Intramuscular hemangiomas

Intramuscular hemangiomas, also known as intramuscular angiomas, intramuscular capillary-type hemangiomas, and intramuscular fast-flow vascular anomalies ⁵, are vascular lesions consisting of benign vascular channels within skeletal muscle.

Terminology

In much of the scientific literature, the term "intramuscular hemangioma" continues to be incorrectly applied to many different entities, most commonly to intramuscular venous malformations.

In this article, as per the ISSVA classification of vascular anomalies (c. 2018), intramuscular hemangiomas refer to the provisionally unclassified vascular anomalies category, which is distinct from venous malformations (VM), arteriovenous malformations (AVM), fibroadipose vascular anomaly (FAVA), PTEN hamartoma of soft tissue (PHOST), and angiomatosis of soft tissue (AST).

Epidemiology

Intramuscular hemangiomas are rare lesions ¹. There is a peak in diagnosis in young adults and adolescents but they are found in a wide age range and most are thought to be congenital. There is no gender predilection ^1,2.

Diagnosis

Intramuscular hemangiomas can be diagnosed by a combination of imaging, pathological and possibly clinical features.

Diagnostic criteria

Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5^th edition) ¹:

• vascular mass consisting of mixed vessel types including lymphatics

• location within the muscle

• often associated with pain aggravated by exercise

Clinical presentation

The typical manifestation is a very slow-growing painless mass ². They can cause chronic pain, especially after exercise ^1-3. Rarely, they can result in functional impairment or deformity ³. There is generally no skin discoloration, warmth or palpable thrill ³. They can remain stable in size over time, show proportional growth to children or grow slowly disproportionately to patient size ².

Pathology

Intramuscular hemangiomas are soft tissue lesions consisting of different types of vascular channels including lymphatic vessels within the muscle often associated with variable amounts of adipose tissue ¹.

Etiology

Their etiology remains unknown at the time of writing ¹. Pathogenic somatic mutations in MAP2K1 and KRAS genes have been described in a subset of cases, which overlap with mutations found in arteriovenous malformations ⁵.

Location

Typical sites of intramuscular hemangiomas are the following  ^1-4:

• lower limb as thigh and calf (most frequent)

• head and neck

• upper limb

• trunk

Macroscopic appearance

Macroscopically, intramuscular hemangiomas often infiltrate the affected muscle diffusely and contain multiple vascular channels, occasionnally with thrombosis and hemorrhage. When capillaries are the predominant vessels, lesions tend to have a solid appearance and yellowish color ¹. Lesions are non encapsulated.

Microscopic appearance

The lesions are composed lobules or sheets of inumerable small, capillary-sized vessels with plump nuclei extending between individual or groups of muscle fibers. They can show occasional mitotic activity and intraluminal papillary tufting, but no cellular atypia ^2,3.

The histological appearance of intramuscular venous malformations reflects their mixed composition of different vessel types ¹:

• large thick-walled veins

• cavernous vascular spaces

• capillaries

• lymphatic

• a variable amount of mature adipose tissue

• possible perineural involvement

Radiographic features

Few cases have been reported in the radiologic literature, making generalizations hard. Being intramuscular and often occuring in younger patients, these lesions are best imaged with ultrasound and MRI.

Plain radiograph

The only recent large series with radio-pathological correlation ² did not report radiographic findings. In four cases of a 1972 article on "small-vessel" type skeletal muscle hemangiomas, considered to be the same entity now known as intramuscular hemangiomas, none of the cases shows radiographic calcification ^2,4.

Ultrasound

On ultrasound intramuscular hemangiomas are described as hypoechoic or isoechoic lesions ², but they can also be hyperechoic relative to skeletal muscle (as in cases 1, 2 and 4 of this article). Contrary to venous malformations, they are not compressible with pressure on the probe. On Doppler, they show pulsatile arterial waveforms, often high resistance ².

CT

Though little has been reported in the literature, they seem to be hypodense on unenhanced CT with early hyperenhancement relative to skeletal muscle. Though early venous drainage signifying arteriovenous shunting is rare, it can occur.

MRI

Intramuscular hemangiomas are generally moderately hyperintense on T2WI, iso- or mildly hyperintense on T1WI and homogeneously hyperenhancing relative to skeletal muscle. They can have peri- and/or intra-lesional flow voids and fat content.

Radiology report

The radiological report should include a description of the following:

• form, location, size and signal characteristics

• lesion margins and transition zone

• relations to surrounding structures

• presence or absence of fat content and flow voids

Treatment and prognosis

Intramuscular hemangiomas are benign. Asymptomatic lesions can be initially treated conservatively. Surgical excision, with or without preoperative embolization has been described for symptomatic cases.

History and etymology

In 1972, a retrospective review of histopathological findings in 89 patients described as having intramuscular hemangiomas classified these based on vessel size and histological features as (1) small vessel, (2) large vessel and (3) mixed types ⁴. The small-vessel type appears identical to what is now referred to as intramuscular hemangioma, whereas the large vessel type is concordant with venous malformations and the mixed types seem to represent to several different entities ².

Tumors previously described as “infiltrating angiolipomas of muscle” or “benign mesenchymoma” are likely intramuscular hemangiomas with marked fatty overgrowth ³.

Differential diagnosis

Imaging appearance cannot reliably distinguish between intramuscular hemangiomas and sarcomas, so biopsy has been recommended in all suspected cases ². Other benign lesions may have similar appearances, such as PTEN hamartoma of soft tissue (PHOST).