Diagnosing Chronic Exertional Compartment Syndrome: Measurement of Intra-Compartmental Pressures Using Noninvasive Subharmonic Aided Pressure Estimation (SHAPE) Versus Shear Wave Elastography (SWE).

Chronic exertional compartment syndrome (CECS) is an innocuous condition seen primarily in 10-60% of young, active people with exercise induced leg pain. Patients present with anterior lower extremity pain that worsens with exercise and resolves after rest. CECS arises from increased intra-compartmental pressure causing impaired tissue perfusion. The average delay in diagnosis and subsequent treatment is 2 years, which has been shown to decrease the success rate of both conservative and surgical therapy. Definitive treatment is with fasciotomy, which has a success rate of up to 95%.

The etiology of CECS is not well understood but is thought to arise from volume expansion of a muscle within a noncompliant space bounded by fascia and bone resulting in insufficient blood flow and a resultant oxygen supply and demand mismatch in that compartment. In some cases the physiological response may lead to a 20% increase in muscle volume. Several risk factors have been identified including pre- existing fascial defects (seen in 40% of CECS patients) and a smaller capillary density to muscle size ratio. In rare cases, CECS may progress to acute compartment syndrome - a surgical emergency requiring emergent fasciotomy.

The gold standard for diagnosing CECS is direct measurement of intra-compartmental pressures with maximum sensitivity and specificity in recent studies of 93% and 74%. However, in rare circumstances, diagnosis can be made on clinical basis alone. Compartment pressure testing is performed by inserting a handheld large gauge needle with a pressure monitor into the muscle and measuring the compartment pressure directly (in mmHg). Direct compartment testing is invasive, painful, and carries a complication risk of neurovascular damage and infection. Furthermore, there is significant variability in this technique with some studies finding more than >5 mmHg difference in 40% of compartmental pressure measurements.

In the diagnostic algorithm of CECS, imaging is primarily used to rule out other more common causes of lower extremity leg pain such as medial tibial stress syndrome (MTSS), stress fractures, and muscle strains. Several non-invasive imaging modalities have been used to diagnose CECS. When compared to pressure testing, MRI showed similar sensitivity, but lower specificity (< 60%). While MRI has shown some diagnostic promise, it is more expensive, less ubiquitous, and less accurate than compartment testing. Near-infrared spectroscopy, which measures hemoglobin O2 saturation of tissues, has shown to have clinically equivalent sensitivies (85%) compared to compartment testing, however, it is not readily available.

A clinical exam alone is insensitive and non-specific in the diagnosis of CECS. Therefore, surgeons must rely on a combination of clinical exam and imaging to determine whether a patient is a surgical candidate. An accurate, non-invasive, and cost effective diagnostic tool does not currently exist for patients with suspected CECS. SWE is a safe, non-invasive and relatively inexpensive modality that has wide ranging diagnostic capabilities. SWE is used measure liver stiffness thereby staging fibrosis in chronic liver disease, following up previously diagnosed hepatic fibrosis and evaluating patients with portal hypertension. Recently, the utility of SWE in musculoskeletal imaging has increased; for example SWE is currently being using to evaluate tendinopahic achilles tendons with some clinical success.