Medical School · Year 3 · Neurology · includes a quiz and discussion video

Seminar 07: Neuromuscular Disorders

Year 3: Neurology Clerkship

Learning Objectives

By the end of this seminar, students will be able to:

1. Classify peripheral neuropathies by pattern and etiology
2. Diagnose and manage common polyneuropathies
3. Recognize neuromuscular junction disorders
4. Evaluate patients with suspected myopathy
5. Identify emergencies such as Guillain-Barre syndrome
6. Apply electrodiagnostic principles to neuromuscular disease

Seminar Outline

I. Approach to Neuromuscular Disease

The clinical evaluation of neuromuscular disorders requires systematic anatomical localization along the motor unit, which extends from the anterior horn cell through peripheral nerve, neuromuscular junction, and muscle. Motor neuron disease, exemplified by amyotrophic lateral sclerosis, produces a characteristic combination of upper and lower motor neuron signs including spasticity, hyperreflexia, weakness, atrophy, and fasciculations. Nerve root involvement produces radicular patterns with dermatomal sensory loss and myotomal weakness corresponding to specific spinal levels. Plexus lesions affect multiple nerve distributions within a single limb, producing patterns that do not conform to individual nerve or root territories. Peripheral nerve pathology produces combined sensory and motor deficits in distributions determined by the affected nerves, whether single nerves in mononeuropathy or diffuse involvement in polyneuropathy.

The neuromuscular junction serves as the interface between motor nerve terminal and muscle fiber, and disorders at this location produce characteristic features distinct from neuropathy or myopathy. Fatigable weakness, in which strength progressively diminishes with sustained or repeated effort, represents the hallmark of neuromuscular junction disorders. Sensory examination remains entirely normal because the junction involves only motor transmission. Muscle diseases characteristically produce proximal weakness affecting shoulder and hip girdle musculature while sparing sensation. Reflexes may be preserved until late in myopathy when muscle bulk is substantially reduced. Elevated serum creatine kinase reflects muscle fiber damage and supports the diagnosis of myopathy when elevated significantly.

Pattern recognition provides an efficient approach to the differential diagnosis of neuromuscular weakness. Distal symmetric sensorimotor symptoms with length-dependent progression, affecting the feet before the hands, suggest polyneuropathy. Proximal symmetric weakness affecting shoulder and hip girdles without sensory involvement points toward myopathy or neuromuscular junction disorders. Asymmetric patterns raise concern for motor neuron disease, mononeuropathy multiplex, or multifocal motor neuropathy. Rapidly ascending weakness developing over days suggests Guillain-Barre syndrome. Fatigable weakness that worsens with activity and improves with rest is characteristic of myasthenia gravis and other junction disorders.

History taking in neuromuscular evaluation should systematically address the distribution of symptoms, the presence and character of sensory complaints, the temporal profile of onset and progression, and relevant medical and family history. Proximal weakness manifests as difficulty rising from chairs, climbing stairs, and reaching overhead, while distal weakness produces foot drop and difficulty with fine motor tasks. Sensory symptoms in neuropathy include numbness, tingling, burning pain, and imbalance from proprioceptive loss. The distinction between true weakness and fatigue or exercise intolerance guides localization. Family history of neuromuscular disease raises suspicion for hereditary conditions including Charcot-Marie-Tooth disease and muscular dystrophies.

<image>Panel A: Motor unit anatomy diagram showing anterior horn cell, axon, neuromuscular junction, and muscle fiber with localization patterns. Panel B: Pattern recognition chart comparing distal symmetric, proximal symmetric, and asymmetric weakness distributions with differential diagnoses. Panel C: Key examination findings table distinguishing nerve, junction, and muscle localization. Panel D: Timeline of symptom progression showing acute, subacute, and chronic neuromuscular presentations with examples.</image>

II. Peripheral Neuropathy Overview

Peripheral neuropathies comprise a heterogeneous group of disorders affecting sensory, motor, and autonomic nerve fibers, classified according to the predominant fiber types involved, the anatomical pattern of involvement, and the underlying pathophysiology. Large fiber sensory neuropathies produce numbness, impaired vibration and proprioception, sensory ataxia, and reduced reflexes, while small fiber neuropathies characteristically cause burning pain, temperature sensory loss, and autonomic symptoms while sparing reflexes and proprioception. Pure motor neuropathies produce weakness and atrophy without sensory symptoms and should raise concern for motor neuron disease or multifocal motor neuropathy. Autonomic neuropathies manifest as orthostatic hypotension, gastroparesis, bladder dysfunction, and sudomotor abnormalities.

