4. Distributive Shock (Anaphylactic, Neurogenic/Spinal)

Quick Recap

Cardiovascular System, Protocol 5/12. Septic shock (the most common distributive cause) has its own dedicated protocol. This protocol covers the distributive shock framework plus full management of anaphylactic shock and neurogenic/spinal shock, the other two major distributive etiologies.

1. Definition

Distributive shock = shock due to profound peripheral vasodilation/loss of vascular tone (low SVR), typically with normal-to-high cardiac output, leading to maldistribution of blood flow and inadequate tissue perfusion despite adequate or even elevated cardiac output. Causes: septic shock (see dedicated protocol), anaphylactic shock, neurogenic/spinal shock, adrenal crisis (see Endocrine protocols), severe pancreatitis (SIRS-mediated), liver failure-associated vasoplegia, drug/toxin-induced vasoplegia (e.g., post-cardiopulmonary bypass vasoplegic syndrome).

2. Pathophysiology (General)

Loss of vascular smooth muscle tone -> peripheral vasodilation -> reduced SVR -> hypotension despite preserved/increased cardiac output. Venodilation also reduces effective circulating volume (relative hypovolemia) even without true fluid loss. Fluids increase preload/venous return partially, but vasopressors (raising SVR) are the primary corrective intervention, distinct from hypovolemic shock where fluid alone is often sufficient.

3. Anaphylactic Shock

Definition

IgE-mediated ("anaphylaxis") or non-IgE-mediated ("anaphylactoid," clinically indistinguishable, previously separately named) systemic hypersensitivity reaction involving mast cell/basophil degranulation (histamine, tryptase, prostaglandins, leukotrienes) or direct complement/mast cell/basophil activation.

Modified NIAID/FAAN diagnostic criteria (95% sensitive when validated) — any ONE of:

1. Acute onset (minutes-hours) with skin/mucosal involvement PLUS respiratory compromise OR reduced BP/end-organ dysfunction
2. Two or more of: skin/mucosal involvement, respiratory compromise, reduced BP/end-organ dysfunction, persistent GI symptoms — occurring rapidly after LIKELY allergen exposure
3. Reduced BP alone after exposure to a KNOWN allergen for that patient

Pathophysiology

Cardiovascular collapse (occurs in ~30% of cases) results from: (a) hypovolemia from increased vascular permeability/capillary leak, (b) peripheral vasodilation, (c) myocardial depression from angioedema/hypoxemia, (d) bradycardia (a distinguishing feature from most other shock states, which are typically tachycardic). ~50% have respiratory symptoms, which can progress to failure from upper airway edema, bronchospasm, and combined cardiogenic/noncardiogenic pulmonary edema. Biphasic reactions occur in up to 20% of patients, typically 1-8 hours after the initial episode (reported up to 72 hours), without re-exposure.

Common Triggers

Immune-mediated: foods (nuts, peanuts, eggs, fish, shellfish, alpha-gal/mammalian meat, cow's milk), antibiotics (especially penicillin — low cross-reactivity with cephalosporins), vaccines, anesthetics, insulin/hormones, antitoxins, blood products, insect stings/bites, snake bites, latex, immune globulins.

Non-immune ("anaphylactoid"): NSAIDs (especially aspirin), opiates, sulfites, radiocontrast media, neuromuscular blockers (curoniums, succinylcholine), antisera, heparin/protamine, hemodialysis membranes.

Immediate Management

• Epinephrine IM 0.3-0.5 mg into the anterolateral thigh, repeated every 5-15 minutes if symptoms persist — the cornerstone therapy, with NO contraindications in anaphylaxis. Epinephrine is well documented to be underused/delayed in practice, and delay is associated with higher rates of biphasic reactions, shock, respiratory failure, hypoxic encephalopathy, and death. IM route preferred (fewer complications than IV) for routine administration; IV reserved for refractory shock.
• Airway: early intubation for persistent respiratory distress, worsening angioedema, bronchorrhea, or refractory hemodynamic instability — these predict a difficult airway, so do not wait. Nebulized epinephrine for persistent stridor, beta-agonists for bronchoconstriction — adjuncts only, must not delay definitive airway management.
• Refractory anaphylaxis (not improving after >=300 mcg cumulative epinephrine):
• Continuous IV epinephrine infusion for persistent hypotension
• IV crystalloid boluses (low-quality evidence but recommended), may need multiple boluses
• Glucagon bolus + infusion if patient is on beta-blockers (blunts epinephrine responsiveness) — monitor for nausea/vomiting
• Methylene blue for refractory vasoplegia
• VA-ECMO consultation for cardiac arrest or truly refractory hypotension unresponsive to all supportive measures
• Adjuncts with limited evidence (do not substitute for or delay epinephrine):
• Corticosteroids: methylprednisolone 1-2 mg/kg IV initial dose, continued up to 4 days — physiologic rationale but recent evidence reviews have refuted the claim that steroids prevent biphasic reactions
• Antihistamines: reasonable adjunct for urticaria/itching symptom control only; poor evidence for preventing biphasic reactions
• Observation period: no consensus on duration; longer observation warranted for patients requiring multiple epinephrine doses, presenting in shock, or with delayed epinephrine administration.
• Discharge: refer to allergist for testing/monitoring; prescribe home epinephrine auto-injector.

