Case reports RGNS 9 Springer-Verlag 1993 Child's Nerv Syst (1993) 9:179-181 Posterior fossa dermoid cyst associated with dermal fistula: report of 2 cases and review of the literature Jan Goffin 1, Chris Plets 1, Frank Van Calenbergh 1, Frank Weyns 1, Tony Van Havenbergh 1, Roger Eeckels 2, Paul Casaer 2, Kathleen Hunninck 2, Guy Wilms 3, Guy Marchal 3, Luk Dejaegher 4 1 Department of Neurology and Neurosurgery, University Hospital Gasthuisberg, Catholic University of Leuven, Herestraat 49, B-3000 Leuven, Belgium z Department of Pediatrics, University Hospital Gasthuisberg, Catholic University of Leuven, Herestraat 49, B-3000 Leuven, Belgium 3 Department of Radiology, University Hospital Gasthuisberg, Catholic University of Leuven, Herestraat 49, B-3000 Leuven, Belgium 4 Department of Neurology, Imeldaziekenhuis, Bonheiden, Belgium Received: 20 December 1991 Abstract. Two cases of dermoid cysts of the posterior fossa in association with a dermal fistula and with differ- ent clinical presentations are reported. The patient in case 1 is a 14-month-old girl with a history of recurrent bacte- rial meningitis. Case 2, a 7-year-old girl, presented with a skin "granuloma" at the inion. Both cases, and a review of the literature, demonstrate the need for a thorough exploration of the cranial and spinal midline skin areas in every newborn and argue for widespread use of nuclear magnetic resonance imaging whenever a suspected mid- line lesion is found. Key words: Dermal fistula - Dermoid cyst - Epidermoid cyst - Meningitis Nuclear magnetic resonance imaging - Posterior fossa Introduction Dermoid cysts and dermal sinuses or fistulae are relative- ly uncommon, usually congenital lesions, which may be found singly or in combination with each other. In the latter case, the dermoid cyst is located at the end of the dermal fistula [6]. Dermoid cysts have to be differentiated from epider- moids: the walls and content of a dermoid include cuta- neous appendages, namely hair, sweat, and sebaceous glands, and their products, all of which are absent from epidermoids, which contain only desquamated cells from an epidermal layer [6]. The regions of preference for epi- dermoids are the base of the skull and the cerebellopon- tine angle, whereas dermoids usually are located at the midline [7] either in the cranial or spinal area. Two cases of dermoid cysts of the posterior fossa in combination with a dermal fistula and with different clin- ical presentations are reported here. The literature is re- viewed. Correspondence to: J. Goffin Case reports Case 1 This 14-month-old girl with a normal psychomotor development until the age of 12 months was admitted on May 3, 1990 with a history of recurrent fever of unknown etiology. Clinical examina- tion showed only some cervical and occipital adenopathy. Despite the absence of clinical meningeal signs, examination of lumbar fluid on May 17, 1990 was compatible with a diagnosis of bacterial meningitis, yet repeated cultures were initially negative, possibly because of the administration of antibiotics since several weeks previously. Eventually, they showed the presence of three anaerobic organisms: Bacteroides fragilis, Propionobacterium acnei, and Pep- tostreptococcus prevotii. Several CT examinations were interpreted as normal, but finally a small isodense lesion was found on the midline in the posterior fossa adjacent to the skull (Fig. 1). On thorough clinical examina- tion a very small skin opening, less than 2 mm in diameter, was detected on the midline at the inion level. Through this skin lesion "fistulography" was performed (Fig. 2), which, without any doubt, revealed a relation with an intracranial cyst, explaining the gate of entrance for the bacterial meningitis. On June 16, 1990 the child was operated upon: a posterior fossa eraniectomy was performed and an intradurally located dermoid cyst, filled with skin debris and pus, was completely resected togeth- er with the fistula and the skin lesion itself. Postoperatively, slowly progressive hydrocephalus developed, which was treated with a ventriculoperitoneal shunt. The final evolution was favorable. Case 2 During the last 6 months before admission, this 7-year-old girl was operated upon elsewhere three times because of the presence and recurrence of a skin "granuloma" on the midline of the cranium at the level of the inion. Clinical examination at admission again re- vealed a skin granuloma at the inion, with a diameter of 7-8 mm. A radiological examination was performed, including computed tomography (Figs. 3, 4) and magnetic resonance imaging (Figs. 5, 6). These showed an intracranial cyst on the posterior fossa midline just beneath the toreular. The cyst was connected to the skin lesion through a bony tunnel. On January 15, 1991 surgery was performed. The skin lesion was circumscribed, a fibrous tunnel, crossing the bone of the skull just