Residency · Residency · Emergency Medicine

Acute Weakness and Neuromuscular Emergencies

Systematic Approach to Weakness in the ED

Localization Framework

The single most important step in evaluating acute weakness is determining where in the nervous system the lesion is located. Upper motor neuron (UMN) lesions involve the brain or spinal cord and produce spasticity, hyperreflexia, a positive Babinski sign, and no fasciculations or atrophy acutely. Lower motor neuron (LMN) lesions involve the anterior horn cell, nerve root, or peripheral nerve and produce flaccidity, hyporeflexia or areflexia, fasciculations, and atrophy. Neuromuscular junction (NMJ) disorders produce fatigable, fluctuating weakness with normal reflexes and no sensory involvement. Myopathies produce proximal weakness with normal to decreased reflexes, no sensory involvement, and an elevated CK.

Key Questions

Several key questions guide the evaluation. The onset — acute (hours to days), subacute (weeks), or chronic (months) — narrows the differential considerably. The distribution — proximal versus distal, symmetric versus asymmetric, ascending versus descending — further refines the localization. Associated features such as sensory loss, pain, autonomic dysfunction, and bulbar symptoms (dysphagia, dysarthria, and diplopia) add diagnostic specificity. The progression pattern is particularly important: ascending weakness suggests GBS, descending weakness suggests botulism, and fluctuating weakness suggests myasthenia gravis.

Guillain-Barre Syndrome (GBS)

Pathophysiology

Guillain-Barre syndrome is an acute inflammatory demyelinating polyneuropathy (AIDP) in its most common subtype. It represents an autoimmune attack on peripheral nerve myelin or axons and is often post-infectious, with Campylobacter jejuni as the most common trigger, followed by CMV, EBV, Zika, influenza, and COVID-19. A post-vaccination association has been reported but is extremely rare.

Clinical Presentation

GBS presents with ascending, symmetric weakness beginning in the lower extremities. Hyporeflexia or areflexia is the hallmark finding. Sensory symptoms — paresthesias and pain — often precede the weakness. The disease typically reaches its nadir at 2 to 4 weeks but can progress to complete paralysis. Autonomic dysfunction, including labile blood pressure, tachycardia or bradycardia, urinary retention, and ileus, is common. The critical concern is respiratory failure, which develops in 20 to 30 percent of patients and requires intubation.

Respiratory Monitoring

Forced vital capacity (FVC) is the single most important parameter — a value below 20 mL/kg predicts the need for intubation. Negative inspiratory force (NIF) less negative than negative 30 cmH2O also predicts respiratory failure. The 20/30/40 rule provides a practical decision aid: FVC below 20 mL/kg, NIF less negative than negative 30, or greater than 30 percent decline from baseline warrants intubation. Serial measurements every 2 to 4 hours are essential. Pulse oximetry alone must not be relied upon because desaturation is a dangerously late finding. The single breath count — inability to count to 20 in one breath — suggests an FVC below 1 liter.

Diagnostic Workup

CSF analysis reveals the classic albuminocytologic dissociation: elevated protein with a normal white blood cell count. However, this may be normal in the first week, and a normal LP does not exclude the diagnosis. Nerve conduction studies and EMG are confirmatory but are not needed emergently for a clinical diagnosis. MRI of the spine can show nerve root enhancement and is primarily used to exclude spinal cord pathology. The disposition of suspected early GBS with a normal workup is challenging — patients can progress rapidly, and safe discharge from the ED is risky even with mild symptoms.

Treatment

IVIG at 0.4 g/kg/day for 5 days is first-line in most centers. Plasmapheresis is equally effective and typically involves 5 exchanges over 2 weeks. Corticosteroids are not effective in GBS, which is notable because this is unlike most autoimmune conditions. Supportive care includes DVT prophylaxis, pain management, respiratory monitoring, and ICU admission for rapidly progressive disease.

Prognosis

Eighty percent of patients achieve independent walking within 6 months. Mortality is 5 to 10 percent, primarily from respiratory failure and autonomic instability. Poor prognostic factors include rapid progression, need for ventilation, the axonal subtype, Campylobacter infection, and older age.

