⚡ Neurogenic Shock vs Autonomic Dysreflexia
Same patient, same spinal cord injury, opposite autonomic crises — one is a complete failure of sympathetic tone, the other is an unregulated explosion of it.
The Key Insight — Same Injury, Opposite Crises
Both conditions occur in patients with high spinal cord injuries (typically cervical or upper thoracic, at or above T6). The cord injury is the same. But the timing is different, the mechanism is opposite, and getting the two confused in an emergency could kill the patient — because the treatments are diametrically opposed.
The Single Most Important Distinguishing Fact
Blood pressure tells you which one you are dealing with:
📉 Neurogenic Shock = HYPOTENSION — sympathetic tone is lost, vessels collapse, BP falls
📈 Autonomic Dysreflexia = HYPERTENSION — sympathetic tone explodes uncontrolled, vessels constrict below the lesion, BP rockets
Both cause bradycardia — from opposite causes (one from unopposed vagal tone on a hypotensive patient; the other from the baroreflex slamming the brakes on a hypertensive patient). This shared bradycardia is the trap. The BP is what tells them apart.
Neurogenic Shock 📉
“The Cut Cables” — Acute sympathetic failure after high spinal cord injury.
What Happens When the Sympathetic Cables Are Cut
Under normal conditions, the brainstem continuously sends descending sympathetic signals down the spinal cord to maintain two things: vascular tone (keeping vessels partially constricted) and cardiac acceleration (maintaining normal heart rate). When a cervical or upper thoracic cord injury severs these descending pathways, both functions collapse simultaneously.
Vasomotor tone lost → Massive vasodilation
Without the sympathetic “drip” to maintain vessel tone, the vascular smooth muscle relaxes completely. Vast volumes of blood pool in the visceral and lower-extremity capacitance vessels (veins). This is a distributive shock — the total blood volume is normal but it is all in the wrong place. The effective circulating volume collapses.
Cardiac sympathetic fibres (T1–T4) cut → Vagus unopposed → Bradycardia
The sympathetic accelerator nerves to the heart originate at T1–T4. When the cord is cut above this level, cardiac acceleration is lost. The Vagus nerve (CN X), which descends via the neck — bypassing the spinal cord entirely — remains fully intact and now drives the heart rate unopposed. The result is bradycardia in the context of hypotension and shock.
Unable to sweat below the lesion → Warm, dry skin
Sweating requires intact sympathetic cholinergic fibres. Below the cord injury, these are disconnected. The patient’s skin below the level of injury feels paradoxically warm and dry despite being in shock. This is the opposite of haemorrhagic shock, where the skin is cold and clammy from sympathetic-driven vasoconstriction and sweating.
The Trauma Bay Trap — Bradycardia in a Shocked Patient
In any trauma patient, hypotension + tachycardia = haemorrhagic shock (sympathetic response to low volume). Hypotension + bradycardia = neurogenic shock (no sympathetic response because the cord is cut). The bradycardia is the pathognomonic clue.
If you miss it: treating neurogenic shock as haemorrhagic shock (transfusing aggressively without vasopressors) risks massive fluid overload and pulmonary oedema, because the blood volume was never actually lost — it is just pooled.
“Fluids First, Then Pressors”
Step-by-Step Management
1. IV Fluid resuscitation — fill the dilated vascular “tank.” Use crystalloid or blood products. This is the first-line treatment to restore effective circulating volume to the pooled vessels.
2. Vasopressors if fluids fail — if BP does not recover with fluids, add vasopressors to squeeze the dilated vessels. Noradrenaline (norepinephrine) is first-line — it provides both alpha-1 (vasoconstriction) and beta-1 (modest inotrope) effects, restoring vascular resistance and cardiac output simultaneously. Phenylephrine (pure alpha-1) is an alternative but may worsen bradycardia via reflex.
3. Atropine for haemodynamically significant bradycardia — if the bradycardia is causing haemodynamic compromise (very slow HR contributing to low output), give atropine (0.5–1 mg IV) to block the unopposed vagal drive.
⚠️ Do NOT over-resuscitate — because blood volume has not been lost (it is merely redistributed), aggressive fluid loading will overflow back into the pulmonary circulation once vasopressors restore tone. Target the minimum fluid needed to achieve organ perfusion, not aggressive volume replacement.
Autonomic Dysreflexia 📈
“The Unregulated Explosion” — Chronic sympathetic storm triggered by a noxious stimulus below the lesion.
Why the Brain Cannot Stop What It Cannot See
Autonomic dysreflexia develops weeks to years after a cord injury at T6 or above. The intact peripheral nerves below the injury remain capable of detecting pain and generating sympathetic signals — but the normal cortical and brainstem inhibition that would suppress an exaggerated reflex cannot descend past the cord lesion.
