Nerve and Muscle Diseases

Patients with neuromuscular disease generally present with weakness, sensory loss, or both of these conditions. Your approach should initially focus on localizing the problem to a specific component of the peripheral nervous system that is involved (e.g., neuropathy or myopathy) and then be directed toward identifying a specific disease process. The major anatomic components of the peripheral nervous system are listed in Table 20.1 .

Table 20.1

Basic Anatomic Subtypes of Neuromuscular Disease

Anatomic Site	Typical Pattern of Motor and Sensory Deficit	Examples
Motor neuron disease	Weakness, wasting, fasciculations; no sensory deficits; hyperreflexia with ALS	ALS, spinal muscular atrophy, polio
Monoradiculopathy	Distribution of a single nerve root (dermatomal pattern)	L5 or S1 root compression herniated disk form
Polyradiculopathy	Distribution of multiple nerve roots	Cauda equine syndrome; carcinomatous meningitis
Plexopathy	Distribution of a nerve plexus	Acute brachial neuritis
Mononeuropathy	Distribution of a single peripheral nerve	Carpal tunnel syndrome
Mononeuropathy multiplex	Multifocal process affecting several discrete peripheral nerves	Vasculitis, leprosy
Polyneuropathy	Diffuse, symmetric, distal stocking-glove pattern; distal hyporeflexia	Diabetic polyneuropathy
Neuromuscular junction disease	Fluctuating weakness with fatigability; no sensory deficits; reflexes preserved	Myasthenia gravis
Myopathy	Diffuse proximal muscle weakness; no sensory deficits; preserved reflexes until late	Polymyositis; muscular dystrophy

ALS, Amyotrophic lateral sclerosis.

Approach to the Patient With Suspected Neuromuscular Disease

History

1.
Clarify the pattern of weakness. Proximal weakness suggests myopathy; distal weakness suggests neuropathy.
2.
Characterize any sensory symptoms. Have the patient identify the exact regions involved and symptom character (sensory loss or unpleasant sensation).
3.
Ask about cramps and muscle twitches (fasciculations). These symptoms point to disease of the motor neuron (amyotrophic lateral sclerosis [ALS]) or muscle (myopathy).
4.
Ask about pain. Pain may be related to a musculoskeletal structure (e.g., herniated disk), or it may be neuropathic or muscular.
5.
Is there any autonomic involvement? Ask about orthostatic dizziness, anhidrosis, visual blurring, urinary hesitancy or incontinence, constipation, and impotence. These symptoms can result from autonomic neuropathy.

Examination

1.
Determine whether the patient has true weakness. Decreased strength needs to be differentiated from limitation arising from pain, and from submaximal effort. Effort-limited weakness is inconsistent and tends to “give way” suddenly.
2.
Map out any sensory deficits. Think in terms of identifying diffuse, distal sensory loss (stocking-glove pattern), as seen in polyneuropathy; focal sensory loss restricted to a single root dermatome or peripheral nerve; or multifocal sensory loss, which suggests mononeuropathy multiplex or a plexus lesion ( Fig. 20.1 ).
3.
Test the reflexes. Loss of deep tendon reflexes suggests peripheral nerve involvement.
4.
Undress the patient to check for wasting and fasciculations (irregular individual muscle twitches). These findings indicate lower motor neuron disease.

Motor Neuron Disease

The clinical hallmarks of anterior horn cell disease are the lower motor neuron signs of weakness, wasting (atrophy), and fasciculations. These signs may be seen alone or in combination with upper motor neuron signs (hyperreflexia, upgoing toes) as in the case of ALS. Sensory disturbances are absent. There are several distinct forms of motor neuron disease:

AMYOTROPHIC LATERAL SCLEROSIS (ALS)

Also known as Lou Gehrig disease, ALS is the most common form of motor neuron disease. It is easily recognized on the basis of progressive weakness, wasting, fasciculations, and upper motor neuron signs. It is familial in 10% of cases. The presence of bulbar involvement (dysarthria, dysphagia, dyspnea) carries a worse prognosis. Median survival after diagnosis is 3 years. ALS is a clinical diagnosis that is supported by exclusion of other causes and evidence of denervation in at least three regions on electromyography (EMG).

