Acute Compartment Syndrome

What it is. Acute compartment syndrome (ACS) is a surgical emergency in which raised pressure within a closed osseofascial compartment compromises local blood flow (capillary perfusion pressure approximately 25mmHg) and the muscle and nerve inside become ischaemic. Without timely fasciotomy the damage becomes irreversible after 6-8 hours of ischaemia.

Causes. Anything that adds volume to a compartment or shrinks the space around it:

1. Fractures (75% of cases) - the tibial shaft is the most common, 36% of all ACS
2. Soft-tissue injury without fracture
3. Arterial injury with ischaemia-reperfusion
4. Burns, especially circumferential
5. Crush injury - direct trauma to muscle
6. Tight casts and dressings - external compression
7. Extravasation of intravenous fluids
8. Prolonged limb compression, as in the drug-overdose position

Who to watch. The scenarios in which the syndrome is most likely, and most likely to be missed:

• Tibial shaft fractures, especially high-energy
• Forearm fractures in children (supracondylar)
• Vascular injury with delayed reperfusion
• Polytrauma, especially the obtunded patient
• The anticoagulated patient, in whom a compartment haematoma is enough

A missed compartment means an incomplete decompression and a persistent compartment syndrome, so the compartments of each region have to be known cold. The leg has 4, the forearm 3 and the thigh 3, and the one most commonly missed is the deep posterior compartment of the leg.

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The cycle. Whatever starts it, the sequence is the same: volume rises inside a space that cannot stretch, pressure follows, and the smallest vessels are the first to close.

Why the pulses stay. ACS is a microvascular problem, not an arterial occlusion. Compartment pressure exceeds capillary perfusion pressure but stays below systolic arterial pressure, typically over 90mmHg, so the arteries remain patent and the pulses are present until very late. Never rule out ACS because a pulse is palpable.

Why delta P, not absolute pressure. The compartment does not care what its pressure is in absolute terms; it cares whether blood can still be driven into it. Perfusion depends on the difference between the diastolic blood pressure and the compartment pressure, which is why the same 35mmHg reading is harmless in a normotensive patient and limb-threatening in a shocked one. The reference values:

• Normal compartment pressure: under 10mmHg
• Delta P (diastolic BP minus compartment pressure) under 30mmHg sustained for more than 2 hours indicates ischaemia and is the threshold for decompression
• An absolute pressure over 30mmHg is common after a tibial fracture and is not by itself an indication for fasciotomy

The duration is part of the threshold, not an optional refinement. A single transient dip below 30 is common after a tibial fracture and frequently self-corrects, and treating it as the diagnosis is what over-operates.

Compartment syndrome is classified by the timing of its onset, by the mechanism that raised the pressure (Matsen), and by the severity it has reached. Classification by location is the anatomy above.

By mechanism (Matsen). Matsen divides the causes into increased content (bleeding, oedema, intravenous extravasation) and decreased compartment size (tight casts, closure of fascial defects, MAST trousers). Both roads lead to the same raised compartment pressure.

A clinical diagnosis. ACS is diagnosed at the bedside, and when clinical suspicion is high fasciotomy is not delayed for a pressure measurement. A single normal pressure does not exclude it, and at-risk patients need serial reassessment rather than one examination.

History. The mechanism of injury and the time since it, which is critical for prognosis. Then the pain: is it out of proportion to the injury, and are the analgesia requirements rising? Ask about numbness or weakness developing.

The 6 P's, in order of appearance. Pain out of proportion is the most sensitive early sign and pain on passive stretch the most specific; a tense compartment is early but subjective. Everything after that is a sign of progression or of damage already done.

Passive stretch, compartment by compartment. Stretch the muscles that live in the compartment you suspect:

• Anterior leg - pain on passive plantar flexion of the toes or ankle
• Deep posterior leg - pain on passive dorsiflexion of the toes
• Volar forearm - pain on passive extension of the fingers
• Dorsal forearm - pain on passive flexion of the fingers

The patient who cannot tell you. In the unconscious, intubated or heavily sedated patient the pain assessment is gone, and these patients are at the highest risk of a missed ACS. Keep a very low threshold for pressure measurement and for fasciotomy. The same applies wherever the examination is unreliable:

• Polytrauma patients, often sedated and ventilated
• Regional anaesthesia, which masks the pain
• Paediatric patients, who cannot articulate it
• Drug intoxication
• Neurological injury

When to measure. In an awake, examinable patient a convincing clinical diagnosis overrides any number. Measurement earns its place where the examination cannot be trusted: McQueen and Duckworth's review (PMID 26814506) puts the sensitivity of clinical findings at only 13 to 64 per cent, against 94 per cent for continuous pressure monitoring at the sustained threshold, with a specificity of 98 per cent for monitoring against 63 to 98 per cent for clinical signs. That is the argument for monitoring the obtunded, intubated, regionally blocked or paediatric patient, not for replacing clinical judgement in the awake one.

