8.2 Neuromuscular Weakness Crises: Myasthenia Gravis & Guillain-Barré Syndrome

8.2 Neuromuscular Weakness Crises: Myasthenia Gravis & Guillain-Barré Syndrome

Clinical Scenario: A 42-year-old female with myasthenia gravis presents to the ICU with progressive dyspnea, dysphagia, and neck flexor weakness after taking ciprofloxacin for a UTI. Vital signs: BP 146/88 mmHg, HR 112 bpm, RR 26 breaths/min, and SpO2 97% on room air. ABG reveals: pH 7.42, PaCO2 38 mmHg, PaO2 86 mmHg, and HCO3 24 mEq/L. Bedside pulmonary mechanics demonstrate Forced Vital Capacity (FVC) of 14 mL/kg and Negative Inspiratory Force (NIF) of $-18\text{ cm H}_2\text{O}$.

Pathophysiology of Neuromuscular Respiratory Failure

Neuromuscular respiratory failure represents an acute ventilatory pump failure resulting from weakness of the diaphragm, intercostals, abdominal wall, and pharyngeal musculature:

• Microatelectasis: Diaphragmatic weakness prevents deep sighs, causing basal microatelectasis and decreased compliance.
• Secretion Retention: Weak abdominal muscles impair cough velocity, predisposing to mucus plugging and aspiration.
• Bulbar Pharyngeal Collapse: Upper airway weakness leads to tongue retroplacement and aspiration.
• The "Normal" ABG Trap: Arterial blood gases and pulse oximetry remain deceptively normal during early diaphragmatic fatigue because tachypnea maintains minute ventilation. PaCO2 elevation occurs only when diaphragmatic exhaustion is complete. A normal PaCO2 in a tachypneic patient is an ominous warning of impending arrest.

Bedside Pulmonary Mechanics & The 20/30/40 Rule

Serial evaluation of bedside pulmonary mechanics is essential for monitoring neuromuscular weakness:

Myasthenia Gravis: Crisis Differentiation & Management

Myasthenia Gravis (MG) is an autoimmune disorder targeting acetylcholine receptors (anti-AChR in ~85%), muscle-specific kinase (anti-MuSK in ~5–10%), or LRP4.

Myasthenic Crisis vs. Cholinergic Crisis

High-Risk Pharmacotherapy & Contraindications in MG

Numerous medications impair neuromuscular transmission:

• Antibiotics: Fluoroquinolones (ciprofloxacin; Black Box Warning), Aminoglycosides (gentamicin; block ACh release), Macrolides (azithromycin), and Colistin.
• Cardiovascular: Beta-blockers, Calcium channel blockers, Class Ia antiarrhythmics (quinidine, procainamide).
• Paralytics: Succinylcholine exhibits unpredictable resistance; non-depolarizing NMBAs (rocuronium, vecuronium) cause extreme hypersensitivity with prolonged paralysis (reduce dose by 50–75%, keep sugammadex ready, or intubate without paralytics).
• Other: Magnesium Sulfate (inhibits calcium influx and ACh release; contraindicated in myasthenic eclampsia) and checkpoint inhibitors.

Acute Disease-Modifying Immunotherapy

• Plasma Exchange (PLEX): 5 exchanges (3–4 L plasma with 5% albumin every other day over 10–14 days). Rapid onset (24–48 hours); preferred in respiratory crisis and anti-MuSK disease.
• Intravenous Immunoglobulin (IVIG): 2 g/kg total dose divided over 2 to 5 days. Onset 4–5 days. Monitor for acute kidney injury and aseptic meningitis.
• Corticosteroids: IV methylprednisolone or oral prednisone. Caution: Can cause transient paradoxical worsening during first 5–10 days; never start without concurrent PLEX or IVIG coverage in an ICU.

Guillain-Barré Syndrome: Critical Care Management

Guillain-Barré Syndrome (GBS / AIDP) is an acute post-infectious polyradiculoneuropathy triggered by molecular mimicry: antibodies against pathogens (Campylobacter jejuni, CMV, EBV) cross-react with peripheral nerve gangliosides (GM1, GD1a):

• Symmetrical, ascending flaccid muscle weakness starting in legs.
• Hyporeflexia or areflexia in affected limbs (hallmark feature).
• Cranial nerve involvement (facial diplegia in >50%) and neuropathic pain.

Dysautonomia & Hemodynamic Instability

Autonomic neuropathy complicates up to 70% of ICU GBS patients:

• Manifestations: Labile blood pressure (hypertension alternating with hypotension), arrhythmias (tachycardia, bradycardia, heart block, asystole), and ileus.
• Management: Arterial line and telemetry. Treat hypertension with short-acting titratable agents (IV esmolol, nicardipine); treat hypotension with low-dose vasopressors. Keep transcutaneous pacing pads at bedside due to vagally mediated asystole during suctioning.

Diagnostic Lumbar Puncture: Albuminocytologic Dissociation

• CSF Findings: Markedly elevated CSF protein ($>45\text{ mg/dL}$, often $100\text{--}400\text{ mg/dL}$) with normal white blood cell count ($<5\text{--}10\text{ cells/}\mu\text{L}$).
• Pitfall: CSF protein may be normal during days 3–7; repeat at 10–14 days if suspicion persists. Pleocytosis ($>50\text{ cells/}\mu\text{L}$) suggests alternative diagnoses.

Disease-Modifying Therapy & Contraindications

• First-Line Therapy: IVIG (2 g/kg over 2–5 days) or PLEX (5 exchanges over 10–14 days) are equally effective. Combining them provides no extra benefit and increases complications.
• Corticosteroids are CONTRAINDICATED: Multiple randomized trials demonstrate that corticosteroids are ineffective in GBS, fail to hasten recovery, and delay axonal regeneration.
• Succinylcholine Danger: Avoid succinylcholine in GBS; an initial 48-hour safe interval is not established. Extrajunctional receptor up-regulation triggers massive potassium release and fatal hyperkalemic cardiac arrest ($K^+ >7\text{--}9\text{ mEq/L}$).