Laboratory Studies
The diagnosis of glomus tumor is primarily clinical, and laboratory workup usually is not needed. If the diagnosis is uncertain in view of the patient's history and physical findings, imaging studies are needed to assess the distal phalanx and the distal interphalangeal (DIP) joint. Various imaging modalities have been used to diagnose glomus tumors (see Imaging Studies). [29, 30, 31, 32]
Imaging Studies
Results of plain radiography are often unremarkable, but radiographs may show a well-circumscribed osteolytic lesion with a sclerotic border or a soft-tissue mass causing bone erosion.
Computed tomography (CT) is indicated for accurately assessing tiny abnormalities of the cortex of the distal phalanx. CT scans demonstrate a nonspecific nodule or mass, either in the soft tissue or within the bone.
For the initial workup, color Doppler ultrasonography (US) is an appropriate first test. US may be helpful for depicting a cystic or a vascular component of a tumor. US and Doppler study yield better visualization of the tumors than standard radiology does; however, these sonographic methods are highly operator-dependent. [33]
Magnetic resonance imaging (MRI) offers whole imaging of the soft parts of the nail unit and the underlying bone. MRI may be performed to localize the tumor before surgery. On T1-weighted images, a glomus tumor appears as a dark, well-delineated mass. T2-weighted images are best for visualizing glomus tumors, which appear as bright and well-delineated masses (see the image below). [31] Because the lesion is richly vascularized, it demonstrates marked contrast enhancement on MRI performed after an intravenous injection of gadolinium-based contrast material.
Intraosseous glomus tumor appears as a bright, well-delineated mass on a T2-weighted MRI.
Although the positive predictive value of MRI for glomus tumor has been found to be high, the negative predictive value is low. [34]
Magnetic resonance angiography (MRA) is especially helpful in diagnosing small lesions that may be missed with other imaging studies. [29, 35]
Other Tests
Dermoscopy may play a useful role in the diagnosis of subungual glomus tumors, in particular through better detection of ramified vessels in bluish spots. [36]
Histologic Findings
On gross examination, a glomus tumor appears as a well-encapsulated soft-tissue lesion. On microscopic evaluation, the tumor usually occurs at the dermal-subepidermal junction and consists of markedly hypertrophied elements of the normal glomus body surrounded by a fibrous capsule. A normal glomus body is made up of vascular structures, smooth-muscle cells, and nerve cells surrounded by uniform epithelioid cells, also known as glomus cells. [37]
Glomus tumor cells stain positively for smooth-muscle actin and weakly express desmin. These cells have also been shown to express CD34. [38]
Glomus tumors with malignant potential may show nuclear atypia, mitotic activity, or necrosis.
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Glomus tumor.
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Subungual hue seen with a superficial glomus tumor.
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Intraosseous glomus tumor appears as a bright, well-delineated mass on a T2-weighted MRI.