The anatomical pattern of neuropathy provides important diagnostic information. Length-dependent polyneuropathies, the most common pattern, produce a symmetric stocking-glove distribution beginning distally and progressing proximally over time, reflecting the greater vulnerability of longer axons. Non-length-dependent patterns with proximal, patchy, or asymmetric involvement suggest inflammatory demyelinating neuropathies, vasculitic neuropathy, or multifocal processes. Mononeuropathy affects a single peripheral nerve, typically at sites of compression or entrapment such as carpal tunnel syndrome. Mononeuropathy multiplex describes the involvement of multiple individual nerves in an asymmetric distribution, raising concern for vasculitis, diabetes, or other systemic conditions.

Electrodiagnostic studies, comprising nerve conduction studies and needle electromyography, provide critical information for characterizing neuropathy. Nerve conduction studies measure the speed of impulse propagation (conduction velocity) and the size of the response (amplitude). Demyelinating neuropathies produce marked slowing of conduction velocities, prolonged distal latencies, conduction block where impulses fail to propagate across a segment, and temporal dispersion of responses. Axonal neuropathies primarily reduce response amplitudes while preserving or only mildly slowing velocities. Needle electromyography assesses the electrical activity of muscle, demonstrating fibrillation potentials and positive sharp waves in denervated muscle, and chronic reinnervation changes including large motor unit potentials in longstanding neuropathy.

The distinction between axonal and demyelinating pathophysiology carries important diagnostic and therapeutic implications. Axonal neuropathies, representing the majority of polyneuropathies, result from metabolic, toxic, and nutritional causes including diabetes, alcohol, chemotherapy, and vitamin deficiencies. The treatment approach focuses on identifying and correcting underlying causes, managing symptoms, and allowing time for axonal regeneration. Demyelinating neuropathies, including Guillain-Barre syndrome and chronic inflammatory demyelinating polyneuropathy, often have autoimmune pathophysiology and may respond to immunomodulatory treatment. Hereditary neuropathies may be either axonal or demyelinating, with Charcot-Marie-Tooth type 1 representing the demyelinating forms and type 2 the axonal forms.

<image>Panel A: Fiber type classification showing large fiber sensory, small fiber sensory, motor, and autonomic neuropathy features with examples. Panel B: Anatomical pattern diagrams illustrating length-dependent, non-length-dependent, mononeuropathy, and mononeuropathy multiplex distributions. Panel C: Nerve conduction study interpretation showing demyelinating versus axonal patterns with example tracings. Panel D: EMG findings diagram showing fibrillation potentials, positive sharp waves, and chronic reinnervation changes.</image>

III. Common Polyneuropathies

Diabetic neuropathy represents the most common cause of polyneuropathy in developed countries, affecting approximately half of patients with long-standing diabetes. The typical presentation is a distal symmetric sensorimotor polyneuropathy with length-dependent numbness, tingling, and burning pain beginning in the toes and gradually ascending. Autonomic involvement commonly accompanies sensory symptoms, producing orthostatic hypotension, resting tachycardia, gastroparesis with early satiety and nausea, neurogenic bladder, and erectile dysfunction. Diabetic amyotrophy, also termed diabetic lumbosacral radiculoplexus neuropathy, presents with severe proximal leg pain followed by weakness and atrophy, often asymmetric in onset, with gradual improvement over months. Management emphasizes glycemic control to prevent progression, neuropathic pain medications, and treatment of autonomic symptoms.

Alcoholic neuropathy develops in the context of chronic excessive alcohol consumption through both direct toxic effects and associated nutritional deficiencies, particularly thiamine. The clinical presentation resembles diabetic neuropathy with distal symmetric sensorimotor symptoms, though painful burning dysesthesias may be particularly prominent. Gait ataxia results from combined sensory and cerebellar dysfunction. Treatment requires alcohol cessation as the essential intervention, thiamine and other B vitamin supplementation, and symptom management. Recovery is possible with abstinence, though improvement may be slow and incomplete, particularly when neuropathy is advanced.

Vitamin B12 deficiency produces a distinctive clinical syndrome affecting both the peripheral and central nervous systems. Peripheral neuropathy manifests as large fiber sensory loss with impaired vibration and position sense, producing sensory ataxia. Central nervous system involvement, termed subacute combined degeneration, affects the dorsal columns and lateral corticospinal tracts of the spinal cord, adding spastic paraparesis to the sensory ataxia. Laboratory evaluation reveals low serum B12 levels, though functional B12 deficiency may occur with normal B12 levels, detected by elevated methylmalonic acid and homocysteine. Causes include pernicious anemia, gastric surgery, malabsorption, and strict vegan diet without supplementation. Treatment with B12 supplementation should begin promptly to prevent irreversible neurological damage.

