Dermoid Cyst in the Temporal Bone Causing Facial Palsy in a Child

A dermoid cyst is a benign tumor composed of ectodermal and mesodermal elements that arises during the early stage of development. While approximately 7% of all dermoid cysts occur in the head and neck, there are rare cases in the temporal bone or middle ear cavity.1, 2 Furthermore, a dermoid cyst in the facial nerve canal is extremely rare, making it difficult to diagnose.3 We present a case of child with a history of facial palsy and a dermoid cyst occurred in the fallopian canal and middle ear cavity. This study was approved by the institutional review board of the Severance Hospital, Yonsei University Health System (IRB #4-2023-1691). A 2 years and 7 months old child with a history of facial palsy that occurred at the age of 3 months presented an abnormal mass lesion on the left eardrum, which was on the same side of the facial palsy (Figure 1A). About 2 years ago, he underwent brain computed tomography (CT) and magnetic resonance imaging (MRI) in the pediatric department to evaluate the cause of facial palsy. No tumors or other abnormal signs were found in the CT and MRI scans. Facial electromyography and electroneuronography at that time reported a 100% degeneration ratio of the left facial nerve. As the cause of facial palsy remained unidentified, the patient received steroid therapy and followed up. When this child visited our clinic, the temporal bone CT scan showed a lobulated and dumbbell-shaped soft tissue mass with erosion of the distal incus in the left posterior mesotympanum (Figure 1B). Erosion and widening of the tympanic-mastoid segments of the left facial nerve canal were also noted. On the temporal MRI, the mass was not enhanced by gadolinium on the T1 images (Figure 1C). The hearing threshold of the affected ear was 45 dB nHL, while the other ear measured 15 dB nHL on the auditory brainstem responses. Based on the imaging and the hearing test results, we suspected benign tumors such as congenital cholesteatoma or facial nerve schwannoma. We planned surgery to remove the tumor due to its potential impact on hearing. Ossiculoplasty was also planned. However, considering the patient's young age and more than 2 years of facial palsy, we decided against facial reanimation surgery or muscle transfer or transposition after thorough discussion of the associated risks and benefits. We conducted a mastoidectomy and identified the enlarged facial nerve canal. The canal was filled with keratin materials and hairs, consistent with a dermoid cyst (Figure 2A). The facial nerve in the mastoid segment and second genu were displaced and changed to fibrotic band without nerve bundles. After the complete removal of the tumor, identified distal nerve ending with normally shaped nerve fibers in the stylomastoid foramen did not respond to a facial nerve stimulation (Figure 2B). And then, we performed ossiculoplasty replacing eroded incus. Pathological examination confirmed the specimen to be a dermoid cyst. Follow-up hearing threshold of the affected ear was 30 dB nHL at 3 months after the surgery. Facial palsy in children can be categorized as congenital or acquired. Congenital facial palsies may result from trauma during delivery, developmental defects, or syndromic diseases such as Möbius syndrome. Acquired facial palsy can be caused by infectious, inflammatory, traumatic, or neoplastic diseases.4 However, about 50% of cases have unknown etiology and classified as Bell's palsy. Consequently, there is a risk that children diagnosed with Bell's palsy may not undergo follow-up and re-evaluation for other potential causes, as seen in our case. Because dermoid cysts or other benign tumors can grow slowly and remain invisible in their early stages, we believe that ongoing follow-up evaluations should be considered, especially in young infants with facial palsy. The entire facial nerve canal should be re-evaluated using thin-slice images from MRI or CT scans. While the duration of the follow-up period may be a matter of debate, we suggest that a minimum of 6–12 months should be warranted, because the time from the onset of facial palsy is crucial for determining the strategy of facial reanimation surgery.5 In a similar case report, a sural nerve graft was successfully applied to facial nerve reanimation in a 9-month-old child, with a favorable outcome.3 Re-evaluation and early detection of the cause of facial palsy in young children may give a chance for better options for facial reanimation surgery. Ji Hyuk Han, study design, data acquisition and analysis, drafting, and reviewing. None. None.