Spinal muscular atrophy

This condition resembles ALS but is limited to pure lower motor neuron degeneration (e.g., no upper motor neuron signs are seen). Spinal muscular atrophy typically has infantile or childhood onset, but adult forms also occur. Adult-onset progression is slower than ALS and is more often hereditary. Testing for the survival motor neuron 1 (SMN1) copy number is available. Nusinersen (Spiranza ®), an antisense oligonucleotide that increases SMN copy number, was approved by the Food and Drug Administration (FDA) in 2016.

Multifocal motor neuropathy

This condition is an immune-mediated motor neuropathy, differentiated by the presence of conduction block on nerve conduction studies (NCSs). The course is protracted over many years, and the weakness is asymmetric; patients progress without treatment. Up to 50% of patients have anti-GM1 antibodies. Treatment with frequent intravenous immunoglobulin (IVIG) is the mainstay of treatment; IVIG is approved by the FDA for this condition.

Other motor neuron diseases

These diseases include poliomyelitis (West Nile causes a polio-like condition), hereditary neurodegenerative diseases, and metabolic systemic storage disorders.

Diagnosis

Diagnostic testing for suspected motor neuron disease should include the following: EMG/NCS, serum electrophoresis, serum immunofixation electrophoresis, quantitative immunoglobulins, and anti-GM1 antibody levels. Cervical spine magnetic resonance imaging (MRI) and lumbar puncture (LP) should be considered. In patients with a paraprotein, bone marrow biopsy may be indicated.

Treatment

ALS is incurable, but riluzole 50 mg by mouth (PO) twice per day, a glutamate antagonist, may slow the progression of the disease. Clinical trials of additional agents are ongoing. Patients with multifocal motor neuropathy may improve with treatment with IVIG 0.5 to 1.0 g/kg given every 2 to 4 weeks.

Monoradiculopathy and Polyradiculopathy

Monoradiculopathies typically result from disk herniation and nerve root compression. They present with a radicular distribution of pain and are discussed in Chapter 18 . Polyradiculopathy involving multiple lumbosacral nerve roots (cauda equina syndrome) presents with low back pain, urinary disturbances, and gait failure.

Plexopathy

Diseases that cause diffuse injury to either the brachial or the lumbosacral plexus lead to regional motor, sensory, and reflex disturbances in one limb. The key to identifying the syndrome is to find a pattern of deficits that cannot be explained by involvement of one nerve root or a single peripheral nerve. EMG and NCS are helpful in confirming and defining the syndrome; weakness is typically more severe in plexopathy than compressive radiculopathy. See Appendix for the anatomy of the brachial and lumbosacral plexus.

Brachial Plexopathy

Upper brachial plexus injury (arising from C5-C7) results in weakness and atrophy of the shoulder and upper arm muscles (Erb palsy). Lower brachial plexus injury (arising from C8 and T1) leads to weakness, atrophy, and sensory deficits in the forearm and hand (Klumpke palsy). The main causes of brachial plexopathy include the following:

1.
Trauma
2.
Idiopathic brachial neuritis (Parsonage-Turner syndrome)

This under recognized syndrome presents with the sudden onset of pain in the shoulder and arm; as the pain resolves over 2 to 4 weeks, weakness and muscle wasting become evident.
3.
Tumor infiltration

Metastatic disease and neurofibroma are most common.
4.
Radiation plexopathy

High-dose irradiation for lymphoma or breast cancer can lead to painless progressive brachial plexopathy 1 to 5 years later. Myokymia (irregular wormlike muscle movement) may be a distinguishing feature.
5.
Cervical rib or bands (thoracic outlet syndrome)

This rare condition is caused by compression of the lower trunk of the brachial plexus as it passes over an abnormal first cervical rib or rudimentary first rib. Patients complain of pain and paresthesias in the C8-T1 distribution of the hand and medial forearm when carrying heavy objects or when raising the arm above shoulder level. Surgical decompression may be helpful in rare cases.

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