Scalloping characteristics in anterior fontanelle dermoid cyst.

Dear sir,

The congenital inclusion dermoid cyst (CIDC) of the anterior fontanelle is a rare but benign and curative lesion, accounting for 0.1–0.2% of all skull tumors.[12] A 3-month-old male infant presented with a gradually increasing swelling located in the midline over the vertex of the head since birth. There were no other associated abnormalities and the child had normal growth and developmental milestones. Clinical examination revealed a soft, cystic, non-compressible 3 × 2.5 × 2.5 cm swelling over anterior fontanelle, covered by normal skin. The lesion was non-pulsatile, not adhering to underlying tissue without any bruit, and transillumination test was positive. Computed tomography (CT) demonstrated an extracranial homogenous hypodense swelling in the midline, overlying and covering anterior fontanelle [Figure 1]. However, apparently there was no intracranial communication. Surgery was done under general anesthesia and the cyst was completely dissected out between the galea aponeurotica and pericranium. There was no tissue connection to the overlying skin or intracranial extension. Any congenital midline lesion of the scalp should inevitably arouse in the clinician's mind the possible existence of some form of intracranial extension.[13] The CIDC is a cystic, soft, and mobile mass, covered by normal skin, without any pain and discomfort or any communication between the cyst and the intracranial cavity.[145] The CDIC of the scalp needs to be differentiated from other cystic lesions of the scalp including encephalocele, meningocele, sebaceous cyst, lipoma, cephalhematoma, lymphangioma or hemangioma, and sinus pericranii.[6-8] Gross examination will give a clue, but may occasionally not differentiate encephalocele from dermoid cyst,[9] and a detailed imaging is required before neurosurgical intervention.[3] Although skull X-ray can reveal changes that include flattening or depression of the skull underneath the lesion with sclerotic margins,[1011] CT scan and magnetic resonance imaging are the best diagnostic exams to confirm the position of the lesion.[1112] Sometimes extracranial cystic lesions with clear contents can have all the features of encephalocele on clinical examination, but a careful evaluation of neuroimaging can help us suspect the diagnosis in such cases.[8] On neuroimaging if the lesion is coming out from inside the cranium - it produces scalloping on the inner table of the cranium with their characteristic imaging appearance.[13] However, if the lesion is extracranial (as in the present case), it will characteristically produce the scalloping over outer table of the cranium.[81011]

Figure 1

Plain CT scan of the brain with bone window showing hypodense, well-defined extracranial swelling over the anterior fontanelle [please note (d) the thin hyperdense outline of the cyst wall (thick arrow) and pattern of scalloping on the outer table (thin arrows)]