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Case presentation

A healthy 51-year-old female presented with a three-day history of sudden onset neck mass, just to the left of midline at the inferior border of the thyroid cartilage. The mass was mildly tender. There was no history of trauma to the area. She denied fevers, chills, dysphagia, odynophagia, otalgia, hoarseness, dyspnea, weight loss, or night sweats. Her primary care provider ordered a thyroid ultrasound which showed focal bulging of the left sternohyoid muscle with a hypoechoic region, consistent with possible phlegmon ( Fig. 1 ). She was referred to our service for further evaluation and a computer-assisted tomography (CT) scan of the neck with contrast was ordered. The CT showed a hyperdense, well-circumscribed, 11 × 7 × 13 mm focus in the left sternohyoid muscle ( Fig. 2 ). An initial differential diagnosis included reactive inflammation of a thyroglossal duct remnant, an abnormal lymph node, and thyroid carcinoma. Overlying vascularity at the site of the mass precluded the ability for routine fine needle aspiration.

Ultrasound image showing a hypoechoic focal bulge of the left sternohyoid muscle. A “steel-cable like pattern” was not appreciated.

Coronal, soft-tissue window, CT of the neck with contrast showing a hyperdense well-circumscribed focus within the sternohyoid. Large venous structure is also seen superficial to the mass, which prevents FNA.

Resection of the anterior neck mass was performed through transcervical approach. A 3-cm horizontal incision was made at the level of the cricoid cartilage. Intraoperatively, the mass was noted to be incorporated into the left sternohyoid muscle without deep attachment to underlying trachea or cricoid. Dissection allowed for resection of the mass with a cuff of normal-appearing muscle and soft tissue surrounding it.

Gross pathologic examination revealed a 22 × 10 × 15 mm pink–gray mass, which was well-circumscribed with infiltrative features. Microscopically, it was composed of spindle cells centrally showing tissue culture fibroblast quality and peripherally maturing to phenotypically spindle forms ( Fig. 3 ). No atypical mitoses were seen. Based on this, a diagnosis of proliferative myositis was made. At 3-month follow-up, the patient was doing well with no evidence of recurrence.

10 ×: The mass is cellular with large fibroblasts and myofibroblasts surrounding individual muscle fibers.

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Discussion

Proliferative myositis was first described by Kern in 1960 . It is a rare, benign tumor that can mimic sarcoma with its sudden onset and rapid growth. Patients typically present at an average age of 50 with a solitary mass, usually located in the proximal limbs, shoulder, or arms . However, in increasingly rare cases, proliferative myositis has been reported in the head and neck region. As of 2008, approximately 25 such cases had been reported, none of which involved the sternohyoid muscle . Reported locations in the head and neck include the sternocleidomastoid , mylohyoid , masseter , buccinator , and temporalis .

The etiology of proliferative myositis is unknown at this time, however several theories have been suggested. In the initial report by Kern, trauma to the muscle was suggested as the inciting factor . Other theories include local ischemia, paracrine myopathy, and trisomy and translocation .

The signs and symptoms of proliferative myositis in the head and neck depend upon the tumor’s size, location, and growth rate. These masses tend to present as a fast-growing lesion that appears centered within skeletal muscle. Associated pain is not uncommon due to the quick growth rate, and there are usually no signs of inflammation or lymphadenopathy on clinical exam .

Following appropriate history and physical exam, radiographic investigation is warranted. Ultrasonography is a particularly valuable imaging modality in this entity. Findings of hypoechoic geometric lines within dense, hyperechoic muscle, described as a “scaffolding” or “steel cable-like” pattern, have been reported as pathognomonic . However, this was not the case with our patient, and prompted evaluation with CT. In addition, clinicians may choose to pursue CT or MRI initially with a rapidly enlarging neck mass. CT imaging shows a poorly demarcated intramuscular lesion with irregular borders . Magnetic resonance imaging (MRI) typically reveals a lesion which is hypo- or isointense to surrounding muscle on T1 weighted sequences, while T2 weighted sequences may show the lesion to be strongly hyperintense, suggestive of an inflammatory process . Definitive diagnosis, however, cannot be made unless tissue is obtained for pathology, either by fine needle aspiration biopsy (FNAB), or by incisional biopsy if the diagnosis remains uncertain. Characteristic findings on histology include; a “checkerboard” pattern of myofibroblasts that infiltrate the surrounding healthy muscle, ganglion-like basophilic giant cells, and no sign of atypical mitosis .

Treatment options for proliferative myositis include watchful waiting or surgical excision. Watchful waiting is favored as reports in the literature have noted that these lesions tend to spontaneously regress, with complete resolution occurring over weeks to months . In cases where the tumor continues to progress, or causes impaired movement or altered cosmetic appearance, or concern for malignancy remains high despite work up (as in our case) then conservative surgical excision is recommended. Radical excision should be avoided as there have been no reports of either recurrence or malignant transformation in proliferative myositis .

In conclusion, proliferative myositis is a rare, benign tumor that infrequently presents in the head and neck region. It can easily be mistaken for malignancy due to its rapid growth, however further work-up, including imaging and FNAB, may suggest the diagnosis. Management can include watchful waiting or surgical resection. Recurrence has not been document to occur. To date no malignant transformation has been reported. We present a case of proliferative myositis of the sternohyoid muscle, not previously described in the literature.