Additional common causes of polyneuropathy warrant consideration in the evaluation of patients with compatible symptoms. Chemotherapy-induced neuropathy, particularly from platinum compounds, taxanes, and vinca alkaloids, represents a significant cause of neuropathy in oncology patients. Uremic neuropathy improves with dialysis or renal transplantation. Hypothyroidism may produce both polyneuropathy and predisposition to entrapment neuropathies including carpal tunnel syndrome. Hereditary neuropathies, particularly Charcot-Marie-Tooth disease, should be suspected in patients with slowly progressive neuropathy, high arches, hammertoes, and positive family history. The evaluation of polyneuropathy should include screening laboratory studies including fasting glucose or hemoglobin A1c, vitamin B12, thyroid function, serum protein electrophoresis, and additional testing guided by clinical features.

<image>Panel A: Diabetic neuropathy spectrum showing distal symmetric, autonomic, and diabetic amyotrophy presentations with clinical features. Panel B: Alcoholic neuropathy mechanism diagram showing toxic and nutritional components with treatment approach. Panel C: B12 deficiency pathway illustrating peripheral neuropathy, subacute combined degeneration, and laboratory diagnosis algorithm. Panel D: Polyneuropathy evaluation checklist showing essential and directed laboratory testing based on clinical features.</image>

IV. Guillain-Barre Syndrome

Guillain-Barre syndrome is an acute inflammatory demyelinating polyneuropathy that represents a neurological emergency requiring prompt recognition and management. The classic presentation consists of rapidly progressive symmetric weakness beginning distally and ascending to involve proximal muscles over days to weeks, with maximal severity reached within four weeks by definition. Reflexes are diffusely diminished or absent even in minimally weak muscles, a finding that helps distinguish Guillain-Barre syndrome from other causes of acute weakness. Sensory symptoms are typically present but milder than motor involvement, with paresthesias and pain common complaints. Autonomic dysfunction occurs in the majority of patients and may produce dangerous complications including labile blood pressure, cardiac arrhythmias, and urinary retention.

The diagnosis of Guillain-Barre syndrome rests on the characteristic clinical presentation supported by cerebrospinal fluid analysis and electrodiagnostic studies. Cerebrospinal fluid classically demonstrates albuminocytologic dissociation, consisting of elevated protein concentration with normal or near-normal cell count, though this finding may not be present in the first days of illness. Nerve conduction studies demonstrate demyelinating features including prolonged distal latencies, slowed conduction velocities, conduction block, temporal dispersion, and prolonged F-wave latencies, though studies performed very early may be normal or show only subtle abnormalities. Anti-ganglioside antibodies are present in some patients and are particularly associated with specific variants.

Several clinical variants of Guillain-Barre syndrome are recognized. The classic acute inflammatory demyelinating polyneuropathy accounts for the majority of cases in Western countries. Acute motor axonal neuropathy and acute motor-sensory axonal neuropathy are axonal variants more common in Asia and associated with anti-GM1 and anti-GD1a antibodies. Miller Fisher syndrome, characterized by the triad of ataxia, areflexia, and ophthalmoplegia, is associated with anti-GQ1b antibodies. The pharyngeal-cervical-brachial variant produces prominent oropharyngeal and upper extremity weakness. An antecedent infection, typically respiratory or gastrointestinal, is reported in approximately two-thirds of patients, with Campylobacter jejuni being the most commonly identified pathogen.

Management of Guillain-Barre syndrome requires intensive supportive care with close monitoring for respiratory and autonomic complications alongside immunomodulatory treatment. Respiratory monitoring with serial forced vital capacity measurements is essential, as respiratory failure requiring mechanical ventilation occurs in approximately twenty to thirty percent of patients. Intubation should be considered when forced vital capacity falls below twenty milliliters per kilogram, negative inspiratory force falls below thirty centimeters of water, or the patient demonstrates respiratory distress or rising carbon dioxide levels. Immunotherapy with either intravenous immunoglobulin or plasma exchange is equally effective and should be initiated promptly in patients with significant weakness. Corticosteroids are not effective in Guillain-Barre syndrome. Deep vein thrombosis prophylaxis is important given immobility. The prognosis is generally favorable, with approximately eighty percent of patients achieving independent ambulation at one year, though recovery may be prolonged and incomplete in some patients.

