Illuminating the unknown: A transilluminant periungual lesion with sonographic clues

CASE DESCRIPTION

A 68-year-old male presented with a 2-month history of an asymptomatic nail plate groove of the left middle finger. Clinical examination revealed longitudinal indentation of the nail plate extending from the proximal nail fold to the nail tip, along with a soft, fluctuant lesion in the proximal nail fold overlying the nail matrix [Figure 1]. To confirm the diagnosis, transillumination was performed using a mobile phone flashlight in a darkened room. Clear light transmission across the lesion created a bright translucent glow that precisely demarcated the lesion boundaries [Figure 2]. High-resolution ultrasonography (Philips Affinity Machine, 15 MHz linear probe) subsequently confirmed a well-defined, anechoic periungual cystic lesion measuring 2.6 × 1.2 × 2.9 mm, originating from the distal interphalangeal joint (DIP) by a characteristic tail-like outpouching and extending distally toward the nail unit [Figure 3]. The lesion was without any internal vascularity on colour Doppler.

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Figure 1: Clinical photograph showing longitudinal indentation of the nail plate (black arrow) and a soft, fluctuant lesion palpable in the proximal nailfold (red arrow).

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Figure 2: Transillumination test using a mobile phone flashlight, demonstrating clear transmission of light across the cyst. The characteristic bright red glow demarcates the boundaries of the cyst and confirms its fluid-filled nature (black arrows).

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Figure 3: High-resolution ultrasonographic image (Philips Affinity Machine, 15 MHz linear probe) showing a well-defined, anechoic periungual cystic lesion in the proximal nailfold (labelled ‘CYST’), with a characteristic tail-like outpouching extending toward the distal interphalangeal joint. Subtle mass effect on the underlying nail matrix is appreciated, explaining the observed nail plate indentation.

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QUESTION

What is the diagnosis?

DIAGNOSIS

Periungual mucoid pseudocyst.

DISCUSSION

Periungual mucoid pseudocysts are common benign lesions of the nail apparatus that typically presents as translucent, skin-coloured cystic swellings near the proximal or lateral nail fold. They often cause longitudinal nail plate grooving due to pressure on the nail matrix.^[1] Historically described as a ‘synovial lesion of the skin’, these cysts are also known as digital mucous cysts, myxoid cysts, synovial cysts, periungual ganglia or mucinous pseudocysts. Their exact pathogenesis remains uncertain, but two mechanisms are recognised.^[1] One type of cyst represents a ganglion-like lesion arising from the herniation of synovium or tendon sheath, commonly associated with degenerative distal interphalangeal joint disease and osteophytes, particularly in older individuals. The other type results from localised mucin overproduction by dermal fibroblasts, without definite joint communication.^[1] In our case, there was no DIP joint osteoarthritis or history of trauma.

Clinically, these cysts usually present as solitary, dome-shaped lesions on the fingers, more frequently in adults, with a female preponderance. Histologically, two patterns are described: a myxomatous type (periungual, mucin-rich, no true epithelial lining) and a ganglion type (closer to the joint, with synovial connection). Management options include intralesional corticosteroids, aspiration, cryotherapy, laser ablation and surgical excision.^[1]

In this case, transillumination and ultrasonography served as simple, non-invasive tools to highlight the cystic, mucin-filled nature of the lesion and aid in diagnosis. The differential diagnosis of periungual nodules includes verruca vulgaris, pyogenic granuloma, glomus tumour, onychomatricoma and malignant lesions such as nodular basal cell carcinoma or subungual melanoma. Transillumination efficiently excludes solid lesions and narrows the differential diagnosis to fluid-filled entities. It operates on the principle that light transmission through tissues varies based on tissue composition and fluid content. In normal fingernails, transillumination produces a uniform red field, while fluid-filled cysts or solid masses interrupt this uniform appearance.

The technique has documented utility in periungual warts, subungual haematomas, glomus tumours and nail psoriasis.^[2,3] A recent overview of nail imaging techniques has highlighted smartphone flashlight transillumination as a practical approach for identifying myxoid cysts and glomus tumours .^[4]