Hemifacial Spasm Caused by Small Epidermoid Tumor, Misinterpreted as Delayed Secondary Hemifacial Spasm Caused by Vestibular Schwannoma Treated with Gamma Knife Surgery.

Hemifacial spasm (HFS), one of the most common hyperactive cranial rhizopathies, is a disorder characterized by spontaneous, intermittent, and repetitive contraction of unilateral facial muscle. The most common cause of HFS is a mechanical compression of the facial nerve at the root exit zone (REZ) by blood vessels located in the brainstem. The incidence of cerebellopontine angle (CPA) tumor-induced HFS ranges from 0.3%-2.5% of all patients with HFS, and the incidence of epidermoid cyst among such tumors is extremely rare (0.2% to 0.25%). We present a patient manifesting HFS induced by a small epidermoid cyst who underwent Gamma Knife surgery (GKS) for ipsilateral vestibular schwannoma (VS) more than 10 years ago.A 59-year-old female was admitted to our hospital with HFS. Ten years before admission, the patient underwent GKS for ipsilateral VS. Enhanced brain magnetic resonance imaging demonstrated that the volume of VS was not increased. Also, no definitive mechanical compression of the facial nerve REZ by blood vessels was detected. We concluded that HFS was secondary to the VS treatment by GKS and decided to operate to remove the tumor and identify REZ. A standardized lateral suboccipital retrosigmoid approach was performed. The cochlear nerve was encased by a tumor mass, and the posterior inferior cerebellar artery (PICA) was displaced by the mass. During tumor excision, we ensured that the VS and displaced PICA did not directly compress the REZ. Interestingly, an unexpected lesion was found with a yellowish viscous mass located between the sixth nerve and ventral side of the seventh nerve REZ. After the tumor removal, a small piece of Teflon was inserted between the PICA and REZ of the facial nerve. Postoperatively, the patient experienced no HFS or facial nerve dysfunction. Histopathologic examination of the lesion revealed an epidermoid cyst.We should consider various possibilities for the cause of HFS and review brain imaging meticulously if patients were treated for CPA tumor with GKS a long time ago and the delayed HFS occurred without changes in tumor volume. HFS induced by epidermoid cyst is rare. Therefore it is critical for the neurosurgeon to ensure the absence of any abnormal contact with the REZ of the facial nerve during surgery for HFS induced by CPA tumors.