<image>Panel A: Guillain-Barre syndrome clinical timeline showing antecedent infection, symptom onset, nadir, and recovery phases with treatment windows. Panel B: CSF albuminocytologic dissociation diagram with typical findings and timing of abnormalities. Panel C: Guillain-Barre syndrome variants table comparing AIDP, AMAN, AMSAN, Miller Fisher, and PCB variant features. Panel D: Respiratory monitoring protocol showing FVC threshold, NIF measurement, and intubation decision algorithm.</image>

V. Chronic Inflammatory Demyelinating Polyneuropathy

Chronic inflammatory demyelinating polyneuropathy represents the chronic counterpart to Guillain-Barre syndrome, distinguished by its progressive or relapsing course extending beyond eight weeks. The clinical presentation includes both proximal and distal weakness, distinguishing it from the purely distal length-dependent pattern of most polyneuropathies. Sensory involvement is typically significant, with large fiber modalities including vibration and proprioception prominently affected. Reflexes are diffusely reduced or absent. The relapsing-remitting form features discrete episodes of worsening followed by partial or complete recovery, while the progressive form demonstrates gradual deterioration over time. Unlike Guillain-Barre syndrome, which is self-limited, chronic inflammatory demyelinating polyneuropathy requires ongoing treatment to maintain stability.

The diagnosis of chronic inflammatory demyelinating polyneuropathy integrates clinical, electrodiagnostic, and cerebrospinal fluid criteria. Clinical criteria require progressive or relapsing weakness and sensory dysfunction in more than one limb developing over at least eight weeks, with hyporeflexia or areflexia. Electrodiagnostic studies must demonstrate demyelinating features including at least one of the following: markedly prolonged distal motor latency, significant reduction in motor conduction velocity, conduction block defined as a reduction in compound muscle action potential amplitude exceeding fifty percent across a nerve segment, or abnormal temporal dispersion. Cerebrospinal fluid protein is elevated in the majority of patients, and the presence of greater than ten white blood cells per microliter should prompt consideration of alternative diagnoses including HIV or Lyme disease.

Several clinical variants of chronic inflammatory demyelinating polyneuropathy are recognized with distinct presentations and sometimes different treatment responses. The typical form presents with symmetric proximal and distal weakness and sensory involvement. Multifocal acquired demyelinating sensory and motor neuropathy, or Lewis-Sumner syndrome, is an asymmetric variant with multifocal conduction block that may mimic mononeuropathy multiplex. Distal acquired demyelinating symmetric neuropathy presents predominantly distally and is often associated with an IgM monoclonal gammopathy and antibodies to myelin-associated glycoprotein. A predominantly sensory variant exists in which motor involvement is minimal despite demyelinating electrodiagnostic features.

Treatment of chronic inflammatory demyelinating polyneuropathy employs immunomodulatory therapies that suppress the immune-mediated demyelination. First-line options include intravenous immunoglobulin, corticosteroids, and plasma exchange, all of which have demonstrated efficacy. Intravenous immunoglobulin is typically administered as an induction course followed by maintenance infusions, with the frequency and dose titrated to maintain clinical stability. Corticosteroids, typically prednisone beginning at one milligram per kilogram daily, provide an alternative initial approach but carry risks of long-term complications. Plasma exchange is effective but requires central venous access and specialized facilities. For patients with inadequate response or requiring steroid-sparing therapy, immunosuppressive agents including azathioprine, mycophenolate mofetil, and rituximab may be employed. Subcutaneous immunoglobulin offers a home-based alternative to intravenous infusion for maintenance therapy.

<image>Panel A: CIDP versus GBS comparison showing time course, clinical features, and treatment differences. Panel B: Electrodiagnostic criteria for CIDP demonstrating demyelinating features with nerve conduction study examples. Panel C: CIDP variant comparison showing typical, MADSAM, DADS, and sensory variant features. Panel D: Treatment algorithm for CIDP showing first-line options, response assessment, and steroid-sparing progression.</image>

VI. Myasthenia Gravis

Myasthenia gravis is an autoimmune disorder of the neuromuscular junction characterized by antibody-mediated impairment of neuromuscular transmission, producing the cardinal feature of fatigable weakness. The term fatigable weakness specifically describes worsening strength with sustained or repeated muscle use, distinguishing myasthenia from the fixed weakness of neuropathy or myopathy. Ocular symptoms including ptosis and diplopia represent the most common initial manifestation, occurring as presenting symptoms in the majority of patients and eventually affecting virtually all patients with generalized disease. Bulbar symptoms including dysarthria, dysphagia, and facial weakness commonly develop, while limb weakness typically affects proximal muscles more than distal. Symptoms characteristically fluctuate, worsening later in the day or with exertion and improving with rest.

