Neurogenic shock
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Neurogenic Shock
Definition
Neurogenic shock is a form of distributive shock caused by loss of sympathetic vasomotor tone to peripheral arterial and venous beds, resulting in inadequate end-organ perfusion. It occurs most commonly after acute traumatic spinal cord injury (SCI) and probably affects fewer than 20% of all SCI patients.
- Schwartz's Principles of Surgery, 11e
- Tintinalli's Emergency Medicine
- Mulholland and Greenfield's Surgery, 7e
Pathophysiology
Loss of Sympathetic Outflow
Injury to the cervical or high thoracic spinal cord disrupts the descending sympathetic pathways. This produces:
| Effect | Mechanism |
|---|---|
| Peripheral vasodilation | Loss of arteriolar and venous vasoconstrictor tone → ↑ vascular capacitance |
| ↓ Venous return | Blood pools in dilated peripheral beds |
| ↓ Cardiac output | Reduced preload |
| Bradycardia | Loss of sympathetic cardiac innervation leaves unopposed vagal (parasympathetic) tone |
| No reflex tachycardia | Disrupted adrenal medullary catecholamine release |
Injuries at C5 and above are most likely to disrupt both cardiac and vasomotor sympathetic fibers. Below C5, reflex tachycardia may still occur. If T1–T4 is compromised, cardiac sympathetic denervation is nearly complete.
- Guyton and Hall Textbook of Medical Physiology
- Sabiston Textbook of Surgery
Other Neurogenic Causes (Non-traumatic)
- Deep general anesthesia → vasomotor center depression
- High spinal/epidural anesthesia → blocks sympathetic outflow
- Prolonged brain ischemia (>5–10 min) → total inactivation of brainstem vasomotor neurons
- Epidural hematoma compressing the cord
- Spinal cord neoplasm
Classic Clinical Presentation
| Feature | Finding |
|---|---|
| Blood pressure | Hypotension |
| Heart rate | Bradycardia (or absent tachycardia) |
| Skin | Warm, flushed, dry (loss of vasoconstriction) |
| Temperature | Hypothermia risk (cannot redirect blood to core) |
| Neurologic | Motor/sensory deficits from SCI |
| Extremities | Warm, flaccid |
Important caveat: Rosen's Emergency Medicine notes that ED patients with acute SCI actually show a range of heart rates and vascular resistances depending on the level of injury and balance of disrupted sympathetic vs. parasympathetic tone. No single presentation fully captures all patients.
Neurogenic Shock vs. Spinal Shock
These two terms are not interchangeable:
| Neurogenic Shock | Spinal Shock | |
|---|---|---|
| Nature | Hemodynamic | Neurological |
| Mechanism | Loss of vasomotor/sympathetic tone | Temporary loss of spinal reflexes below injury |
| Findings | Hypotension, bradycardia | Flaccidity, areflexia, loss of voluntary movement |
| Resolution | 24–48 hours with treatment | Days to weeks |
- Tintinalli's Emergency Medicine; Sabiston Textbook of Surgery
Diagnosis
Neurogenic shock is a diagnosis of exclusion. In any trauma patient, hypotension must never be presumed neurogenic until hemorrhage, tension pneumothorax, and cardiogenic shock are excluded.
Key clues:
- Bradycardia + hypotension (vs. tachycardia + hypotension in hemorrhagic shock)
- Warm, dry skin
- Evidence of SCI (vertebral fracture, neurologic deficit)
- In penetrating SCI, 74% of hypotensive patients have hemorrhage as the cause — only 7% have true neurogenic shock
The severity of cardiovascular dysfunction correlates with injury completeness: patients with complete motor injuries are >5× more likely to need vasopressors than those with incomplete lesions.
Management
Step 1 — Airway & Ventilation
Secure airway and ensure adequate oxygenation first.
Step 2 — IV Fluid Resuscitation
- Treat the relative hypovolemia from vasodilation with IV crystalloids
- Most patients respond to volume restoration alone with resolution of hypotension
- Caution: Loss of pulmonary vasomotor tone predisposes to pulmonary edema with excessive fluids — monitor cardiac preload carefully
Step 3 — Vasopressors (if fluids insufficient)
- Consider only after hypovolemia is excluded and neurogenic shock is confirmed
- Dopamine: first-line historically
- Phenylephrine (pure α-agonist): increases peripheral vascular tone and venous return
- Norepinephrine: increasingly preferred, especially in bradycardic patients (provides both α-vasoconstriction and β1-chronotropy)
- Atropine: for sinus bradycardia secondary to high cervical SCI, combined with vasopressor therapy
The need for vasopressors is typically brief (24–48 hours).
Step 4 — Spinal Cord Perfusion Optimization
Hypotension worsens secondary SCI by reducing blood flow to an already ischemic cord. Patients should be managed in the ICU with attention to blood pressure, oxygenation, and hemodynamics to optimize spinal cord perfusion and potentially improve neurologic outcomes.
Key Points Summary
- Distributive shock from loss of sympathetic vasomotor tone (most commonly traumatic SCI at cervical/high thoracic levels)
- Classic triad: hypotension + bradycardia + warm vasodilated skin
- Diagnosis by exclusion — rule out hemorrhage first
- Treatment: fluids → vasopressors (norepinephrine or phenylephrine preferred; add atropine for bradycardia)
- Distinct from spinal shock (which is a neurological, not hemodynamic, phenomenon)
- Treat aggressively to protect the injured spinal cord from secondary ischemic insult
Sources: Schwartz's Principles of Surgery 11e | Tintinalli's Emergency Medicine | ROSEN's Emergency Medicine | Mulholland and Greenfield's Surgery 7e | Sabiston Textbook of Surgery | Guyton and Hall Textbook of Medical Physiology
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