Define Compartment Syndrome. Describe its diagnosis and treatment.

The Mechanism — a Vicious Cycle in a Closed Box

A muscle compartment is bounded by inelastic fascia — so it is, in effect, a closed box of fixed volume. Anything that increases the contents (bleeding, oedema after fracture or reperfusion, a crush injury, a burn) or decreases the volume (a tight cast, a bandage, prolonged limb compression) raises the intracompartmental pressure. Once that pressure exceeds the pressure in the capillaries and small veins (which is low), venous outflow ceases — so blood continues to arrive but cannot leave. Congestion worsens the oedema, which raises the pressure further, which worsens the congestion: a self-amplifying vicious cycle. The result is tissue ischaemia — and irreversible muscle and nerve necrosis begins within 4 to 6 hours.

WHY the Pulse Is the Most Dangerous Sign of ALL

The single fact that most often costs a patient their limb is understanding why the pulses remain present in compartment syndrome — and why waiting for them to disappear is a catastrophic error. The compartment pressure needed to occlude the capillaries and veins is far lower than systolic arterial pressure. So for the pressure to abolish the distal pulse, it would have to exceed systolic pressure entirely — which almost never happens before the limb is already dead. Hence: a normal pulse, a warm foot and a normal capillary refill DO not exclude compartment syndrome — they are entirely compatible with muscle that is dying at that moment. The classic 'five Ps' (pain, pallor, paraesthesiae, paralysis, pulselessness) are a dangerously misleading teaching, because four of them are late. The diagnosis rests on the first P.

The Signs That Matter — and the Diagnosis

Pain out OF proportion to the injury — severe, escalating, and requiring increasing doses of analgesia (a patient whose opioid requirement is climbing is sounding an alarm) — is the earliest and most important sign. Pain ON passive stretch of the muscles in the compartment (e.g. Passive extension of the toes in an anterior leg compartment syndrome) is the most sensitive sign. Then: a tense, swollen, woody compartment; and paraesthesiae in the distribution of the nerve traversing it (an early sign of nerve ischaemia). Paralysis and pulselessness are pre-terminal. It is a clinical diagnosis. Compartment pressure monitoring is useful chiefly in the unconscious, the child, or the patient with a nerve block or spinal anaesthesia — who cannot report pain (and in whom the diagnosis is therefore most often missed): a delta pressure (diastolic BP minus compartment pressure) below 30 mmHg indicates decompression.

Treatment — and the Consequence of Delay

This is a surgical emergency measured in hours. Immediately: remove all constricting dressings and split the cast down TO skin (a split cast that is not opened through the padding does nothing); place the limb at heart level (not elevated — elevation reduces the arterial inflow pressure and worsens the ischaemia); give oxygen and analgesia; and correct hypotension. Then: urgent, complete fasciotomy of all compartments — through long incisions (a two-incision, four-compartment fasciotomy in the leg), left open and dressed, with delayed closure or skin grafting. If diagnosed and decompressed in time, function is preserved. If missed, the dead muscle is replaced by fibrous tissue which contracts — producing volkmann'S ischaemic contracture (a fixed, useless, clawed limb) — and the myoglobin released causes rhabdomyolysis with acute kidney injury and hyperkalaemia.

WHY a Nerve Block Can Cost a Limb

A modern and increasingly important danger is understanding why regional anaesthesia — a nerve block or an epidural — given for pain relief after a fracture, may mask the one sign that would have saved the limb. The entire diagnosis of compartment syndrome rests on pain: pain out of proportion, escalating pain, and pain on passive stretch. A patient with an effective regional block feels nothing — so the alarm never sounds. The surgeon and nurses, seeing a comfortable patient, are reassured; the analgesic requirement does not climb; and the muscle dies silently. Numerous cases of missed compartment syndrome have followed exactly this course. Hence: regional blocks must be used with great caution — or avoided — in high-risk injuries (tibial fractures, crush injuries, forearm fractures in children, vascular injury and reperfusion), and where they are used, the limb must be monitored with compartment pressure measurement rather than relying on symptoms. The same applies to the unconscious, the intubated, the intoxicated and the very young child — all of whom cannot report pain, and in all of whom the diagnosis is most often missed.

WHY It Also Occurs Without a Fracture

A further point that catches the unwary is understanding why compartment syndrome does not require a fracture at all — and is therefore easily missed in patients who have not been to theatre. It can be caused by anything that raises the compartment contents or reduces its volume. Hence it occurs after a crush injury (even without fracture); after prolonged limb compression — the classic case being a patient found unconscious after an overdose, having lain on their own limb for hours; after reperfusion following a vascular injury or arterial repair (the reperfused muscle swells dramatically); after a burn (a circumferential eschar acts exactly like a tight cast, and needs escharotomy); after intense exercise (chronic exertional compartment syndrome); after a snake bite; after intravenous extravasation; after intramuscular bleeding in a haemophiliac or a patient on anticoagulants; and — iatrogenically — from a tight cast or bandage. In every one of these, the same rule applies: escalating pain and pain on passive stretch mean compartment syndrome until proved otherwise.

Pulses persist until very late — never wait for them to disappear.

• Compartment syndrome: raised pressure within a closed fascial compartment → venous outflow ceases → a self-amplifying vicious cycle of congestion and oedema → ischaemia. Irreversible muscle and nerve necrosis begins in 4–6 hours.
• The pulses remain present — compartment pressure rarely exceeds systolic pressure. A normal pulse, warm foot and normal capillary refill DO not exclude IT. Do not wait for the 'five Ps' — four of them are late.
• The signs that matter: pain out OF proportion (earliest — an escalating analgesic requirement is an alarm) and pain ON passive stretch (most sensitive); a tense, woody compartment; paraesthesiae. It is a clinical diagnosis.
• Measure the pressure in those who cannot report pain (the unconscious, the child, and — crucially — the patient with a nerve block or epidural): decompress if the delta pressure (diastolic BP − compartment pressure) is <30 mmHg.
• Treat immediately: remove all dressings and split the cast down TO skin; keep the limb AT heart level (do not elevate); then urgent, complete fasciotomy of all compartments, left open. Missed → volkmann'S ischaemic contracture, and rhabdomyolysis with acute kidney injury and hyperkalaemia.