The immunopathology of myasthenia gravis involves antibodies directed against components of the neuromuscular junction. Antibodies against the nicotinic acetylcholine receptor are present in approximately eighty-five percent of patients with generalized myasthenia and a smaller proportion of those with purely ocular disease. These antibodies impair neuromuscular transmission through several mechanisms including receptor blockade, accelerated receptor internalization, and complement-mediated damage to the postsynaptic membrane. Antibodies against muscle-specific kinase are present in approximately five to ten percent of patients, particularly those with prominent bulbar and respiratory involvement, atrophy, and a tendency toward myasthenic crises. Antibodies to lipoprotein-related protein 4 and other components are found in smaller proportions, while approximately five to ten percent of patients remain seronegative.

The diagnostic evaluation of suspected myasthenia gravis includes serological testing, electrodiagnostic studies, and assessment for thymoma. Acetylcholine receptor antibodies are highly specific when positive but may be negative in ocular myasthenia and some generalized cases. Muscle-specific kinase antibodies should be tested when acetylcholine receptor antibodies are negative. Repetitive nerve stimulation demonstrates a decremental response, defined as a greater than ten percent reduction in compound muscle action potential amplitude between the first and fourth or fifth stimulus, reflecting the failure of neuromuscular transmission with repeated activation. Single-fiber electromyography is the most sensitive diagnostic test, demonstrating increased jitter (variability in transmission time) and blocking. Computed tomography or magnetic resonance imaging of the chest should be performed to evaluate for thymoma, present in ten to fifteen percent of patients, while thymic hyperplasia is found in the majority of younger patients.

The association between myasthenia gravis and thymic pathology has important therapeutic implications. Thymoma, when present, should be surgically removed regardless of myasthenia severity due to its malignant potential. Thymectomy is also recommended for patients younger than sixty-five years with generalized acetylcholine receptor antibody-positive myasthenia gravis even without thymoma, as it has been shown to improve outcomes and reduce the need for immunosuppression. The benefit of thymectomy in muscle-specific kinase antibody-positive myasthenia is less established. Thymic hyperplasia, characterized by germinal center formation within the thymus, is present in sixty to seventy percent of patients, particularly younger women, and may contribute to the autoimmune process.

<image>Panel A: Myasthenia gravis clinical features showing ocular, bulbar, respiratory, and limb involvement with characteristic fluctuation pattern. Panel B: Antibody profiles comparing AChR-positive, MuSK-positive, and seronegative myasthenia with clinical associations. Panel C: Repetitive nerve stimulation diagram showing normal response versus decremental response with amplitude measurements. Panel D: Thymic pathology spectrum showing normal thymus, thymic hyperplasia, and thymoma with management implications.</image>

VII. Myasthenia Gravis Treatment

The management of myasthenia gravis employs a tiered approach beginning with symptomatic treatment, adding immunosuppression for inadequate control, and utilizing thymectomy and rescue therapies as appropriate. Pyridostigmine, an acetylcholinesterase inhibitor, provides symptomatic relief by increasing the availability of acetylcholine at the neuromuscular junction. The typical starting dose of sixty milligrams three times daily is titrated based on response and tolerability, with most patients requiring doses between 180 and 480 milligrams daily. Cholinergic side effects including diarrhea, abdominal cramping, increased salivation, and bradycardia may limit dose escalation. Pyridostigmine alone may be sufficient for patients with mild ocular myasthenia, but most patients with generalized disease require immunosuppressive therapy.

Immunosuppressive treatment aims to reduce the autoimmune attack on the neuromuscular junction. Corticosteroids, typically prednisone, represent the most commonly used initial immunosuppressive agent, producing improvement in the majority of patients within weeks to months. An important caveat is that corticosteroids may produce transient worsening of weakness in the first one to two weeks of treatment, so initiation in patients with significant respiratory or bulbar involvement requires careful monitoring, often in a hospital setting, with low starting doses gradually increased. Steroid-sparing agents including azathioprine, mycophenolate mofetil, and cyclosporine are added for patients requiring high-dose steroids or experiencing intolerable steroid side effects. These agents have delayed onset of effect, typically requiring months to achieve full benefit. Rituximab has demonstrated particular efficacy in muscle-specific kinase antibody-positive myasthenia gravis.

Thymectomy plays an important role in the management of myasthenia gravis, particularly in younger patients with generalized disease. For patients with thymoma, surgical removal is mandatory regardless of age or myasthenia severity. For patients without thymoma who have generalized acetylcholine receptor antibody-positive disease and are younger than sixty-five years, thymectomy improves clinical outcomes and reduces the need for immunosuppression. The procedure should ideally be performed when the patient is clinically stable, with optimization of myasthenia control beforehand. Robotic and video-assisted thoracoscopic approaches have reduced the morbidity compared to transsternal approaches. The benefit of thymectomy accrues over years, with continued improvement seen even after the first year.

