Autonomic Dysreflexia & the Unstable Body

The body below the lesion, off its leash

On the earlier rungs of this track you learned to read the cord with the ASIA exam and to grade it with the ASIA Impairment Scale — to ask how high the injury sits and whether it is complete or incomplete. Those questions mapped the *voluntary* losses: which muscles will not move, where sensation stops. This guide turns to a quieter system that the same injury also severs — the autonomic nervous system, the automatic machinery that runs blood pressure, heart rate, sweating, and temperature without your ever thinking about it. When the cord is cut, that machinery below the level keeps running, but it has lost the brain that used to conduct it.

Here is the piece of anatomy that makes everything in this guide click. The body's accelerator — the sympathetic nervous system, which clamps blood vessels and speeds the heart — leaves the spinal cord through a narrow band of segments, roughly the chest and upper-back levels (T1 through about L2). The brake — the parasympathetic system carried by the vagus nerve — bypasses the cord entirely and reaches the heart and gut directly. So picture the brain as a manager sitting up top, sending calming, modulating orders down the cord to that sympathetic band. A high injury, above the band, cuts the manager off from his whole sympathetic workforce. The workforce is still there, still able to act — but now no one upstairs can tell it when to stop.

Autonomic dysreflexia: a true emergency

Now follow one closed loop and you will understand the most dangerous complication in all of spinal cord medicine. [[autonomic-dysreflexia|Autonomic dysreflexia]] is a sudden, runaway surge of blood pressure, set off by something the person often cannot even feel. It threatens almost anyone whose injury sits at or above roughly the T6 level — that is, above where most of the sympathetic accelerator leaves the cord. Trace it as a story. Somewhere below the lesion, a noxious irritation begins — most commonly an over-full bladder, perhaps from a kinked or blocked catheter. The bladder wall stretches and screams pain signals up the cord. The person feels nothing, because that message can no longer reach the brain. But it does reach the sympathetic band, and it triggers a massive reflex: vessels across the whole lower body clamp shut, and blood pressure climbs, fast and high.

The brain *does* notice the soaring pressure — sensors in the neck arteries shout the alarm — and it fires back the only correction it knows: relax the vessels, slow the heart. But its calming orders run down through the cord, and the injury blocks them from reaching the clamped-down lower body. So the brake works only *above* the lesion. The result is a body split in two. Below the line, vessels stay viciously clamped and the pressure keeps climbing. Above the line, the brain over-applies what brake it can reach: the heart slows, vessels in the head and neck dilate. That split is exactly why the warning signs cluster above the lesion — a pounding headache, a flushed and sweaty face, blotchy red skin and a stuffy nose, while the skin below stays pale, cold, and bristling with goosebumps.

Be clear about why this is an emergency and not a curiosity. A person with tetraplegia may have a resting blood pressure that is normally low; in dysreflexia it can rocket far above their baseline within minutes. A jump that a healthy person's vessels would shrug off can, in this body, burst a vessel in the brain — a stroke, a seizure, even death — and it can do so while the person looks merely flushed and complains of headache. This is the honest weight of it: a blocked catheter, something so mundane, can kill through a chain of reflexes the person cannot feel and cannot consciously stop.

Reading the signs and breaking the loop

Because the loop is driven by an irritation the person cannot feel, the response is wonderfully logical: do not chase the blood pressure with drugs first — hunt down and remove the irritation, and the reflex collapses on its own. The single most important first move is also the simplest: sit the person upright. Gravity drains blood into the slack lower body and pulls the pressure down at once, which is the exact opposite of the instinct to lie an unwell person flat. Then loosen anything tight, and search the body below the lesion for the culprit, starting with the bladder, which causes the large majority of episodes.

1. Sit the person upright and lower their legs — gravity itself starts pulling the dangerous pressure down.
2. Loosen every tight thing — clothing, an abdominal binder, leg straps, a tight shoe — anything pressing or constricting.
3. Hunt the trigger, bladder first: check the catheter for a kink or block; a full neurogenic bladder is the usual culprit.
4. If the bladder is not the cause, look next to the bowel (a loaded rectum from neurogenic bowel), then to skin, a pressure sore, an ingrown nail, or anything else amiss below the lesion.
5. Keep monitoring the blood pressure; if it stays dangerously high after the trigger is removed, that is when medical treatment with fast-acting blood-pressure drugs is added — never as the first or only step.

