Clinical Analysis: 40F, Generalized Weakness x 8 Days, No Comorbidities

Localizing the Lesion First

Differential Diagnoses (Prioritized)

1. Inflammatory Myopathies - TOP PRIORITY in a 40-year-old woman

• Classic presentation: subacute proximal weakness of arms and legs over weeks-months; can present acutely
• Core criteria: subacute proximal weakness + elevated serum creatine kinase + endomysial inflammation on muscle biopsy
• No skin rash (distinguishes from dermatomyositis)
• Normal or mildly elevated CK at presentation is possible
• Goldman-Cecil Medicine: "Most patients with dermatomyositis, polymyositis, or immune-mediated necrotizing myopathy present with subacute proximal weakness of the arms and legs progressing over months, though these diseases may present acutely."

• Identical muscle weakness pattern to PM, but with characteristic skin lesions (heliotrope rash, Gottron papules, shawl sign, periungual telangiectasias)
• Must specifically look for these rashes - their presence clinches the diagnosis
• Important: associated with occult malignancy in adults - screening is mandatory
• Goldman-Cecil Medicine: "Because dermatomyositis is associated with malignancy, appropriate laboratory and radiologic studies should be performed to search for underlying malignancy in all newly diagnosed patients."

• Acute or subacute proximal weakness identical to PM/DM
• Associated with anti-HMGR or anti-SRP antibodies
• May be paraneoplastic
• Muscle biopsy shows necrotic/regenerating fibers without significant inflammation

2. Endocrine Myopathies

• Proximal muscle weakness, muscle stiffness, exercise intolerance
• TSH not mentioned in workup - this is a critical gap
• Reflexes may be slow (delayed relaxation), serum CK elevated
• Firestein Rheumatology: "Myopathy due to thyroid disease is characterized primarily by proximal muscle weakness and muscle wasting... more than 75% of hypothyroid patients have neuromuscular symptoms."

• Proximal weakness with normal or low CK
• Thyrotoxic periodic paralysis causes episodic weakness with hypokalemia - though her potassium is implied normal
• TSH is mandatory

• Insidious proximal weakness, predominantly legs
• Normal serum muscle enzymes
• Look for other features: moon face, striae, hypertension, hirsutism

3. Myasthenia Gravis

• Weakness that fluctuates and worsens with exertion (fatigable weakness)
• Proximal limbs + bulbar (ptosis, diplopia, dysarthria, dysphagia)
• Reflexes typically preserved - fits this case
• Onset peak in women of reproductive and middle age
• Anti-AChR or anti-MuSK antibodies
• This is a neuromuscular junction disorder, not a true myopathy - but presents very similarly

4. Viral / Infectious Myositis

• Acute/subacute myopathy post-viral illness (influenza, Coxsackie, COVID-19, EBV, HIV)
• Associated with elevated CK, myalgias
• No fever in this patient reduces (but doesn't exclude) this; viral prodrome may have resolved

5. Metabolic Causes - Partially Excluded

• Hypokalemia, hyperkalemia - excluded by electrolytes (though not explicitly stated)
• Hypophosphatemia - should be checked
• Normal Ca, Mg, renal function already ruled out several metabolic causes
• Note: normal calcium does NOT rule out hypokalemia (need separate serum potassium)

6. Medication / Toxin-Induced Myopathy

• Statins (statin myopathy, IMNM) - check medication history
• Alcohol-related myopathy
• Other drugs: hydroxychloroquine, colchicine, fibrates

7. Paraneoplastic Syndrome

• Subacute generalized weakness in a middle-aged woman without prior illness raises the possibility of an underlying malignancy (breast, ovary, lung)
• PM/DM themselves are paraneoplastic in a proportion of cases

Summary: Most Likely Diagnoses

1. Polymyositis / Inflammatory myopathy - most likely
2. Hypothyroid myopathy - high on list since TSH not checked
3. Myasthenia Gravis - must exclude
4. Dermatomyositis with malignancy - screen for skin rashes and malignancy
5. Viral/post-infectious myositis - history of preceding illness?

Investigations

Management Approach

Initial / General

• Admit if weakness is progressing or affecting respiratory muscles - check FVC and NIF
• Monitor for respiratory involvement (both PM/DM and MG can cause respiratory failure)
• Determine if she can swallow safely (bulbar function)

Specific (once diagnosis confirmed)

• High-dose corticosteroids: Prednisolone 1 mg/kg/day (up to 60-80 mg/day) as first line
• Steroid-sparing agents: Methotrexate or Azathioprine added early to reduce steroid dose
• IVIG for refractory cases or DM
• Rituximab for anti-Jo-1 or anti-SRP positive disease
• Mandatory malignancy screening before immunosuppression

• Treat the underlying thyroid disease
• Hypothyroid: Levothyroxine replacement - weakness resolves with normalization of thyroid function
• Hyperthyroid: Anti-thyroid drugs / RAI / surgery; treat the thyrotoxicosis

• Pyridostigmine (acetylcholinesterase inhibitor) as symptomatic therapy
• Prednisolone + immunosuppression (azathioprine/mycophenolate)
• IVIG or plasmapheresis for acute exacerbations
• CT chest to look for thymoma; thymectomy indicated if present (and in generalized MG < 60 years)

• Supportive care, NSAIDs for myalgias
• Rest and hydration
• Monitor CK for rhabdomyolysis

Monitoring

• Regular assessment of muscle power (MRC scale)
• Respiratory function (FVC, peak cough flow)
• Swallowing assessment
• Repeat CK levels to track response to treatment

Sources: Goldman-Cecil Medicine (26th ed.), Firestein & Kelley's Textbook of Rheumatology, Bradley and Daroff's Neurology in Clinical Practice, AAFP - Muscle Weakness in Adults (2020)