Women's Health Alliance 300 20^th Avenue, Suite 401 Nashville, TN 37203, USA Dept. of Ultrasound

Synonyms: None

Definition: An epidermal scalp cyst is an extracranial cystic mass with no intracranial extension and no calvarial defect. The mass is filled with laminated keratin and is lined with differentiated cornified squamous epithelium¹⁰. Epidermal scalp cysts are benign in nature.

Case report: A 19-year-old G₂P₁ presented for routine anomaly scan. The patient had a normal tri-screen result. She had one previous child with isolated choroid plexus cysts and no other clinical history of note. A transabdominal and transvaginal ultrasound examination revealed a single intrauterine pregnancy at 17 weeks gestation. A cystic extracranial mass was noted over the anterior fontanelle measuring 10mm. A thin echogenic membrane was seen at the base of the mass thought to represent the periosteum or soft tissue under the mass. There was no intracranial extension of the mass. The mass was mobile and not attached to underlying structures. No defect was seen in the calvarium, and the intracranial anatomy was unremarkable. A detailed scan of the rest of the fetus was unremarkable. The diagnosis of an epidermal cyst was therefore suspected. The fetus was scanned again at 18, 22 and 37 weeks with no change in appearance of the mass. A spontaneous vaginal delivery of a normal female infant was performed at 39 weeks.

Several views of the cyst. Note the lack of anomalies in the brain and the lack of connection to the brain or ventricles. There are also 2 identical video-clips ( 2.6 MB MPEG video-clip as well as a 4.7 MB video-clip). Some users have problems playing MPEG clips. If this is your case select the second (larger) clip.

Postnatal clinical examination showed the cystic lesion of the scalp. Ultrasonic assessment at 1 month of age confirmed the cystic lesion.

Sagittal view over the cyst

Transverse views (note the interhemispheric fissure). Also in the energy and velocity Doppler image note flow in the superior sagittal sinus.

And the cute little girl:

Prevalence: There are only three cases of epidermal scalp cysts reported prenatally [3,5]. Dermoid and epidermoid cysts account for 23 percent of all scalp lesions and there is a 2:1 female preponderance. The ethnic distribution is equal.

Etiology: Unknown.

Pathogenesis: Epidermal cysts are congenital tumors formed from pockets of ectoderm sequestered between the third and fifth week of embryonic life as the neural grove closes. This mechanism explains why most lesions are midline. They are typically found in the occipital region and over the anterior fontanelle [1].

Sonographic findings: An epidermal scalp cysts presents as an extracranial cystic or heterogenous mass with no associated defect in the calvarium. Recognition of the absence of a defect in the calvarium is perhaps the most reliable aid in making the diagnosis. The mass is mobile and not attached to the underlying structures. The mass tends to remain cystic during the pregnancy, however, there is one reported case where the original cystic lesion regressed to a dense tissue mass.

Implications for targeted examinations: When an extracranial cystic mass is diagnosed parentally, a cephalocele is usually the primary consideration. If no defect is identified in the calvarium, the differential diagnosis would include a cystic hygroma, edema, teratoma, lipoma, or hemangioma [4]. It is important to make an accurate diagnosis of the mass. An in-utero diagnosis of a cephalocele may lead to the termination of the pregnancy where as an epidermal scalp cyst is benign in nature. A careful search for a defect in the calvarium, normal intracranial contents, and associated anomalies is warranted.

Differential diagnosis: The differential diagnosis would include a cephalocele, meningocele, cystic hygroma, subcutaneous edema, cervical teratoma, lipoma, mesenchymal sarcoma, or hemangioma.

Associated anomalies: There are no reported associated malformations.

Prognosis: There are three cases of epidermal scalp cyst reported prenatally [3,5]. In two of the three cases, the cyst remained a small anechoic mass throughout the gestation as it did in our experience. One case reports the original cystic lesion had regressed to a dense tissue mass by 26 weeks gestation [3]. In all cases, no reported associated anomalies were identified. The prognosis is excellent. No intervention is required.

Recurrence risk: All cases are sporadic; therefore the recurrence risk is low [1,2].

Management: Recognition of extracranial masses is important because of their benign nature. It is crucial to identify if a defect is present in the calvarium. When the bony defect is small the diagnosis is very difficult. A transvaginal scan with a high-frequency transducer or MRI may further delineate the mass and confirm that there is no intracranial extension. There have been at least two reported cases where the pregnancy was terminated for benign lesions. One was an occipital hemangioma and the other was an epidermal scalp cyst [6,7]. The presence of an extracranial mass must lead to a careful consideration of the differential diagnosis. When prenatal diagnosis of an epidermal scan cyst is made with confidence, no further intervention is warranted.

References:

1. Okaro E, Broussin B, Ville Y, Prenatal diagnosis of atypical cystic lesions of the fetal scalp. Ultrasound Obstet Gynecol 1998 Dec;12(6):442-4.
2. Shahabi S, Busine A, Prenatal diagnosis of an epidermal scalp cyst simulating an encephalocele. Prenat Diagn 1998 Apr;18(4):373-7.
3. Pryce DW, Khine M, Verbov JL, Van Velzen D, Scalp cyst with heterotopic brain tissue. Br J Dermatol 1993 Aug;129(2):183-5.
4. Witkowski JA, Parish LC, Taping of a cyst: a method to facilitate removal of a scalp cyst. Int J Dermatol 1974 Jul-Aug;12(4):226.
5. Nyberg DA, Mack LA. The spine and neural tube defects, In Nyberg DA, Mahoney SB, Pretorius DH, eds. Diagnositc Ultrasound of Fetal Abnormalities; Text and Atlax. Chicago: Year Book Medical, 1990:152-60.
6. Pearce JM, Griffin D, Campbell S. The differential prenatal diagnosis of cystic hydromata and encephalocele by ultrasound examination. J Clin Ultrasound 1985;13:317-20.
7. Winter TC 3