The mirror image: orthostatic hypotension

Dysreflexia is blood pressure surging too high; its mirror image is blood pressure falling too low. [[orthostatic-hypotension-sci|Orthostatic hypotension]] is the lightheaded, grey-out, near-faint feeling that strikes when the person is raised upright. Here the disconnected sympathetic system fails in the *opposite* direction. In an intact body, standing up makes blood pool in the legs; the brain instantly senses the dip and orders the leg vessels to clamp, holding pressure steady. After a high cord injury, that order cannot get through. Blood sinks into the slack lower body, pressure sags, and too little reaches the brain — so the person greys out the moment the wheelchair tilts up or the therapist raises the plinth.

Picture an early rehabilitation session: a woman with a recent cervical injury, who has lain flat in an intensive care bed for weeks, is tilted upright for the first time. Within seconds she turns pale, sweaty, and faint — not from fear but from physics, blood draining out from under a brain that cannot defend its own supply. The team tilts her back down, and the colour returns. This is why people are brought upright *gradually*, over days, on tilt-tables and reclining wheelchairs, while the body slowly relearns to cope. Compression stockings, an abdominal binder, and patience are the everyday tools — concrete, low-tech, and far more useful here than any drug. Encouragingly, this particular instability usually eases over the early weeks and months as the body adapts; dysreflexia, by contrast, tends to stay for life.

A body that can no longer hold its own temperature

The same broken conductor disturbs a third automatic system: temperature. [[impaired-thermoregulation|Impaired thermoregulation]] follows the now-familiar logic — the brain's thermostat can no longer command the body below the lesion. Think of how a healthy body holds 37°C. When hot, it widens skin vessels and sweats to shed heat; when cold, it clamps those vessels and shivers to make heat. Both responses are routed through the sympathetic system down the cord. Above a high injury they still work; below it, they are cut off. The larger the body area stranded below the lesion — greatest in tetraplegia, where the injury is high and the marooned territory vast — the less of the body can defend its own temperature.

The practical consequence is that the body drifts toward the temperature of the room around it, a little like a cold-blooded animal — clinicians sometimes call this poikilothermia. On a hot day the person cannot sweat enough below the lesion and overheats; in the cold they cannot clamp and shiver enough below it and chill. A summer afternoon outdoors, or an over-air-conditioned ward, is not a comfort question but a safety one. The remedy is once again concrete and unglamorous: dress for the weather deliberately, seek shade and shelter, sip cool or warm drinks, and watch the environment rather than trusting a body that can no longer warn you. Notice the unifying thread across this whole guide — high pressure, low pressure, lost temperature control are three faces of one fact: below the lesion, the body's automatic government has lost contact with its capital.

WHAT FAILS        TOO HIGH                 TOO LOW
----------------------------------------------------------
Blood pressure    Autonomic dysreflexia    Orthostatic
  (sudden surge,            hypotension
   noxious trigger)         (greys out on
                            sitting up)

Temperature       Overheats in the heat    Chills in the cold
  (can't sweat / clamp below the lesion -> drifts toward
   the room temperature)

ONE ROOT CAUSE: brain's automatic control is cut off from
the sympathetic system below the level of the injury.

Why this belongs at the centre of rehabilitation

It is tempting to file autonomic instability under "medical complications" and hand it to the doctors while therapy gets on with the visible work of moving and walking. That would be a mistake, and here is the honest reason. Rehabilitation does not cure the lesion — you have met this limit on every rung — it rebuilds a life around it. But a person cannot relearn transfers, drive a power chair, or sit through a day of training matched to their injury level if a full bladder might tip them into a hypertensive crisis, or if standing up greys them out, or if an afternoon in the sun overheats them. Autonomic stability is the ground the rest of rehabilitation stands on; without it, the visible work cannot even begin.

So weave one honest hope through the caution. None of this is a counsel of fear. These instabilities are predictable, they follow rules you can now trace from the anatomy, and they are managed every day by people who live full, mobile, working lives with spinal cord injury. The bladder is emptied on a schedule so it never over-fills; the upright position is reclaimed gradually; the wardrobe is matched to the weather. Mastery here is not the heroics of an emergency — it is the quiet competence of a person and a team who understand a rewired body well enough that the emergency, most days, simply never arrives.