Hemifacial spasm (HFS) is a relatively rare movement disorder most commonly caused by vascular compression of the facial cranial nerve at its root exit zone from the brainstem. It is characterized as involuntary intermittent twitching of the muscles of the face. Microvascular decompression (MVD) was initially introduced by Gardner and Sava⁶⁾. and was popularized by Jannetta et al.⁹⁾. Since its introduction as a treatment for HFS, MVD via retrosigmoid suboccipital approach has become the treatment of choice due to its excellent results and low operative complications. Potential complications following MVD include hearing deficit, facial palsy, cerebrospinal fluid (CSF) leakage, low cranial nerve palsy, hemorrhage, cerebellar injury, meningitis, and death^12,17). The delayed post-operative dural arteriovenous fistula (AVF) is very rare complication of this approach and has rarely been reported to date. In this report we present a case of delayed postoperative dural AVF occurring at the surgical site after retrosigmoid suboccipital craniotomy for MVD in a patient with HFS.

A 49-year-old man, ten months prior to his present admission, had been operated in our institution for left HFS caused by vascular compression of the facial nerve by the left anterior inferior cerebellar artery (AICA) (Fig. 1). There were no evidence of pathologic lesions such as hemorrhage, sinus thrombus, vascular malformation, tumors in preoperative imaging studies, including computed tomography (CT) and magnetic resonance imaging (MRI). A left retrosigmoid suboccipital craniotomy was performed to alleviate the spasm. The size of the craniotomy was relatively small and about twice the size of a quarter coin. At the time of the operation, we did not expose the transverse and sigmoid sinuses of the patient. At the end of the operation, we performed a watertight dural closure. The muscle and scalp were closed in the anatomical layers. The patient's hemifacial spasm completely disappeared postoperatively. Ten months after the operation, the patient complained of amnesia and dysarthria. The patient had no history of head trauma before and after operation. CT showed intracranial hemorrhage (ICH) on left temporoparietal subcortical area (Fig. 2A) and CT angiography (CTA) showed the abnormal enhanced vessels on left transverse-sigmoid sinus (Fig. 2B). Magnetic resonance angiography (MRA) clearly showed a dural AVF at the site of the previous surgery (Fig. 2C, D). Digital subtraction angiography (DSA) demonstrated a dural AVF of the left transverse-sigmoid sinus fed by the left occipital artery (Fig. 3A). After superselective catheterization of the left occipital artery, which supplied the affected sinus of the dural AVF, embolization with Onyx® (total 3.5 cc; ev3, Irvine, CA, USA) was performed successfully (Fig. 3B). One year later, follow-up DSA did not showed any recurrence of lesion (Fig. 3C). The patient's condition was in normal without recurrence of hemifacial spasm.

Dural AVF is low pressure, spontaneously occurring communications between vessels of internal and external carotid origin that supply the dura and a dural venous sinus. Dural AVFs are relatively rare, with a prevalence of 0.15 per 100000 people per year, and constitute 6-15% of all intracranial AVFs with most being located in the posterior fossa²⁾. Although the etiology of dural AVFs is still controversial, they are generally classified into congenital or acquired. Acquired dural AVFs have been reported to develop secondarily to sinus thrombosis^8,19,21), head injury^4), cerebral infarction^7), and craniotomy¹⁸⁾. Among the variable causes, acquired dural AVF after craniotomy had rarely been reported^18,21,22). Although the time of occurrence, location, and type of surgery were variable according to the reports, sinus thrombosis, thrombophlebitis, and late intrasinus hypertension following sacrifice of the venous sinus were suggested as underlying mechanism of dural AVF¹⁹⁾.

In our case, the dural AVF developed at the site of the previous craniotomy was not associated with sacrifice of the transverse-sigmoid sinus or trauma. Also, there were no evidence of dural sinus thrombosis in the preoperative imaging studies. There was one report which addressed the symptomatic dural AVF after craniotomy for the trigeminal neuralgia surgery¹⁸⁾. Different from previous report, our case was unique in nature that the initial presentation of the AVF was intracranial hemorrhage, which was confirmed in the CT scan. Maybe our case is the first report in which the dural AVF after craniotomy was initially presented as ICH. Besides, this initial presentation of dural AVF would lead to difficulties in exact diagnosis. At this point, CTA and MRA was very effective tool which confirmed the dural AVF as a cause of ICH.

The first option of treatment for dural AVF is transarterial embolization^1,10). Although the coil embolization is an attractive option, transarterial Onyx® embolization has recently been recommended as a safe therapeutic alternative. It is very effective with type I and II dural AVF through the Cognard classification because it reduces the shunted blood flow and facilitates subsequent transvenous embolization or surgery. However, Onyx® embolizations¹⁵⁾ have limitations and risks, including cardiac Onyx® embolization^5), reflexive bradyarrhythmia^16,20), visual hallucination, and cranial nerve palsy³⁾. If the transarterial or transvenous embolization failed, we considered the next treatment option to be surgical interruption of the fistulas^13,21) or radiosurgery^11,14). In our case, the dural AVF was completely obstructed during initial procedure and there were no leakage or evidence of complication in the immediate and late (12 months) follow-up angiography. Therefore, no further surgeries were needed and the patient was benefitted from this minimally-invasive procedure.

The dural AVF is rare complication after retrosigmoid suboccipital craniotomy and intensive monitoring for the complication would be warranted after microvascular decompression.