Rapid immunotherapies including intravenous immunoglobulin and plasma exchange provide relatively quick improvement and are reserved for specific situations. These therapies are indicated for myasthenic crisis, preoperative optimization before thymectomy or other surgery, and as a bridge during initiation of slower-acting immunosuppression. The effect of these treatments is temporary, typically lasting several weeks, so they are not suitable as long-term monotherapy. Intravenous immunoglobulin is administered at two grams per kilogram divided over two to five days. Plasma exchange typically involves five to seven exchanges performed every other day. Complement inhibitors including eculizumab and neonatal Fc receptor antagonists including efgartigimod represent newer therapeutic options for refractory disease.

<image>Panel A: Treatment pyramid showing symptomatic therapy base, immunosuppression middle tier, and rescue therapies at the apex with appropriate indications. Panel B: Pyridostigmine dosing and side effect management with adjustment strategies. Panel C: Corticosteroid initiation protocol showing low-start approach for high-risk patients and monitoring for transient worsening. Panel D: Thymectomy indications and outcomes showing benefit timeline and patient selection criteria.</image>

VIII. Myasthenic Crisis

Myasthenic crisis represents a life-threatening complication of myasthenia gravis defined by respiratory failure requiring mechanical ventilation or airway protection. Crisis occurs in approximately fifteen to twenty percent of myasthenia gravis patients, typically within the first two years of diagnosis, though it may occur at any time. Precipitants include infection, particularly respiratory infections, surgical procedures, medication changes, tapering of immunosuppression, and exposure to medications that impair neuromuscular transmission. Drugs that may precipitate or worsen myasthenia include aminoglycoside antibiotics, fluoroquinolones, magnesium, beta-blockers, and certain cardiac antiarrhythmics. Crisis may also occur de novo as the presenting manifestation of myasthenia gravis.

Assessment of respiratory function is critical in patients with myasthenic weakness and relies on bedside pulmonary function testing rather than arterial blood gas analysis. Forced vital capacity and negative inspiratory force provide objective measures that predict the need for mechanical ventilation before respiratory failure develops. The rule of twenty-thirty-forty provides guidance: forced vital capacity below twenty milliliters per kilogram, negative inspiratory force weaker than minus thirty centimeters of water, or a decline of more than forty percent from baseline should prompt consideration of intubation. Single-breath count, the number of integers a patient can count in one breath, provides a rapid bedside assessment correlating with forced vital capacity. Arterial blood gas abnormalities represent late findings in neuromuscular respiratory failure because patients can maintain oxygenation and carbon dioxide elimination until weakness is profound.

Management of myasthenic crisis requires intensive care unit admission with close monitoring, respiratory support, immunotherapy, and identification and treatment of precipitants. Intubation should be performed electively when respiratory function declines rather than emergently after respiratory arrest. Intravenous immunoglobulin or plasma exchange should be initiated promptly, as both are equally effective. Some experts favor plasma exchange for its more rapid onset of effect. Pyridostigmine is typically held during crisis to reduce respiratory secretions and because the benefit is minimal in the setting of severe weakness. Identification and treatment of the precipitating factor, particularly infection, is essential. Corticosteroids may be initiated or increased with awareness of potential transient worsening. Most patients require mechanical ventilation for days to weeks before recovering sufficient strength for extubation.

Lambert-Eaton myasthenic syndrome represents an important disorder of the neuromuscular junction with distinct features from myasthenia gravis. This condition results from antibodies directed against voltage-gated calcium channels on the presynaptic motor nerve terminal, impairing acetylcholine release. Patients experience proximal weakness, particularly in the legs, along with autonomic symptoms including dry mouth, constipation, and erectile dysfunction. A distinguishing clinical feature is post-exercise facilitation, in which strength transiently improves immediately after brief maximal effort. Reflexes are typically diminished but may briefly normalize after exercise. Approximately fifty to sixty percent of patients have an underlying malignancy, most commonly small cell lung cancer, making cancer screening mandatory. Electrodiagnostic testing shows low compound muscle action potential amplitudes that increment by more than one hundred percent with high-frequency repetitive stimulation or immediately after brief exercise. Treatment addresses the underlying malignancy when present and includes symptomatic therapy with 3,4-diaminopyridine and immunosuppression.