Noxious stimulus below the lesion
Something painful or irritating occurs below the cord lesion level. The most common trigger by far (>80%) is a distended bladder — typically from a kinked, blocked, or overfilled urinary catheter. Other triggers: faecal impaction, tight clothing or a pressure sore, a urinary tract infection, an ingrown toenail, even a uterine contraction in labour.
Sympathetic reflex arc fires via splanchnic nerves (T5–L2)
The pain signal ascends the cord and triggers a massive sympathetic outflow through the splanchnic nerves. The signal hits the level of the cord lesion and cannot reach the brain — so the brain cannot send inhibitory signals back down. The sympathetic reflex fires completely unregulated and unstoppable from above.
Massive vasoconstriction BELOW the lesion → Severe hypertension
The unregulated sympathetic storm causes intense vasoconstriction of all vessels below the cord lesion. This drives BP to life-threatening levels — systolic pressures of 200–300 mmHg are not uncommon. This can cause hypertensive stroke, retinal haemorrhage, and cardiac failure.
Brain detects ↑↑BP → Fires the Vagus → Bradycardia + vasodilation ABOVE the lesion
The baroreceptors at the carotid sinus and aortic arch detect the dangerous hypertension and send an emergency signal to the brainstem. The brain’s only intact output pathway is the Vagus nerve (CN X), which bypasses the spinal cord via the neck. The vagal response drives profound bradycardia. Simultaneously, the brain dilates vessels above the lesion via intact vasodilator pathways — causing the characteristic flushing, sweating, and pounding headache above the injury.
The examination of a patient in autonomic dysreflexia is genuinely striking — the body looks as though it belongs to two different patients split at the level of the cord lesion:
| Region | Appearance | Why |
|---|---|---|
| Above the lesion | Flushed, sweating face and neck. Blurred vision. Pounding headache. Nasal congestion. Piloerection. | Brain-mediated vasodilation via intact pathways above the cord, trying to reduce BP. Sweating above lesion intact (sympathetic fibres unaffected here). |
| Below the lesion | Pale, cool, dry skin. No sweating. Goosebumps (piloerection from sympathetic storm). | Intense sympathetic vasoconstriction below the lesion — vessels clamped down. No sweating because the thermoregulatory response is overwhelmed by vasoconstriction. |
| BP | Severely elevated — e.g., 210/110 mmHg. A life-threatening emergency. | Unregulated sympathetic surge with no central inhibition. |
| HR | Bradycardic — paradoxically slow despite hypertension. | Baroreflex activates vagus to try to lower BP. The heart rate tells the brain the problem is on the high side; the bradycardia differentiates this from other hypertensive emergencies. |
This Is a Medical Emergency — Systolic BP >150 in SCI = Act Now
Uncontrolled autonomic dysreflexia can cause haemorrhagic stroke, hypertensive encephalopathy, and death within minutes. The clock starts when BP rises. Do not wait for the BP to come down by itself.
Management: “Sit Up, Strip Down, Search and Spray”
1. Sit the patient upright immediately — sitting up uses gravity to induce orthostatic venous pooling in the legs, reducing venous return and cardiac output, which lowers BP within seconds. This is the fastest intervention available and must happen first.
2. Remove tight clothing, compression devices, check for pressure areas — any mechanical trigger below the lesion must be removed. Loosen anything that could be causing pressure.
3. Check the urinary catheter first — blocked catheter is the cause in >80% of cases. Check the entire catheter tubing for kinks, check the bag is draining, change the catheter if in doubt. If no catheter → in-and-out catheterisation immediately. If catheter is working → perform bladder scan (overfilled bladder despite catheter = blocked).
4. Check for bowel impaction — digital rectal examination to identify faecal impaction. If impaction is found, use topical lignocaine gel before manual evacuation to reduce further stimulation.
5. Antihypertensives if BP remains dangerous — use rapid-acting, short-acting agents only (so effect is immediately reversible once the trigger is found):
🩺 GTN spray (sublingual glyceryl trinitrate) — first-line, immediate vasodilation
🩺 Nifedipine (bite and swallow/sublingual) — calcium channel blocker, rapid onset
⚠️ Never use long-acting antihypertensives — if the trigger is found and removed, the BP will crash and a long-acting drug will then cause severe hypotension.
