Core clinical reasoning
This patient has a closed tibia–fibula fracture treated with closed reduction and a long-leg plaster cast. One hour after
4 mg of IV morphine, his pain has risen from
4/10 to
9/10. The toes are pink and warm, capillary refill is
2 seconds, and the dorsalis pedis pulse is palpable and equal to the opposite side. However, pain increases sharply with gentle passive downward bending of the toes.
Pain that is out of proportion to the injury, poorly relieved by opioids, and worsened by passive stretch of the compartment muscles is the earliest and most reliable indicator of acute compartment syndrome. The normal pulse, color, and capillary refill do not rule out the problem because these findings often remain intact until muscle and nerve damage is already advanced.
Key point! In acute compartment syndrome, arterial flow is usually preserved even when tissue perfusion pressure is critically low. The dorsalis pedis pulse and a
2-second capillary refill reflect arterial inflow, not the microvascular perfusion inside the compartment. Therefore, normal distal pulses are a late-preserved finding and cannot be used to exclude compartment syndrome.
Why passive stretch pain matters
The lower leg has four osteofascial compartments: anterior, lateral, deep posterior, and superficial posterior. When tissue pressure inside a closed compartment rises above capillary perfusion pressure, muscle ischemia begins.
Ischemic muscle becomes sensitive to stretch because stretching increases metabolic demand and further compresses the already hypoxic tissue.
In this patient, bending the toes downward passively stretches the anterior compartment muscles, including the
tibialis anterior and the long toe extensors. The sharp pain with this maneuver is a direct clinical sign of rising pressure in the anterior compartment, which is the most commonly affected compartment after tibial fractures.
The classic sequence of compartment syndrome is pain, then paresthesia, then paresis, and only later pallor and pulselessness. By the time pulses disappear, irreversible muscle necrosis may have already occurred.
Why the other options are incorrect
| Option | Why it is not the best answer |
|---|
| 1. Pain on moving the toes suggests a deep calf vein clot | Deep vein thrombosis typically causes calf swelling, warmth, and tenderness along the deep venous system, not sharp pain specifically triggered by passive toe stretch. It also does not produce the characteristic pain-out-of-proportion pattern seen here. |
| 2. The pulse and 2-second refill rule out a circulation problem | This is the most dangerous misconception. Pulses and capillary refill reflect arterial inflow and remain normal in early compartment syndrome. Relying on them delays diagnosis and risks limb loss. |
| 4. This is expected fracture pain needing a higher morphine dose | Fracture pain typically improves with immobilization and opioids. Pain that escalates despite adequate analgesia and is provoked by passive muscle stretch is a red flag, not an indication to simply increase the opioid dose. |
Pathophysiology of compartment syndrome
Compartment syndrome occurs when
interstitial tissue pressure within a closed osteofascial space exceeds the perfusion pressure needed to maintain capillary blood flow. Normal tissue pressure is below
10–12 mmHg. When pressure rises, venous outflow is compressed first, which further increases interstitial pressure and reduces arteriovenous pressure gradient. The result is progressive muscle and nerve ischemia.
Common causes after extremity trauma include fracture bleeding, soft-tissue swelling, and restrictive dressings or casts. In this patient, the long-leg cast itself may be contributing to external compression, and the fracture hematoma is adding internal volume to the compartment.
Watch out! A cast that is too tight or a limb that continues to swell inside a rigid cast can precipitate compartment syndrome even after a successful closed reduction. The cast may need to be split or bivalved, and the surgeon must be notified immediately.
Clinical assessment priorities
The earliest findings are
pain out of proportion and
pain with passive stretch. Other signs include a tense, firm compartment on palpation, paresthesia in the distribution of nerves traversing the compartment, and later motor weakness. The classic “five Ps” — pain, paresthesia, paresis, pallor, pulselessness — are a late and unreliable framework because pulselessness and pallor appear only after irreversible damage has begun.
In the emergency setting, serial clinical examination is essential. If compartment syndrome is suspected, compartment pressure measurement may be performed, but clinical suspicion alone is sufficient to warrant immediate surgical evaluation.
The nurse’s next action is to report the findings now and prepare for urgent intervention, which may include splitting the cast, elevating the limb only to heart level, and preparing for fasciotomy if pressure remains elevated. Elevating the limb above heart level is avoided because it reduces arterial perfusion pressure and worsens ischemia.
Nursing implications for licensure exams
A common exam scenario presents a patient with a fracture in a cast who has severe pain unrelieved by opioids, with intact distal pulses. The correct response is always to suspect compartment syndrome and notify the provider immediately, not to administer more analgesia or reassure based on normal perfusion findings.
The key discriminator in this question is the combination of
9/10 pain despite recent IV morphine and the sharp increase in pain with passive toe flexion. These two findings together override the reassuring pulse and capillary refill data.
Key point! Normal neurovascular checks — pink toes, palpable pulse, capillary refill under
3 seconds — do not exclude compartment syndrome. The earliest and most reliable sign is pain that is disproportionate and worsened by passive stretch of the affected compartment muscles.