SCIENTIFIC
Proliferative Myositis Presenting as a Rapidly Enlarging Neck Mass in a 2-Month-Old
AUTHORS: Benjamin W Clark, BS 1
Anderson G White, BS 1 Adam Van Horn, MD 2
1
Marshall University School of Medicine, Huntington, WV
2
University of Michigan, Department of Otolaryngology, Head and Neck Surgery, Ann Arbor, MI
CORRESPONDING AUTHOR: Benjamin Clark clark579 @ marshall. edu
DISCLOSURES:
This work was presented at the 2023 Society for Ear, Nose and Throat Advancement in Children Annual Meeting, Charleston, SC, November 30- December 2, 2023.
ETHICS STATEMENT: Permission obtained from parent.
FUNDING: None
CASE PRESENTATION
A full term, otherwise healthy 2-monthold male presented with a right neck mass. The mass was not present at birth and rapidly grew approximately three centimeters in less than a month prior to presentation. On physical examination, the infant was well appearing with a firm, well circumscribed and mobile right-sided neck mass, with no overlying skin change. The mass seemed incorporated within the body of the sternocleidomastoid( SCM) muscle, which was non-tender to palpation. There were no limitations of range of motion with neck extension, flexion, and rotation, nor was there preferential neck posture. Differential diagnosis at this time included proliferative myositis, fibromatosis colli, and rhabdomyosarcoma. Ultrasound and computed tomography studies were obtained, revealing a well-defined mass
ABSTRACT
Proliferative myositis is an uncommon benign soft tissue lesion characterized by rapid growth within skeletal muscle tissue. We describe a case of proliferative myositis in a 2-month-old male who presented with a painless, rapidly enlarging mass in his right sternocleidomastoid muscle. A biopsy of the mass rendered the diagnosis of proliferative myositis. The mass was observed with full resolution after
within the right SCM, displacing the surrounding tissues rather than infiltrating( Figure 1). Cytology from fine needle aspiration revealed atypical spindle cells, prompting an incisional biopsy for definitive diagnosis. Histopathological findings included proliferation of benign fibroblasts and myofibroblasts, accompanied by distinctive ganglion-like polygonal cells and spindle cells. Immunohistochemical staining showed S-100 and keratins were negative, while smooth muscle actin stained positively in myofibroblasts and ganglion-like cells( Figure 2). Given this process occurred within the muscle body, a diagnosis of proliferative myositis was rendered. The patient was observed and follow up magnetic resonance imaging showed regression of the mass( Figure 3). The lesion then fully resolved on physical examination four months following the incisional biopsy, and the patient maintained normal SCM strength and tone.
DISCUSSION
Proliferative myositis is a rare, benign proliferative process resulting in a soft tissue mass. Sometimes termed a pseudosarcomatous lesion, proliferative myositis can present as a rapidly enlarging mass concerning for malignancy. 1 Proliferative myositis most often presents in adult patients in the upper extremity or trunk. Head and neck involvement is less frequently seen, and pediatric cases are rare. 2 The etiology of proliferative myositis remains unknown. Preceding trauma has been observed as a correlation in multiple cases, and alternative causes
four months, highlighting the importance of considering proliferative myositis in the differential diagnosis of rapidly enlarging soft tissue masses and emphasizing successful non-surgical management. Further understanding and awareness of this rare diagnosis are crucial for appropriate management strategies. To our understanding, our patient is the youngest reported with this disorder.
FIGURE 1
Axial computed tomography scan with intravenous contrast demonstrating a mass within the right sternocleidomastoid muscle.
FIGURE 2
Immunohistochemical staining for smooth-muscle actin, 200x magnification.
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