What are the clinical presentations of Lambert-Eaton Myasthenic Syndrome (LEMS) and polymyositis?

Clinical Presentations of Lambert-Eaton Myasthenic Syndrome (LEMS) and Polymyositis

Lambert-Eaton Myasthenic Syndrome (LEMS) and polymyositis present with distinct clinical features that require careful assessment for accurate diagnosis and timely treatment to improve morbidity, mortality, and quality of life outcomes.

Lambert-Eaton Myasthenic Syndrome (LEMS)

Key Clinical Presentation

• Proximal muscle weakness: Primarily affects hip and shoulder muscles, causing difficulties in standing up, lifting arms, and moving around 1
• Muscle weakness pattern: Characteristically craniocaudally progressive, predominantly at the hip girdle 2
• Areflexia or hyporeflexia: Reduced or absent muscle stretch reflexes 3
• Autonomic dysfunction: Common features include dry mouth, constipation, and erectile dysfunction 4
• Post-exercise facilitation: Distinctive feature where brief exercise temporarily improves muscle strength 1

Diagnostic Features

• Approximately 60% of LEMS cases are paraneoplastic, most commonly associated with small cell lung cancer (SCLC-LEMS) 1
• 40% have non-tumor etiology associated with underlying autoimmune disease (NT-LEMS) 1
• Presence of antibodies against P/Q-type voltage-gated calcium channels (VGCCs) in most patients 2, 4
• Characteristic electromyographic findings: low-amplitude muscle responses that increase dramatically (>100%) after brief exercise 3

Important Distinctions

• Unlike myasthenia gravis, ocular and bulbar symptoms are less common and less severe 3
• LEMS should trigger an intensive search for malignancy, especially in older patients with smoking history 3
• LEMS can be a clinically important early indicator of cancer 1

Polymyositis

Key Clinical Presentation

• Symmetric proximal muscle weakness: Primarily affecting the shoulders and hips 2
• Progressive weakness pattern: Difficulties in standing up, lifting arms, and moving around 2
• Muscle pain (myalgia): Present in severe cases 2
• Systemic symptoms: May include fever, weight loss, and fatigue 2
• Absence of skin rash: Unlike dermatomyositis, polymyositis does not have cutaneous manifestations 5

Diagnostic Features

• Elevated muscle enzymes, especially creatine kinase (CK), which is often markedly elevated 2
• Elevated inflammatory markers (ESR, CRP) 2
• Distinct immunohistopathologic phenotype involving CD8+ cytotoxic T cells invading non-necrotic muscle fibers 2
• Electromyography (EMG) shows muscle fibrillations indicative of myopathy 2
• MRI can show increased intensity and edema in affected muscles 2
• Muscle biopsy may be required to confirm diagnosis 2

Key Differences and Diagnostic Pitfalls

Distinguishing Features

1. Pattern of weakness:

   • LEMS: Weakness improves after brief exercise (post-exercise facilitation)
   • Polymyositis: Weakness is persistent and does not improve with exercise

2. Associated findings:

   • LEMS: Autonomic symptoms and areflexia are prominent
   • Polymyositis: Systemic inflammatory symptoms may be present

3. Laboratory findings:

   • LEMS: Anti-VGCC antibodies, normal or mildly elevated CK
   • Polymyositis: Markedly elevated CK, no specific antibody pattern (though myositis-specific antibodies may be present)

Common Pitfalls to Avoid

• Misdiagnosing LEMS as polymyositis due to similar proximal weakness pattern 6
• Failing to screen for underlying malignancy in LEMS patients 1
• Overlooking the rare co-occurrence of both conditions in the same patient 6
• Confusing polymyositis with polymyalgia-like syndrome, which has pain but not true weakness 2
• Delaying diagnosis of LEMS in patients with pre-existing autoimmune disorders 6

Clinical Approach to Suspected Cases

For Suspected LEMS:

1. Perform detailed neurological examination focusing on:

   • Proximal muscle strength before and after brief exercise
   • Deep tendon reflexes before and after exercise
   • Autonomic symptoms assessment

2. Order diagnostic tests:

   • Electromyography with repetitive nerve stimulation
   • Anti-VGCC antibody testing
   • Comprehensive malignancy screening, especially for SCLC

For Suspected Polymyositis:

1. Perform comprehensive muscle examination focusing on:

   • Symmetric proximal muscle strength
   • Presence of muscle tenderness
   • Absence of skin manifestations

2. Order diagnostic tests:

   • Muscle enzymes (CK, aldolase)
   • Inflammatory markers (ESR, CRP)
   • Myositis-specific autoantibody panel
   • MRI of affected muscles
   • EMG
   • Consider muscle biopsy for definitive diagnosis

Early recognition of these distinct clinical presentations is crucial for appropriate management and improved outcomes, particularly since LEMS may be an early indicator of underlying malignancy requiring urgent attention.