Ultrasonographic Patterns and Pathological Correlation in the Diagnosis of Subcutaneous Nodules

Panniculitis refers to a group of conditions involving inflammation of the fat cells in the subcutaneous tissue. It can result from various causes, such as inflammation, infection, vasculitis (inflammation of blood vessels), or lymphoproliferative disorders (abnormal lymphocyte proliferation). In general, it is challenging to determine the cause of subcutaneous tissue inflammation based solely on skin manifestations, as patients often present with similar symptoms, such as subcutaneous lumps, tenderness, and redness, usually found in the lower limbs. Therefore, diagnosis often requires a comprehensive patient history taking, physical examinations, and sometimes biopsy for histopathological examination to confirm the diagnosis. Occasionally, additional tests, including immunohistochemistry, cultures, or polymerase chain reaction (PCR), may be necessary for greater diagnostic accuracy.

Since 1997, Ackerman has proposed diagnostic guidelines for panniculitis based on histopathological findings, initially categorizing it according to the site of inflammation into septal panniculitis and lobular panniculitis. Further classification considers whether vasculitis is present. However, even with physical examination and histopathological analysis, accurately classifying panniculitis remains challenging. In 2007, Requena and colleagues highlighted limitations in biopsy, such as the selection of lesions with differing characteristics or lesions at different stages, resulting in varying histopathological features. Moreover, the depth or insufficient amount of tissue collected for biopsy may reduce diagnostic accuracy.

Since 1979, medical advancements have led to the increased use of ultrasound in dermatological diagnosis. Low-frequency ultrasound machines (2 MHz) were initially used to examine skin lesions, such as skin inflammation or subcutaneous nodules. In 1983, Miyauchi and colleagues began using higher-frequency ultrasound (10 MHz) to study ultrasound characteristics compared to histopathological results in patients presenting with subcutaneous or dermal nodules. Over time, the use of higher-frequency ultrasound became more common, as it provides better detail and convenience, being a simple, safe, and cost-effective procedure.

For panniculitis, research has shown that ultrasound can assist in diagnosis. In 2020, Romani and colleagues studied the correlation between ultrasound findings and histopathology in 62 cases of panniculitis. Septal panniculitis typically shows thickening of the septa with hypoechoic areas resembling a jigsaw pattern, difficult to compress, increased blood flow, and possible clotting in veins. Lobular panniculitis shows hyperechoic fat with irregular borders, without septal thickening or increased vascularity, and may show fat necrosis or extension into muscles and adjacent tissues. However, further research is needed for clear differentiation between types of panniculitis based on ultrasound features.

Subsequent studies have used ultrasound to describe various forms of panniculitis, including those caused by gout, erythema nodosum, lupus erythematosus, and subcutaneous T-cell lymphoma, among others.