The Stryker STIC device is the one most commonly used in practice. The highest pressure is usually within 5cm of the fracture, so that is where to measure; measure all the compartments, and repeat the readings in borderline cases.

1. Sterilise the skin
2. Insert the needle into the compartment at 90 degrees
3. Inject a small amount of saline
4. Read the pressure on the digital display
5. Measure at multiple points, within 5cm of the fracture site

An arterial line transducer is more accurate but less practical: a standard arterial line configuration zeroed at the level of the compartment, an 18G needle into the compartment, flushed with saline and read. Whitesides' technique, the original method, used a mercury manometer with intravenous tubing and saline; it is more cumbersome and less accurate than modern methods and is now mainly of historical interest.

Where the threshold comes from. McQueen and Court-Brown monitored the anterior compartment continuously for 24 hours in 116 tibial diaphyseal fractures (PMID 8898137). In the first 12 hours alone 53 patients exceeded an absolute 30mmHg and 30 exceeded 40mmHg, yet only 3 patients (2.6%) developed true acute compartment syndrome, and none of the 116 had any sequelae at six-month review. Decompressing on an absolute threshold of 30mmHg would have sent 50 patients (43%) to an unnecessary fasciotomy, and even a 40mmHg threshold would have sent 23%; a delta P of 30mmHg missed no cases.

The error runs in both directions. Treating a high absolute pressure as an automatic indication over-operates on a scale of roughly 4 in 10 monitored tibial fractures, and a fasciotomy is not a benign investigation but a wound that needs closure or grafting. Waiting for an absolute number to look alarming in a hypotensive trauma patient under-calls the genuine emergency: a patient with a diastolic pressure of 50mmHg and a compartment pressure of 25mmHg has a delta P of 25, and this indicates ACS even though the absolute reading is under 30. Quote delta P as the threshold and the absolute reading only as a component of it. The study's own limits apply: single centre, anterior compartment only, and continuous monitoring equipment that is not universally available.

Other investigations.

• Bloods - CK (elevated with muscle damage), renal function, coagulation
• Urine - myoglobinuria (dark urine)
• No role for imaging in the acute diagnosis; do not delay for CT or MRI

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Volkmann's ischaemic contracture is the end result of untreated volar forearm ACS: the ischaemic muscle necroses, fibroses and contracts, and the hand is left in the classic claw-hand posture of flexed wrist, extended MCP joints and flexed IP joints. The fingers extend as the wrist is flexed, the cascade sign. Treatment is reconstruction: muscle slide, tendon lengthening or free muscle transfer.

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Rhabdomyolysis releases myoglobin, turns the urine dark, and can lead to acute kidney injury. Manage it with aggressive intravenous fluids to a urine output over 1ml/kg/hr, alkalinise the urine with sodium bicarbonate, and monitor renal function and electrolytes for the risk of hyperkalaemia.

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What to monitor. CK, which peaks at 24-72 hours; urine output and colour; renal function (creatinine); potassium, because cell lysis causes hyperkalaemia; and the wound.

What decides the outcome. Time to fasciotomy is the most important factor, then the completeness of the fasciotomy, the severity of the underlying injury, and the patient's age and comorbidities.

Medicolegal. Missed or delayed compartment syndrome is a common cause of medical litigation in orthopaedics, and delay is the most common cause of it. Documentation of serial clinical assessments, pressure measurements and the timing of intervention is essential, and early involvement of senior colleagues is prudent.

Global epidemiology:

ACS is a worldwide trauma problem driven by high-energy mechanisms (road traffic, falls, sport, crush and industrial injury). Distribution shifts with regional injury patterns and the time-to-presentation differs sharply between well-resourced trauma systems and rural or limited-resource settings, where delayed transfer worsens outcomes.

Major guidance, side by side:

Registry & population evidence:

There is no dedicated compartment-syndrome registry; the evidence base is built on prospective and cohort series (chiefly the Edinburgh series - PMIDs 8898137, 8898136, 10755426). National trauma registries (e.g. the UK TARN and equivalents) capture ACS as a complication of long-bone trauma and consistently identify young men with tibial shaft fractures as the highest-risk cohort, mirroring the published series.

Global practice variation: