystrophy, motor neuron disease (MND), or chronic neuropathy. Muscle bulk is usually normal during the acute stage of PM, MG, or acute demyelinating polyneuropathy (such as GBS).

3. Muscle tone

often is normal in patients with muscle or neuromuscular junction diseases such as PM or MG. Tone is decreased (flaccid) in disorders of nerve function such as MND and GBS.

4. Key muscles.

Examination of certain muscles can aid in narrowing the differential. For example, neck flexor and hip flexor muscles are compromised early in MG and PM.

5. Sensory features.

Sensory symptoms occur among patients with polyneuropathy (GBS) or plexopathy. Sensory examination is normal among patients with primary muscle disease, MND or neuromuscular junction disease.

6. Muscle stretch reflexes

are normal in patients with neuromuscular junction disease or primary muscle disease and are diminished or absent in patients with acute polyneuropathies such as GBS.

II. DIFFERENTIAL DIAGNOSIS

A useful approach in the evaluation of acute muscle weakness is to localize the site of lesion along the “motor unit,” which consists of all the muscle fibers innervated by a single anterior horn cell. This discussion is limited to the most frequent conditions causing acute muscle weakness, particularly those leading to generalized muscle weakness.

A. Acute anterior horn cell disease.

1. Acute anterior poliomyelitis

does not occur in the United States but can be seen in endemic areas in other countries. It typically follows a prodrome of systemic symptoms such as fever, nausea, vomiting, constipation, muscle pain, and headaches. Muscle weakness develops a few days after the prodromal stage with asymmetric weakness.

2. The West Nile virus (WNV)

is associated with many disorders of the neurologic system. One of the neurologic manifestations of West Nile virus infection is an acute anterior horn cell myelitis. Rarely (0.1%), patients infected with WNV will develop acute flaccid paralysis in a focal or segmental distribution. Electromyography/nerve conduction studies (EMG/NCS) reveals evidence for MND and CSF analysis reveals a pleocytosis, elevated protein, and elevated IgM West Nile titers. The prognosis is poor.

B. Acute polyradiculoneuropathy.

1. GBS

is an acute inflammatory demyelinating polyradiculoneuropathy (AIDP). It begins with lower-extremity paresthesia followed by ascending symmetric muscle weakness. Proximal muscles weakness may be more prominent in rare cases.

Muscle stretch reflexes are universally absent or diminished. Bifacial peripheraltype weakness is frequent. Labile blood pressure, tachycardia, and other autonomic disturbances may occur as a result of involvement of the autonomic nervous system. Early in the course of the disease, the only EMG abnormality may be the absence of F waves as a result of proximal root involvement; later, the EMG shows changes consistent with demyelination. CSF examination shows elevation of protein with minimal or no pleocytosis.

2. HIV infection.

Acute inflammatory demyelinating polyneuropathy similar to GBS can occur in patients with HIV infection. CSF pleocytosis is common.

3. Cauda equina syndrome.

This is an acute polyradiculoneuropathy of the conus medullaris and lumbosacral nerve roots. Cauda equina syndrome manifests as lower extremity neuropathic pain, sensory disturbance, bowel and bladder dysfunction, and asymmetric BLE weakness. There are many causes, including neoplastic invasion, cytomegalovirus infection, and acute compression, such as from an epidural hematoma. Evaluation includes emergent imaging by MRI, often followed by lumbar puncture to evaluate the CSF.

C. Acute plexopathy.

1. Acute idiopathic brachial plexopathy

is an uncommon disorder characterized by shoulder pain followed by weakness of shoulder girdle muscles, although distal arm muscles can be involved as well. Pain is a very important part of this syndrome. There are familial (hereditary) and sporadic cases. A history of a preceding febrile illness resulting from viral infection or vaccination is common. The diagnosis is made by clinical presentation and EMG.

2. Acute lumbosacral plexopathy,

also known as diabetic amyotrophy or acute femoral neuropathy, is an acute inflammatory lesion of the lumbosacral plexus. Patients have weakness, sensory changes, and severe neuropathic pain, which can be asymmetric, due to unequal involvement of the various roots within the plexus. It sometimes occurs in
the setting of poorly controlled diabetes, but it can be a sign of carcinomatous infiltration or vasculitis and may be idiopathic. Diagnosis is made with clinical examination, EMG, and lumbar puncture.

3. Other acute forms of plexopathy.

Acute plexus lesions can occur in patients who have sustained closed or open trauma to the brachial plexus, as in traction injuries. Neoplastic involvement, radiation, and orthopedic procedures also can cause plexus damage. Traumatic plexus injuries may follow gunshot wounds, needle punctures, and insertion of intravenous lines.

D. Acute neuropathy.

1. GBS.

See III.B.

2. Lyme’s disease.

Acute demyelinating polyneuropathy can occur among patients with Lyme’s disease. Lyme’s disease can manifest with peripheral facial palsy and an ascending-type paralysis from the lower extremities, such as inpatients with GBS. CSF is abnormal, showing elevation of protein; but unlike GBS, there is a moderate degree of lymphocytic pleocytosis.

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