Intramuscular myxoma

Last revised by Joshua Yap on 7 May 2024

Intramuscular myxomas are a rare benign type of soft tissue myxoma that is mesenchymal in origin.

The incidence is reported to be 1 in 1,000,000 and most frequently diagnosed in females (~57%) between 40-70 years  1.

The typical presentation is that of a slowly enlarging mass(es) in 64% of patients 1. The lesions are a deeply seated mass confined to skeletal muscle and without malignant potential. A painful mass will be present in ~55% of patients 1,3

Myxomas have been reported to range in size from 1-17 cm 1,2. Intramuscular myxomas can be solitary or multiple, when multiple myxomas are present they are associated with monostotic or polyostotic fibrous dysplasia, known as Mazabraud syndrome 1,3-5.

Intramuscular myxomas can be located in any skeletal muscle group however most commonly occur in the quadriceps (65%), hip adductors (~35%), gluteus muscles (~20%), gastrocnemius and upper arm 1,3. When occurring in conjunction with fibrous dysplasia they are typically located in the vicinity of the bone lesions. Both fibrous dysplasia and intramuscular myxomas most commonly affect the pelvic girdle and lower limbs (75%). The upper limbs are affected in less than a quarter of reported cases.

  • lesions are generally hypocellular and hypovascular although areas of increased vascularity and cellularity may be present 1

  • there are loose reticulin fibers within an abundant myxoid stroma 1,6

  • the cells have a stellate shape with small hyperchromatic pyknotic nuclei and scanty cytoplasm

  • there is the absence of nuclear atypia, mitotic figures or necrosis 6

There may be a link with postzygotic mutations of the GNAS1 gene located on chromosome 20q13.2-q13.3, particularly when associated with Mazabraud syndrome 3-5GNAS1 encodes the alpha subunit of G-protein (GSa) involved in cell proliferation 3-5. This mutation affects the mesenchymal precursor cells at the early stages of commitment 5.

Typically seen as a well-circumscribed intramuscular mass. Some may have partially ill-defined borders.

Most commonly demonstrate normal findings (~ 55%) or a non-specific soft-tissue mass (45%) 1.

Demonstration of a well-defined hypoechoic to near anechoic mass to the surrounding soft tissue and often shows a heterogeneous echotexture 1. Posterior acoustic enhancement may be observed in a significant portion of cases. There may be some internal echoes and increased through transmission 1.

In ~85% of cases, there is a sonographic bright rim sign of increased echogenicity around the myxoma (i.e. peripheral rim of echogenicity) 1. Frequently noted is the bright cap sign seen as a triangular hyperechoic area adjacent to at least one of the poles of the mass 1,12. The lesion is hypovascular or avascular on color Doppler ultrasound.

Intramuscular myxomas appear as well-demarcated, homogeneous and hypodense ovoid lesions. The soft tissue mass attenuation is higher than that of water and less than that of surrounding tissue 1. They usually show mild diffuse enhancement or peripheral and septal enhancement seen in approximately 50% of cases 1.

MRI features include:

  • intra-tumoral cysts may be present in a small proportion of cases

  • pseudocapsule, perilesional/rim of fat and perilesional edema may be present with bright cap sign 11,12

  • T1: hypointense to muscle

  • T2: hyperintense

  • T1 C+ (Gd): four different patterns have been described 10

    • peripheral enhancement

    • peripheral and patchy internal enhancement

    • peripheral and linear internal enhancement

    • heterogeneous internal enhancement

Intramuscular myxomas may show F18-FDG avidity on PET-CT 4,9.

As the myxomas are benign, conservative treatment is recommended 10. Myxomas should be excised if they cause pain, pressure symptoms, neurological symptoms or interfere with functionality.

A case study described the use of bisphosphonate therapy providing a clear reduction in the diameter of intramuscular myxoma after the use of zoledronic acid for 4 years 5.

Histopathological investigation should be performed when there is an uncertainty of diagnosis to exclude other differentials including primary malignancy or metastatic disease 10.

The most common complication of myxomatous surgical excision is recurrence which occurs in ~30% of cases at a median of 8.5 years (range: 1.9-16 years) 3. Highly cellular lesions have higher recurrence rates however size, increased number and younger age at diagnosis are not significantly associated with recurrence 3.

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