Full Comparison — Side by Side
| Feature | Neurogenic Shock 📉 | Autonomic Dysreflexia 📈 |
|---|---|---|
| Timing | Acute — occurs within hours of the injury | Chronic — weeks to years after injury |
| Mechanism | Descending sympathetic pathways severed → complete loss of tone | Noxious stimulus below lesion → unregulated sympathetic reflex with no central inhibition |
| Autonomic state | Sympathetic FAILURE — lost tone | Sympathetic EXPLOSION — unregulated surge |
| Blood pressure | 📉 HYPOTENSION — vessels dilated | 📈 SEVERE HYPERTENSION — vessels clamped below |
| Heart rate | 🐢 Bradycardia — vagus unopposed (no sympathetic cardio-acceleration) | 🐢 Bradycardia — baroreflex activates vagus to combat ↑↑BP |
| Skin below lesion | Warm, flushed, DRY — vessels dilated, no sweating | Pale, cool, dry — vessels constricted by sympathetic storm |
| Skin above lesion | Normal (injury is below brain) | Flushed, SWEATING, piloerection — brain’s vasodilator response |
| Headache | None — hypotension, no high-pressure headache | Pounding, severe — from hypertensive vasodilation above lesion |
| Trigger | The spinal cord injury itself | Noxious stimulus below injury — #1 cause is blocked urinary catheter (>80%) |
| Level required | Cervical or upper thoracic (above T4 to cut cardiac sympathetics) | At or above T6 (to involve splanchnic sympathetics) |
| Treatment | IV Fluids → Noradrenaline/Phenylephrine + Atropine for bradycardia | Sit up + Remove trigger (check catheter first) + GTN spray/Nifedipine |
| Fluids? | ✅ YES — fill the dilated tank (cautiously) | 🚫 NO — giving fluids to a hypertensive patient is dangerous |
| Antihypertensives? | 🚫 NO — patient is already hypotensive | ✅ YES — GTN/Nifedipine if trigger removal fails |
| Analogy | “Cut cables” — the engine is off, everything collapses | “Runaway train” — engine at full throttle with no brakes from above |
The Lethal Treatment Mix-Up
If you confuse the two conditions and give the wrong treatment:
Treating neurogenic shock as dysreflexia → giving antihypertensives to a hypotensive patient → cardiovascular collapse
Treating autonomic dysreflexia as neurogenic shock → giving IV fluids and vasopressors to a hypertensive patient → hypertensive stroke or heart failure
Always check the blood pressure first. Everything else follows from that single number.
🧠 Mnemonics — All in One Place
LOW BP + Bradycardia + Warm dry skin = Neurogenic Shock
Acute phase. Cables cut. Sympathetic failure. Treat with fluids + vasopressors. Do NOT give antihypertensives.
HIGH BP + Bradycardia + Flushed above / Pale below = Autonomic Dysreflexia
Chronic phase. Sympathetic explosion. Sit up + find trigger + GTN/Nifedipine. Do NOT give fluids or vasopressors.
BOTH cause Bradycardia — BP is the ONLY differentiator
Bradycardia in neurogenic shock = vagus unopposed (no sympathetic). Bradycardia in AD = baroreflex response to dangerously high BP. Same sign, different emergency.
Warm, dry skin below the lesion
Vessels dilated + no sweating below = paradoxically warm. The opposite of haemorrhagic shock (cold clammy).
hIpotension (Hypotension)
Distributive shock from massive vasodilation. Blood pooled in capacitance vessels. Volume not lost — just pooled.
Slow heart (Bradycardia)
T1–T4 cardiac sympathetic fibres cut. Vagus unopposed. Bradycardia + hypotension in a trauma patient = always think neurogenic shock.
Early (Acute phase)
Occurs in the acute injury phase. Resolves over days–weeks as the autonomic system partially reorganises.
Sit Up — FIRST action, immediately lowers BP via orthostasis
Gravity pools blood in legs, reduces venous return and cardiac output, BP drops within seconds.
Strip Down — remove tight clothing, check pressure areas
Any constriction below the lesion is a potential trigger. Remove it all.
Search — check urinary catheter FIRST (>80% of cases)
Check entire tubing for kinks. Check bag is draining. If blocked → change catheter. No catheter → in-and-out cath immediately. Also check for bowel impaction.
Spray — GTN spray or Nifedipine if BP still dangerous
SHORT-ACTING agents only. Never long-acting — when trigger is removed BP will crash and a long-acting drug will then cause severe hypotension.
Neurogenic Shock — Warm everywhere below the lesion
Vessels dilated, no sweating = uniformly warm and dry below the level of injury. Head-to-toe warm.
Autonomic Dysreflexia — Split: Flushed/sweating ABOVE, pale/cool BELOW
The two halves look completely different. Above lesion: red, sweaty, headache. Below lesion: pale, cold, no sweat. The split is the visual diagnosis.
CN X (Vagus) bypasses the spinal cord via the neck
In both conditions, the vagus is the only intact autonomic pathway from the brain to the body. In neurogenic shock it is unopposed (drives bradycardia). In dysreflexia it is the brain’s only tool to fight the BP (also drives bradycardia). Shared bradycardia — different crisis.
In dysreflexia, the brain is “blind” — it senses BP via baroreceptors but cannot send inhibitory signals down the broken cord
It fires the vagus and dilates vessels above the lesion — the only weapons it has. Below the cord, the sympathetic storm continues unchecked.
📝 Test Your Knowledge
Select a category to begin. Questions are shuffled every time.