Differential Diagnosis for Fluctuant Mass under the Helix

Single most likely diagnosis

• Furuncle or Carbuncle: A fluctuant mass that appears suddenly, especially after a period of increased bacterial colonization (e.g., during sleep), is highly suggestive of a furuncle (boil) or carbuncle, which are infections of the hair follicle. The initial sharp pain and subsequent improvement in pain as the lesion fluctuates are consistent with the natural history of these conditions.

Other Likely diagnoses

• Epidermoid Cyst: These are benign cysts that can become inflamed and present as a fluctuant mass. They are common and can occur anywhere on the body, including the scalp. The sudden onset could be due to a rupture or infection of the cyst.
• Infected Sebaceous Cyst: Similar to epidermoid cysts, sebaceous cysts can become infected, leading to a fluctuant, painful mass. The location under the helix makes this a plausible diagnosis given the abundance of sebaceous glands in this area.
• Abscess: A collection of pus that can occur anywhere, including under the skin, an abscess could present as a fluctuant mass. The absence of redness or fever does not rule out an abscess, especially if it is not yet fully developed or is partially drained.

Do Not Miss (ddxs that may not be likely, but would be deadly if missed)

• Malignancy (e.g., Squamous Cell Carcinoma): Although less likely, any new or changing mass, especially in sun-exposed areas like the helix, warrants consideration of malignancy. Early detection is crucial for treatment and prognosis.
• Necrotizing Fasciitis: This is a severe infection of the skin and subcutaneous tissues that can present with pain out of proportion to the physical findings. The absence of fever and redness does not exclude this diagnosis, and any delay in diagnosis can lead to severe consequences.

Rare diagnoses

• Pilar Cyst: These are benign cysts that originate from the hair follicle and can present as a firm or fluctuant mass. They are less common than epidermoid or sebaceous cysts but should be considered in the differential diagnosis of scalp masses.
• Dermoid Cyst: A congenital cyst that can present at any age, dermoid cysts can become infected and fluctuant. They are less common and might not be the first consideration but should be kept in mind, especially if other diagnoses are ruled out.