Differential Diagnosis (broad, since diagnosis is clinical)

Urticaria alone, status asthmaticus, vancomycin infusion reaction ("red man syndrome"), scombroid fish poisoning, carcinoid syndrome, pheochromocytoma, mastocytosis, vocal cord dysfunction, panic attack (diagnosis of exclusion). Transfusion-related anaphylaxis specifically occurs in IgA-deficient individuals reacting to IgA in donor plasma — prevent with washed RBCs in future transfusions; treat with IM/SC epinephrine, IV hydrocortisone, IV antihistamine, airway protection, salbutamol nebulization, IV fluids; stop the transfusion immediately and return the unit to the blood bank.

Investigations

Diagnosis is clinical — do not delay treatment for labs. Serum tryptase (peaks 60-90 min post-onset, elevated for 1-6h) and serum histamine (falls within 30-60 min, so timing matters) can retrospectively confirm the diagnosis but do not inform immediate management. No trigger is identified in up to 60% of cases.

4. Neurogenic (Spinal) Shock

Definition

Distributive shock following acute spinal cord injury (typically cervical or high thoracic) due to loss of sympathetic outflow/innervation to the heart and peripheral vasculature, causing both vasodilation (hypotension) and often bradycardia (unopposed vagal tone) — distinguishing it from most other shock states, similar to anaphylaxis in this regard.

Must be a diagnosis of exclusion — hemorrhagic and cardiogenic causes must be ruled out first in any trauma patient, since concurrent traumatic hemorrhage is common and takes management priority.

Diagnosis

Known/suspected spine or spinal cord injury (typically cervical/high thoracic level) + hypotension unresponsive to appropriate fluid resuscitation, often with bradycardia rather than the compensatory tachycardia seen in hypovolemic/septic shock.

Management

• Exclude hemorrhagic/cardiogenic shock first (per Hypovolemic Shock and Cardiogenic Shock protocols) — treat any identified hemorrhage as the priority
• Cautious fluid resuscitation as an initial step
• Vasopressors/inotropes as needed once hemorrhage excluded, with norepinephrine as first-line choice, targeting MAP >85 mmHg specifically in acute spinal cord injury (higher target than typical shock resuscitation, reflecting evidence that maintaining higher spinal cord perfusion pressure may improve neurologic outcomes in the acute post-injury period)
• Address bradycardia if hemodynamically significant (atropine, or pacing if refractory)
• Coordinate with spine/neurosurgery for definitive spinal stabilization

5. Other Distributive Shock Causes (Brief)

Adrenal crisis: see Endocrine System protocols for full detail — hydrocortisone stress dosing is the definitive treatment alongside fluid resuscitation.

Severe pancreatitis / burns / liver failure: SIRS-mediated vasoplegia managed with the same general distributive shock principles (fluids + vasopressors), with disease-specific management per their dedicated protocols.

Post-cardiopulmonary bypass vasoplegic syndrome: methylene blue, hydroxocobalamin, and vasopressin have been used as adjuncts to catecholamine vasopressors in refractory post-bypass vasoplegia — managed in coordination with cardiac surgery/anesthesia.

6. Consultation Matrix

7. Monitoring Framework

Continuous hemodynamic monitoring, watch for biphasic anaphylactic reactions during extended observation, serial neurologic exams in spinal cord injury-associated neurogenic shock, watch for bradyarrhythmia in neurogenic shock.

8. Complications

Anaphylaxis: biphasic reaction, hypoxic encephalopathy from delayed epinephrine, difficult airway from angioedema. Neurogenic shock: prolonged hypotension worsening secondary spinal cord injury, bradyarrhythmia. Prevention: prompt epinephrine administration (anaphylaxis), MAP target maintenance (neurogenic shock). Rescue: ECMO for refractory anaphylactic shock, pacing for refractory bradycardia.

9. Documentation & Medicolegal Checklist

10. Key Guidelines

Joint AAAAI/ACAAI Practice Parameter Update on Anaphylaxis (Shaker et al., J Allergy Clin Immunol 2020); World Allergy Organization Anaphylaxis Guidelines (Simons et al., 2015); NIAID/FAAN consensus definition of anaphylaxis.

11. Controversies

Routine corticosteroid use in anaphylaxis is widely practiced despite recent systematic reviews refuting the claim that it prevents biphasic reactions — physiologic rationale persists but the evidence base has weakened considerably. Optimal observation period post-anaphylaxis remains unstandardized given poor-quality biphasic reaction incidence data. MAP target in acute spinal cord injury (>85 mmHg) is based on limited evidence and is not universally adopted with the same threshold across all guidelines.

12. References

1. McCauley M. Anaphylactic Shock. Washington Manual of Critical Care, 4th ed, 2025 (Ch. 5).
2. Other Injuries Requiring ICU Care (Neurogenic/Spinal Shock). Washington Manual of Critical Care, 4th ed, 2025 (trauma chapter section).
3. Shaker MS, Wallace DV, Golden DBK, et al. Anaphylaxis—a 2020 practice parameter update, systematic review, and GRADE analysis. J Allergy Clin Immunol. 2020;145(4):1082-1123.
4. Alqurashi W, Ellis AK. Do corticosteroids prevent biphasic anaphylaxis? J Allergy Clin Immunol Pract. 2017;5(5):1194-1205.
5. Francuzik W, Dolle-Bierke S, Knop M, et al. Refractory anaphylaxis: data from the European Anaphylaxis Registry. Front Immunol. 2019;10:2482.
6. Simons FER, Ebisawa M, Sanchez-Borges M, et al. 2015 update of the evidence base: World Allergy Organization anaphylaxis guidelines. World Allergy Organ J. 2015;8(1):32.

See also: Septic Shock (dedicated protocol, Cardiovascular System) for the most common distributive shock etiology.