【General Internal Medicine】What are the causes of bilateral leg weakness?

Weakness in both legs—also termed bilateral lower extremity weakness—can arise from a wide range of underlying conditions, spanning neurological, muscular, metabolic, vascular, and systemic causes. A thorough clinical evaluation is essential to determine the origin, as the differential diagnosis is broad and often time-sensitive.

Neurological etiologies are among the most critical to consider. Spinal cord disorders—including compressive lesions (e.g., herniated disc, spinal stenosis, or metastatic epidural tumor), inflammatory conditions (e.g., transverse myelitis), or demyelinating diseases (e.g., multiple sclerosis)—commonly present with symmetric or asymmetric bilateral leg weakness, often accompanied by sensory changes, bowel or bladder dysfunction, or spasticity. Peripheral neuropathies—particularly those that are length-dependent and symmetric, such as diabetic or chronic inflammatory demyelinating polyneuropathy (CIDP)—may cause distal weakness, but typically spare proximal strength early on. Central nervous system involvement, such as stroke affecting bilateral motor pathways (e.g., brainstem or high cervical infarction) or neurodegenerative disorders like amyotrophic lateral sclerosis (ALS), may also manifest with progressive bilateral leg weakness.

Muscular causes include primary myopathies such as polymyositis, dermatomyositis, or inherited muscular dystrophies, which often present with proximal-predominant weakness, fatigue, and elevated serum creatine kinase (CK). Electrolyte disturbances—especially severe hypokalemia, hypophosphatemia, or hyponatremia—can induce acute or subacute generalized weakness, including in the lower limbs. Endocrine disorders, including untreated hypothyroidism (myxedema), hyperthyroidism (thyrotoxic myopathy), or Cushing syndrome, may also contribute to muscle fatigue and proximal weakness.

Vascular insufficiency—such as bilateral iliac or aortoiliac occlusive disease (e.g., Leriche syndrome)—can produce claudication, rest pain, and eventually atrophy and weakness due to chronic ischemia. Systemic illnesses, including advanced heart failure, chronic kidney disease, malignancy-associated paraneoplastic syndromes, or severe malnutrition, must also be considered, particularly when weakness is accompanied by weight loss, fever, or constitutional symptoms.

A targeted history—including onset (acute vs. subacute vs. chronic), progression, associated symptoms (e.g., numbness, gait instability, urinary retention, rash, or dysphagia), medication review (e.g., statins, corticosteroids), and comorbidities—is foundational. Physical examination should assess muscle bulk, tone, strength distribution (proximal vs. distal), reflexes, sensation, gait, and signs of systemic disease. Diagnostic workup may include blood tests (CBC, electrolytes, CK, TSH, glucose, HbA1c, vitamin B12, ESR/CRP), nerve conduction studies and electromyography (NCS/EMG), lumbar spine MRI, brain or cervical spine imaging as indicated, and occasionally muscle or nerve biopsy.

Given the potential for serious, treatable, or rapidly progressive conditions, new-onset or worsening bilateral leg weakness warrants prompt medical evaluation. Early diagnosis and intervention—whether surgical decompression for spinal cord compression, immunomodulatory therapy for inflammatory neuropathies or myositis, or correction of metabolic derangements—can significantly impact functional outcomes and prognosis.