<image>Panel A: Myasthenic crisis precipitant categories including infection, surgery, medications, and immunosuppression changes with specific examples. Panel B: Respiratory monitoring protocol showing FVC, NIF, and single-breath count thresholds with decision points for intubation. Panel C: Crisis management algorithm showing ICU admission, intubation criteria, immunotherapy choice, and precipitant workup. Panel D: Lambert-Eaton myasthenic syndrome comparison with myasthenia gravis showing clinical features, antibodies, electrodiagnostics, and cancer association.</image>

IX. Myopathies

Myopathies comprise a diverse group of muscle diseases characterized by weakness predominantly affecting proximal musculature, with preservation of sensation and typically maintenance of reflexes until late in the disease course. The clinical pattern of proximal weakness manifests as difficulty rising from a seated position, climbing stairs, and raising arms overhead. Gower's sign, in which patients use their hands to push up their thighs when rising from the floor, reflects hip girdle weakness. Serum creatine kinase is elevated in most myopathies, reflecting muscle fiber damage, though the degree of elevation varies considerably. The neck flexors are commonly affected, producing difficulty lifting the head from a pillow. Facial involvement occurs in some myopathies but is absent in others, providing a useful diagnostic feature.

The inflammatory myopathies represent an important group of acquired, potentially treatable conditions. Polymyositis presents with symmetric proximal weakness developing subacutely over weeks to months, with elevated creatine kinase typically in the thousands. Dermatomyositis shares the muscle features of polymyositis but is distinguished by characteristic cutaneous findings including heliotrope rash, a violaceous discoloration of the upper eyelids, and Gottron papules, scaly erythematous lesions over the metacarpophalangeal and interphalangeal joints. Dermatomyositis carries an increased risk of underlying malignancy, particularly in adults over forty, necessitating cancer screening. Inclusion body myositis differs from other inflammatory myopathies in its clinical features, pathology, and poor response to immunotherapy. Patients are typically older than fifty and demonstrate an asymmetric pattern with prominent involvement of finger flexors and knee extensors, producing a characteristic weakness pattern with grip weakness and falls due to quadriceps involvement. Immune-mediated necrotizing myopathy, associated with antibodies to signal recognition particle or HMG-CoA reductase, may occur in association with statin exposure.

The muscular dystrophies are inherited disorders characterized by progressive muscle degeneration. Duchenne muscular dystrophy, the most common childhood muscular dystrophy, results from X-linked mutations in the dystrophin gene producing absent dystrophin protein. Boys present between three and five years with progressive proximal weakness, calf pseudohypertrophy, and markedly elevated creatine kinase, becoming wheelchair-dependent by adolescence and dying from respiratory or cardiac complications. Becker muscular dystrophy results from dystrophin gene mutations producing reduced or dysfunctional rather than absent protein, with a milder phenotype and later onset. Myotonic dystrophy, an autosomal dominant condition caused by CTG trinucleotide repeat expansion, produces the unique combination of weakness with myotonia (delayed muscle relaxation), along with multisystem involvement including cataracts, cardiac conduction abnormalities, and endocrine dysfunction. Limb-girdle and facioscapulohumeral muscular dystrophies represent additional forms with specific patterns and genetic bases.

Metabolic myopathies result from defects in muscle energy metabolism, typically presenting with exercise intolerance, cramps, and myoglobinuria. Glycogen storage diseases, including McArdle disease (myophosphorylase deficiency), impair glycogen utilization, producing fatigue and cramps with exertion and the characteristic second wind phenomenon in which exercise tolerance improves after brief rest. Lipid storage myopathies affect fatty acid oxidation, producing symptoms with prolonged exercise. Mitochondrial myopathies result from mutations in mitochondrial or nuclear DNA affecting the respiratory chain, producing variable phenotypes often including ptosis and ophthalmoparesis, exercise intolerance, and multisystem involvement. Muscle biopsy in mitochondrial disease classically shows ragged red fibers representing subsarcolemmal mitochondrial accumulation.

<image>Panel A: Myopathy clinical pattern showing proximal weakness distribution, Gower's maneuver, and typical CK elevation ranges by myopathy type. Panel B: Inflammatory myopathy comparison showing polymyositis, dermatomyositis, and inclusion body myositis features with skin findings illustrations. Panel C: Muscular dystrophy overview showing Duchenne, Becker, myotonic, and limb-girdle dystrophies with inheritance patterns and key features. Panel D: Metabolic myopathy presentation with exercise intolerance mechanism and second wind phenomenon in McArdle disease.</image>

X. Diagnostic Approach to Myopathy

The evaluation of suspected myopathy integrates clinical assessment, laboratory testing, electrodiagnostic studies, and often muscle biopsy to establish diagnosis and guide treatment. Serum creatine kinase is the most important laboratory marker, typically elevated in myopathy though the degree varies considerably. Very high elevations in the tens of thousands suggest active inflammatory myopathy, rhabdomyolysis, or some dystrophies, while mild elevations may occur with less destructive processes or in advanced disease with little muscle remaining. Aldolase and lactate dehydrogenase are less specific markers of muscle injury. Myositis-specific antibodies, including anti-Jo-1 and other antisynthetase antibodies, anti-Mi-2, anti-MDA5, anti-SRP, and anti-HMGCR, help classify inflammatory myopathies and have prognostic implications. Genetic testing has become increasingly important for diagnosing muscular dystrophies and other inherited myopathies.

