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Background
Rectus sheath hematoma (RSH) is defined as a collection of blood within the rectus sheath, which is divided into three types, based on the extent of posterior extension of the hematoma.

Educational Goals / Teaching Points
Type I RSH are confined within the rectus muscle and are unilateral. Type II are also limited to within the rectus muscle, but additionally cross the midline or dissect along the transversalis fascial plane. Type II RSH can be unilateral or bilateral. Type III is typically below the arcuate line and extends into the peritoneum and prevesical space. The etiologies of RSH can vary, but trauma, coughing, and strenuous physical activity are common causes. Other less common etiologies include anticoagulant therapy and vascular malformations. Clinical manifestations of RSH can include abdominal pain, swelling, and a palpable mass. However, these symptoms overlap with many intraabdominal pathologies, making imaging crucial for accurate diagnosis. Relevant laboratory measurements for RSH include a complete blood count, coagulation studies, and blood typing and cross-matching. Although there is no established treatment algorithm for RSH, one should consider the patient’s clinical status and RSH classification to guide treatment. Conservative therapy is typically first-line for all types of RSH in patients without coagulopathy. For patients with coagulopathy, treatment is centered around addressing the etiology of the coagulopathy. For example, stopping anticoagulation/antiplatelets or transfusing with coagulation factor concentrate, fresh frozen plasma, or packed red blood cells. Additionally, reversal of any offending medication (i.e., vitamin K, protamine sulfate) may be necessary. Angioembolization is a last-line therapy for persistent bleeding in RSH cases, and surgical intervention carries risks and is generally avoided unless there are specific indications.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Ultrasound and CT are reliable imaging modalities for identifying RSH. Case series have shown ultrasound is around 80% sensitive and specific, while CT is near 100% sensitive and specific. On ultrasound, RSH is typically a heterogeneous collection involving the rectus muscle which may be difficult to delineate from other abdominal wall structures. Ultrasound appearance can vary depending on the chronicity and extent of the hematoma. In the acute phase, CT demonstrates a hyperdense collection within the rectus muscle. Subacute to chronic RSH can have a heterogeneous hyper- and hypodense appearance. Additionally, CTA with delayed images is important to determine if there is active bleeding, indicated by extravasation/pooling of contrast.

Conclusion
The prognosis for RSH is excellent, with most cases resolving spontaneously within weeks to months. Interventional radiologists could embolize an RSH bleed via the inferior epigastric artery, but most cases do not require that intervention. As a radiologist, it is important to have an understanding of rectus sheath and fascial anatomy to identify and ensure prompt management/resolution of this potentially life-threatening pathology.