EPIDERMOID CYST

Epidermoid cysts are small, reddish or white bumps that can appear anywhere on the skin. They may also (rarely) occur in bone. When present under the fingernail, they are called subungual epidermoid cysts. In this location they can be single but are more often multiple. Usually, only one digit is affected, especially the thumb. Epidermoid cysts are slow growing, typically painless, and almost always benign. Therefore, if a cyst grows rapidly, the diagnosis should be reassessed. They very rarely develop into skin cancer. Although epidermoid cysts usually do not require treatment, they may become inflamed, leading to swelling and pain. They also can rupture and become infected. There are no preventive measures.  There are multiple synonymous terms used to identify the epidermoid cyst. Other names included epidermal inclusion cyst, wen, steatoma and sebaceous cyst.  Some authors identify the sebaceous cyst as a separate and distinct lesion because its cells secret sebum into the cyst while the invaginated epidermis of the epidermoid cyst fills the lesion with layers of keratin.  Clinically the epidermoid cyst is typically a firm (almost hard) white small mass which can have layers like an onion when cut.  The sebaceous cyst is filled with a liquid or pasty sometimes foul-smelling material (sebum).  In a practical sense probably, these lesions represent a continuum that all related to congenitally or traumatically invaginated epidermis.^1,3,4

Pathophysiology

• Congenital epidermoid cysts may be due to abnormal ectodermal implantation during embryogenesis.
• The acquired variety occurs as a result of trauma or skin disorder (eg, acne).
• The cysts form when epidermal cells penetrate deep into the skin and multiply rather than follow the normal course and shed.
• The cyst wall comprises these cells, which secrete keratin into the cyst.

Related Anatomy

• Epidermoid cysts are most commonly located on the face, neck, and trunk but also see in the fingers, thumb and hand.
• Intraosseous epidermoid cysts occur most frequently in the distal phalanges and the skull.
• In extremely rare cases, epidermoid cysts can develop in the brain, spleen, testes, liver, kidney, and cecum. These cysts are usually found incidentally, while performing imaging studies for another reason.

Incidence and Related Conditions

• Risk factors for epidermoid cysts, which may be congenital or acquired, include history of acne, post-pubertal life stage, rare genetic disorders (eg, Gardner syndrome), injury/trauma (eg, surgery)
• The typical epidermoid cyst is usually not brought to the attention of a physician, so overall incidence is unknown.
• The term “sebaceous cyst” is often used interchangeably with “epidermoid cyst,” but the sebaceous cysts are a different entity and are less common. Sebaceous cysts arise specifically from oily sebaceous glands of the skin.  Sebaceous cysts usually have an edge located in the most superficial layer of the skin.  They are truly cystic and frequently contain a milky or pasty material. They can drain spontaneously and get secondarily infected.

Differential Diagnosis

• Boil
• Brodie abscess
• Glomus tumor
• Sebaceous cyst

Due to their benign status, epidermoid cysts are rarely reported to a physician. Therefore, typical patient characteristics have been difficult to collect. In the hand, epidermoid cysts are more common in males and on the palmar aspect, and age usually is post puberty. The typical patient presents because the cyst has become symptomatic (eg, inflamed or infected, signified by redness, tenderness, and/or discharge) or enlarge enough to interfere with grip. The patient may have experienced a recent injury or trauma (eg, surgery), and questioning may reveal a history of acne. If the cyst is intraosseous, the patient is likely to be a manual worker. 

SURGICAL COMPLICATIONS

• Scarring  
• Recurrence (particularly after incision and drainage where the cyst wall was not removed)

• Incision and drainage: recurrence common
• Excision: good results with recurrence uncommon
• In one 27-year study of 101 epidermoid cysts excised from the hand, the post-excision cyst-recurrence rate was 11%.²
• Laser therapy produces the least amount of scarring 

• Many benign and malignant processes can affect the subungual region.
• Dermoscopy shows subungual epidermal cysts as well-demarcated, ring-shaped figures.  
• Differentiating interosseous epidermoid cysts from other bone tumors of the hands is difficult.
• Classic Surface in Subcutaneous cysts:
  • Small, round bump just under the skin
  • Locations commonly the face, trunk, or neck
  • Blackhead plugging the center of the cyst rarely in the hand
  • Purulent discharge (keratin forming a thick, yellow pus) with unpleasant odor
  • Redness, swelling, and tenderness
• In Subungual cysts:
  • Clubbing
  • Enlargement of the distal phalanx
  • Onycholysis
  • Pachyonychia
  • Pain
  • Ridging of the nail
  • Subungual hyperkeratosis
• Additional work-up options include microscopic examination of skin scraping, biopsy for laboratory analysis and/or dermoscopy

Cited Articles

1. Mayo Clinic. Epidermoid cysts. 2014. Accessed January 30, 2017 at  http://www.mayoclinic.org/diseases-conditions/sebaceous-cysts/basics/definition/con-20031599
2. Lincoski CJ, Bush DC, Millon SJ. Epidermoid cysts in the hand. J Hand Surg Eur Vol 2009;34(6):792-6. PMID: 19786411
3. Gross S. Sebaceous cysts correlation of clinical and pathological diagnoses n three hundred cases. JAMA (1953): 152(9): 813-814.
4. Marks JG and Miller JJ. (2013). Lookingbill and Marks Principles of Dermatology (5^th ed.). Chapter 7: Dermal and Subcutaneous Growths. Elsevier (Saunders). London, New York. 2013.

Case Reports

1. Hsieh C-H, Tien Y-C, Chien S-H. Intraosseous epidermoid cyst of distal phalanx of fingers. Scholarly Research Exchange Vol 2008, Article ID 963687. Accessed February 24, 2017 at https://www.hindawi.com/archive/2008/963687/abs/
2. Park JY, Kim YW, Lee KY, Sung JY. Epidermoid cyst of the cecum. Ann Coloproctol 2015;31(1):37-9. PMID: 25745626

Reviews

1. Cianci P, Tartaglia N, Altamura A, et al. A recurrent epidermoid cyst of the spleen: report of a case and literature review. World J Surg Oncol 2016;14:98. PMID: 27036391
2. Hassani FD, Bouchaouch A, El Fatemi N, et al. Pineal epidermoid cyst: case report and review of the literature. Pan Afr Med J 2014;18:259. PMID: 25489364
3. Willard KJ, Cappel MA, Cozin SH, Abzug JM. Benign subungual tumors. J Hand Surg Am 2012;37:1276–86. PMID: 22560606

Classic

1. Bobra ST, Matzinger K, McCluskey LU. Epidermoid cyst of the terminal phalanx of the thumb. Can Med Assoc J 1964;90:1464-6. PMID: 14158557