Electrodiagnostic testing provides supportive evidence for myopathy and helps exclude neuropathic conditions. Nerve conduction studies are typically normal in pure myopathy. Needle electromyography demonstrates characteristic myopathic features including short-duration, small-amplitude, polyphasic motor unit action potentials and early recruitment, reflecting the loss of muscle fibers within motor units and compensatory recruitment of additional units to generate force. Increased insertional activity with fibrillation potentials and positive sharp waves occurs in inflammatory, necrotizing, and some dystrophic myopathies, reflecting muscle fiber irritability. Myotonic discharges, consisting of waxing and waning trains of discharges producing a characteristic sound on the audio speaker, are diagnostic of myotonic disorders.

Muscle biopsy provides definitive histopathological diagnosis in many cases and is particularly valuable when the etiology remains uncertain after clinical and laboratory evaluation. The biopsy site should be selected based on clinical involvement, typically a moderately affected muscle, avoiding severely atrophied or recently needled muscles. Light microscopy assesses fiber size variability, internal nuclei, necrosis, regeneration, inflammation, and structural abnormalities. Immunohistochemistry for dystrophin and other proteins aids in diagnosing muscular dystrophies. Electron microscopy evaluates ultrastructural abnormalities in storage diseases and mitochondrial myopathies. Inflammatory myopathies demonstrate characteristic patterns: perimysial inflammation and perifascicular atrophy in dermatomyositis, endomysial inflammation with invasion of non-necrotic fibers in polymyositis, and rimmed vacuoles with inflammatory infiltrates in inclusion body myositis.

Treatment of inflammatory myopathies relies primarily on immunosuppressive therapy. High-dose corticosteroids, typically prednisone at one milligram per kilogram daily, represent first-line treatment for polymyositis, dermatomyositis, and immune-mediated necrotizing myopathy. Response is monitored through clinical strength assessment and creatine kinase levels, with steroid tapering once improvement occurs. Steroid-sparing agents including methotrexate, azathioprine, and mycophenolate mofetil are added for inadequate response or to facilitate steroid reduction. Intravenous immunoglobulin is effective particularly in dermatomyositis and may be used as first-line or rescue therapy. Inclusion body myositis, unfortunately, responds poorly to immunosuppression, and management focuses on physical therapy to maintain function and address swallowing and fall prevention. Cancer screening is important in dermatomyositis given the association with malignancy. Exercise programs are beneficial across myopathy types and should be incorporated into management.

<image>Panel A: Myopathy laboratory evaluation showing CK interpretation, myositis-specific antibody panel, and genetic testing indications. Panel B: EMG findings in myopathy showing myopathic motor unit potentials versus normal and neuropathic patterns, with myotonic discharge waveform. Panel C: Muscle biopsy technique showing site selection, processing, and key histopathological findings by disease type. Panel D: Inflammatory myopathy treatment algorithm showing corticosteroid induction, response assessment, steroid-sparing progression, and IBM management differences.</image>

Summary

• Neuromuscular localization: nerve (distal weakness, sensory loss, hyporeflexia), NMJ (fatigable weakness, no sensory), muscle (proximal weakness, preserved sensation, elevated CK)
• Polyneuropathy patterns: length-dependent (distal symmetric) versus non-length-dependent (patchy, proximal suggests inflammatory)
• Electrodiagnostics: axonal neuropathy (reduced amplitude) versus demyelinating (slow velocity, conduction block)
• Guillain-Barre syndrome: acute ascending weakness, areflexia, CSF albuminocytologic dissociation; treat with IVIG or plasma exchange; monitor respiratory function closely
• CIDP: chronic course exceeding eight weeks, proximal and distal weakness; responds to IVIG, corticosteroids, or plasma exchange
• Myasthenia gravis: fatigable weakness, AChR or MuSK antibodies, decremental response on repetitive stimulation
• Myasthenic crisis: monitor FVC (intubate if less than 20 mL/kg), NIF, and single-breath count; treat with IVIG or plasma exchange
• Lambert-Eaton myasthenic syndrome: proximal weakness with post-exercise facilitation, autonomic symptoms, associated with small cell lung cancer in fifty to sixty percent
• Myopathy: proximal weakness, elevated CK, preserved reflexes; inflammatory myopathies treatable with immunosuppression
• Inclusion body myositis: older patients, finger flexor and quadriceps weakness, poor response to immunotherapy

Key Terms

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