Myasthenia Gravis (MG) and Myasthenic Crisis

Pathophysiology

Myasthenia gravis is mediated by autoimmune antibodies against acetylcholine receptors (AChR-Ab, present in 85 percent) or muscle-specific kinase (MuSK-Ab, present in 5 to 8 percent) at the neuromuscular junction. It has a bimodal age distribution, affecting young women in their 20s to 30s and older men in their 60s to 70s. It is associated with thymic abnormalities: thymoma in 10 to 15 percent and thymic hyperplasia in many others.

Clinical Presentation

The defining feature of myasthenia gravis is fatigable weakness that worsens with repetitive use and improves with rest. Ocular symptoms — ptosis and diplopia — are the most common initial presentation, occurring in 50 percent of patients. Bulbar symptoms include dysarthria, dysphagia, nasal speech, and jaw fatigue with difficulty chewing. Generalized disease produces proximal limb weakness and neck flexor weakness. Reflexes are normal (because the disease is at the neuromuscular junction, not the nerve or muscle), and sensation is normal.

Myasthenic Crisis

Myasthenic crisis is respiratory failure due to weakness of the respiratory muscles and constitutes a medical emergency. Common triggers include infection (the most common), surgery, medication changes, and medication non-compliance. The same 20/30/40 rule used in GBS applies: FVC below 20 mL/kg or NIF less negative than negative 30 cmH2O warrants intubation. BiPAP may serve as a bridge, but preparation for intubation should not be delayed. Succinylcholine should be avoided for intubation because myasthenia gravis patients are resistant to it; rocuronium at a reduced dose is used instead.

Medications That Worsen MG — CRITICAL KNOWLEDGE

Several medication classes can precipitate or worsen myasthenic crisis and must be avoided: aminoglycosides, fluoroquinolones, and macrolides among the antibiotics; beta-blockers and calcium channel blockers; magnesium sulfate; phenytoin; D-penicillamine; botulinum toxin; and neuromuscular blocking agents (patients have increased sensitivity to non-depolarizing agents).

Treatment

Myasthenic crisis is treated with IVIG (0.4 g/kg/day for 5 days) or plasmapheresis (5 exchanges). Chronic management includes acetylcholinesterase inhibitors (pyridostigmine), immunosuppression (prednisone, azathioprine, or mycophenolate), and thymectomy. Cholinergic crisis — over-treatment with acetylcholinesterase inhibitors causing excessive acetylcholine — presents with SLUDGE symptoms (salivation, lacrimation, urination, defecation, GI distress, and emesis) and is treated by holding pyridostigmine. Differentiating cholinergic crisis from myasthenic crisis was traditionally done with the edrophonium test, though this is rarely performed now.

Transverse Myelitis

Pathophysiology

Transverse myelitis is an inflammatory demyelination of the spinal cord. It can be idiopathic, post-infectious, or associated with multiple sclerosis, neuromyelitis optica (NMO/NMOSD), or sarcoidosis.

Clinical Presentation

The onset is acute to subacute (hours to days) with bilateral, usually asymmetric, motor, sensory, and autonomic dysfunction. A sensory level on examination is the key finding. Back pain at the level of the lesion is common, and urinary retention is often an early feature.

Diagnostic Workup

MRI of the spine with gadolinium reveals T2 signal abnormality within the cord, often spanning more than 2 vertebral segments. It is critical to distinguish transverse myelitis from compressive myelopathy (epidural abscess, tumor, or hematoma), and MRI is essential for this differentiation. CSF analysis shows pleocytosis and elevated protein. NMO-IgG (aquaporin-4 antibody) and MOG antibody testing should be considered. MRI of the brain evaluates for MS-associated demyelinating lesions.

Treatment

High-dose IV methylprednisolone (1 g/day for 3 to 5 days) is first-line. Plasmapheresis is used if the condition is steroid-refractory. IVIG is an alternative. The underlying etiology should be addressed, and it is important to note that NMO treatment differs from MS treatment.

Other Neuromuscular Emergencies

Botulism

Botulism presents with descending paralysis — the opposite direction from GBS. Cranial nerves are affected first, producing diplopia, ptosis, and bulbar weakness, followed by descending involvement of the limbs and respiratory muscles. Types include foodborne (from contaminated food), wound (associated with injection drug use), and infant (associated with honey ingestion). Autonomic involvement includes dry mouth, constipation, and urinary retention. Diagnosis is clinical, with confirmation by toxin assay (which takes days), so treatment is empiric. Treatment consists of botulinum antitoxin (heptavalent) and supportive care, including prolonged ventilatory support if needed. Public health authorities should be contacted immediately.

