Background: Spontaneous neck hematoma are rare, and can potentially be life threatening if they affect airway. Here we report a case of spontaneous Sternocleidomastoid hematoma in a patient who was initiated on enoxaparin therapy after developing Atrial fibrillation during the hospital course.  It is known that anticoagulation is needed in patients with new onset atrial fibrillation to decrease the risk of thromboembolic events. However, use of anticoagulants in critically ill patients poses a huge risk of bleeding. We present a rare case of a patient on therapeutic dose Enoxaparin for anticoagulation for Atrial fibrillation, with CHADS2-VASC of 4 and HAS-BLED of 2, who went on to develop a massive intramuscular sternocleidomastoid hematoma. One of the known major complications is hemorrhage (1-10%), with a handful of cases of spontaneous anterior abdominal wall hematoma being reported until date with varying duration of enoxaparin use. We are reporting the first case of Sternocleidomastoid hematoma in the setting of Enoxaparin use. 

CASE: A 85-year-old man with a past medical history of hypertension, hyperlipidemia, diabetes mellitus   who presented to the hospital for altered mental status and urinary incontinence. His labs were significant for a WBC of 15 and a positive urinalysis. His urine and blood cultures were positive for Methicillin-resistant Staph Aureus (MRSA). On hospital day 3, he developed new onset atrial fibrillation. Echocardiogram showed systolic dysfunction with EF 40-45% with mild to moderate mitral regurgitation. Given his high risk CHADS2-VASC and HAS-BLED scores, he was started on therapeutic Enoxaparin. On hospital day 9, the patient suddenly developed an expanding neck hematoma with new concerns for neck pain and dysphagia. Vital signs at the time, including pulse oximetry, were stable with no concern for airway compromise. A CT neck revealed a large right sternocleidomastoid intramuscular hematoma. The patient was transferred to the ICU for close airway monitoring for concerns of hematoma expansion and need for emergent intubation. Vascular surgery was consulted with no planned intervention. Enoxaparin was discontinued and Protamine was given for reversal. Fortunately, the patient did not develop any further complications except for pain in the neck. He remained hemodynamically stable and was transferred out of the ICU 24-hours later. His hemoglobin remained stable as well and he as eventually discharged on antibiotics, Cardizem, and Amiodarone. There were plans to re-start anticoagulation in the near future with outpatient cardiology follow-up. 

DISCUSSION: Low molecular weight heparins (LMWH) have several advantages compared to unfractionated heparin including longer biological half-life, increased bioavailability and predictable pharmacokinetics negating the need for monitoring. Its clinical use is widespread ranging from treating Non-ST segment elevation myocardial infarctions to thromboprophylaxis in post-operative hip arthroplasties. However, LMWHs including enoxaparin are associated with major and minor bleeding risk.  Commonly reported enoxaparin-induced bleedings are in the abdominal wall and rectus sheet, attributed to the location of the site injection. Major bleedings events from Enoxaparin use in the setting of atrial fibrillation are rare (1.6%). The case presented in this article highlights a very rare bleeding complication seen in patients on therapeutic Enoxaparin. The importance of recognizing this potential complication is in the rapidity of subsequent management in order to prevent life-threatening hemorrhage. 

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