Rare case of Intramuscular endometriosis of forearm: a report

Endometriosis is a disorder that commonly affects females of reproductive age and is defined as the presence of endometrial glands or stroma outside the uterine cavity. Patients typically present with cyclical pain during menses. Endometriosis can be characterized as endopelvic or extra pelvic depending on the sites involved.

Smith et al reported a case of a 40-year-old, right-hand dominant, female who presented with a painful mass in her right proximal forearm. She was ultimately diagnosed with intramuscular endometriosis and underwent surgical excision.

A 40-year-old, right-hand-dominant, female (G3P3003) presented to the Orthopedic Department with an 18-month history of an intermittently painful mass in the medial aspect of her right forearm. The mass would become increasingly tender and firm in the days before menses and during menstruation. Her past medical history was notable for relapsing remitting multiple sclerosis (MS), hypertension, dyslipidemia, anxiety, and morbid obesity. She denied any excessively heavy or painful menses.

Physical examination revealed a small palpable mass in the right flexor pronator musculature and was tender to palpation. Previous imaging, including radiographs of the right forearm demonstrated no acute osseous or soft tissue abnormality. Magnetic resonance imaging (MRI) without intravenous contrast showed an oval intramuscular mass in the anterior compartment of the proximal forearm, measuring approximately 2.2 × 2.1 × 3.0 cm. The mass demonstrated heterogenous T2 signal and cystic changes. There was no invasion of deeper musculature or fascia.

On ultrasound imaging, the mass was circumscribed and hypoechoic. The mass also demonstrated internal doppler flow. Unfortunately, the ultrasound guided core needle biopsy was nondiagnostic. Therefore, the patient elected to undergo open biopsy with frozen section diagnosis, which was consistent with endometriosis.

The patient was referred to Obstetrics/Gynecology for further evaluation and to discuss treatment options. Additional history obtained at that time revealed that she had delivered all three of her children by uncomplicated Cesarean section. Physical examination did not result in any concern for pelvic endometriosis. Dienogest, a progestin-based medication, was recommended with the goal of suppressing the proliferation of the endometrium to decrease symptoms. As the patient was still working to achieve remission of her MS with medications, she did not wish to add another medication at that time and declined the progestin medication.

Approximately 18 months after initial presentation to the orthopedic oncology clinic, the patient’s symptoms worsened with pain now radiating into the elbow, distal forearm, and hand. She elected to trial Dienogest; however after several months, this did not improve her symptoms. After discussion with the orthopedic surgeon, the patient ultimately decided to undergo surgical excision of the mass. At 6 months postoperatively, the patient remains significantly improved compared to pre-resection. She does have very mild soreness at the forearm at the time of menstruation but states it does not bother her.

The authors commented – “In summary, we report an extremely rare case of intramuscular endometriosis in the proximal forearm. Extremity endometriosis may be included in the differential diagnosis of soft tissue tumors when symptoms of pain or a palpable mass occur in the extremities of women of reproductive age, particularly those masses with a cyclic pattern, which coincides with the menstrual cycle.”

Further reading:

Intramuscular endometriosis of the forearm: a case report Kira L. Smith et al Skeletal Radiology https://doi.org/10.1007/s00256-024-04648-w