Tick Paralysis

Tick paralysis presents with ascending paralysis mimicking GBS, caused by a neurotoxin in tick saliva (Dermacentor species). The key difference from GBS is rapid improvement within hours of tick removal. A thorough skin examination including the scalp must be performed to find and remove the tick. CSF and nerve conduction studies are normal, which distinguishes tick paralysis from GBS.

Spinal Cord Compression

Spinal cord compression is a surgical emergency regardless of etiology. Epidural abscess presents with the classic triad of fever, back pain, and neurologic deficit and requires emergent MRI. Epidural metastatic disease presents with a cancer history, back pain, and progressive weakness. Epidural hematoma presents with anticoagulation use, sudden back pain, and weakness. All require emergent MRI and decompression. Dexamethasone 10 mg IV should be given for suspected malignant cord compression while awaiting imaging.

<image>A comparison table diagram showing the key differentiating features of four neuromuscular emergencies: GBS, myasthenic crisis, botulism, and transverse myelitis. For each condition, the diagram shows: direction of weakness progression (ascending, fatigable, descending, sensory level), reflex pattern (areflexia, normal, diminished, UMN pattern below level), sensory findings (paresthesias, none, none, sensory level), autonomic features (labile BP/HR, none, dry mouth/constipation, bladder dysfunction), respiratory monitoring (FVC/NIF, FVC/NIF, clinical, depends on level), key diagnostic test (CSF protein/NCS, AChR antibodies/EMG, toxin assay, MRI spine), and treatment (IVIG/PLEX, IVIG/PLEX, antitoxin, IV steroids).</image>

<image>A clinical illustration showing the bedside respiratory assessment for neuromuscular respiratory failure. Panel 1: Patient performing forced vital capacity (FVC) testing at the bedside with a handheld spirometer, with a gauge showing the danger zone below 20 mL/kg. Panel 2: Patient performing negative inspiratory force (NIF) testing with a manometer, showing the threshold of -30 cmH2O. Panel 3: The "20/30/40 rule" displayed as a visual decision aid: FVC less than 20 mL/kg, NIF less negative than -30 cmH2O, or greater than 30-40% decline from baseline equals intubate. Panel 4: Single breath count technique with the patient counting out loud, noting that inability to reach 20 suggests FVC less than 1 liter.</image>

Clinical Pearls

Localization is everything — determining whether the lesion is at the UMN, LMN, NMJ, or myopathy level should precede test ordering. GBS presents with ascending weakness and areflexia, and CSF protein may be normal in the first week, so a normal LP does not exclude the diagnosis. The 20/30/40 rule applies to both GBS and myasthenic crisis: FVC below 20, NIF less negative than negative 30, or greater than 30 to 40 percent decline from baseline means intubation is needed. Pulse oximetry must not be relied upon for respiratory monitoring in neuromuscular disease because desaturation is a dangerously late finding. Aminoglycosides, fluoroquinolones, and magnesium must be avoided in myasthenia gravis because these medications can precipitate crisis. Steroids do not work in GBS, which is an important exception among autoimmune conditions. Descending paralysis should prompt consideration of botulism, while ascending paralysis should prompt consideration of GBS or tick paralysis. A thorough skin examination should always be performed in patients with acute ascending paralysis because tick paralysis reverses rapidly with tick removal. Suspected spinal cord compression (from epidural abscess, tumor, or hematoma) requires emergent MRI, not CT.

References

• Fokke C, et al. Diagnosis of Guillain-Barre syndrome and validation of Brighton criteria. Brain. 2014;137:33-43.
• Hughes RA, et al. Immunotherapy for Guillain-Barre syndrome. Cochrane Database Syst Rev. 2014;9:CD001446.
• Sanders DB, et al. International consensus guidance for management of myasthenia gravis. Neurology. 2016;87:419-425.
• Wendell LC, Levine JM. Myasthenic crisis. Neurohospitalist. 2011;1:16-22.
• Transverse Myelitis Consortium Working Group. Proposed diagnostic criteria and nosology of acute transverse myelitis. Neurology. 2002;59:499-505.

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