Hemifacial spasm (HFS) is a neuromuscular movement disorder characterized by brief or persistent involuntary contractions of the muscles innervated by the facial nerve [1]. The exact pathophysiology of HFS remains unclear, but two hypotheses have been discussed. In the peripheral hypothesis, the compression of nerve fibers by adjacent vessels may transmit abnormal nerve signals and cause HFS [2]. The central hypothesis explains that HFS is created by vessel compression to the facial nucleus on the brainstem; then, the backfiring signal (antidromic nerve conduction) might stimulate the facial nerve to cause spasm [3]. Except in cases of secondary HFS caused by trauma, demyelinating disorders, tumors, and infections [4], it is believed that the cause of HFS is chronic compression of adjacent vessel to the root exit zone (REZ) of the facial nerve [5]. The average age of HFS is mostly between the 5th and 6th decades of life [1,6] but a lower incidence is reported in those aged 30 years and under [7-9]. Since only a few studies on young-onset hemifacial spasm (YOHFS) have been reported, little is known. Because chronic nerve influences may cause HFS, the pathophysiology of YOHFS may be different from that of typical HFS. We analyzed the characteristics of 151 patients with YOHFS, which is, to the best of our knowledge, the largest cohort reported to date.

A total of 4,363 patients underwent microvascular decompression (MVD) surgery for HFS between April 1997 and March 2021. Among them, 151 patients were aged 30 years or younger at the time of surgery (3.5%). The diagnosis was made based on clinical history, physical examination, and radiologic findings. All patients underwent magnetic resonance imaging to identify vessel compression at the facial nerve REZ.

The assessment was performed by reviewing the medical records. The present study protocol was reviewed and approved by the Institutional Review Board of Konkuk University Medical Center (approval No. 2021-08-002). The patients’ sex, age, duration of symptoms at the time of surgery, past medical history, severity of the symptoms, improvement of spasms, changes in lateral spread response (LSR) during surgery, type of compression, changes in brainstem auditory-evoked potential (BAEP), facial nerve indentation, and offending vessels were analyzed. Postoperative complications were also analyzed.

The severity of HFS was categorized into four different groups, as described in a previous report [10]. Grade 1 patients complained of localized spasms around the periocular area. Grade 2 patients showed involuntary movement spreading to other parts of the ipsilateral face and with the orbicularis oris, zygomaticus, frontalis, or platysma muscle affected. Grade 3 patients experienced interference with vision because of frequent tonic spasms. Grade 4 patients had disfiguring asymmetry and continuous contraction of the orbicularis oculi muscles affecting the opening of the eye.

Facial electromyography and BAEP were monitored throughout the surgery to detect injury to the 7th and 8th nerves. The changes in the amplitude and latency of BAEP were continuously reported to the operator by an expert neurophysiologist. The LSR was monitored to evaluate successful MVD. We categorized the LSR changes into five types: Type 1A (disappearance of the LSR after opening of the dura), 1B (LSR loss after cerebrospinal fluid [CSF] drainage), 2A (LSR loss immediately after MVD), 2B (delayed LSR loss after MVD), or 3 (no signal change after MVD).

Compression types were categorized into six groups. In the arachnoid type, arachnoid trabeculae between the vessel and the brainstem cause the vessel to be tightly tethered to the facial nerve. In the branched type, the facial nerve is caught between the compressing vessel and its branch. In the perforator type, perforating arteries from the compressing vessel cause compression by tethering the vessel to the brainstem. In the loop type, the vascular loop of the offending vessel compresses the facial nerve. In the sandwich type, the facial nerve is sandwiched between two different vessels. In the tandem type, one vessel compresses another vessel, which in turn compresses the nerve [5,11].

The indentation of the facial nerve compression by the offending vessels was analyzed under a microscope and categorized into three groups: grade 1 (minimal or no indentation of the facial nerve), grade 2 (moderate indentation), or grade 3 (severe indentation causing discoloration of the facial nerve) [5].

The improvement in spasm after MVD was categorized into three groups by the degree of the disappearance of spasm and was evaluated immediately post-operation, at a 1-month follow-up, and at a 1-year follow-up. Group 1 comprised patients with complete disappearance of spasm, group 2 comprised those whose symptoms remained under 50% compared to the preoperative state, and group 3 comprised those who remained ≥50% symptomatic or even had aggravation of symptoms after surgery.

We find five previous studies in the literature of patients with YOHFS who underwent MVD in PubMed search [7,8,12-14]. Two studies included 10 or less than 10 patients and largest cohort reported by Chang et al. [8] included 33 patients. All five studies examined the number of patients, sex, age of onset or age at surgery, sides of HFS, surgical outcome and the most common offending vessels. Those previous reported literatures are summarized in Table 3.

We have found some common features of YOHFS through comparison with these other studies. Female predominance was found in three published studies and in our study (male-to-female ratio, 1:1.5). However, Jho and Jannetta [7] showed an equal male-to-female ratio, and Karki et al. [13] showed male predominance. Their studies had small patient groups of 10 and 6, respectively. Regarding the size of the patient groups, we could assume that YOHFS has female predominance as well as HFS in the general population. However, the overall male-to-female ratio reported in other studies shows a more definite female predominance (1:1.9-2.4) [5,15,16] but YOHFS shows a more balanced male-to-female ratio. These sex differences in YOHFS and HFS in the general population might be because HFS is a facial muscle disorder related to facial expressions and cosmetic issues; as women tend to be more interested in their cosmetic appearance, they may be more likely to visit a clinic regarding facial muscle disorders than men.

According to Singh et al. [17], the decline in estrogen and progesterone levels is related to peripheral neuropathy. Postmenopausal patients with peripheral neuropathy had lower estrogen and progesterone levels than postmenopausal patients without peripheral neuropathy. In addition, Suzuki et al. [18] reported that estrogen plays a role in neuroprotection. Estrogen protects against cell death during cerebral ischemia and influences cell proliferation. Thus, the role of hormone changes in aging females can also increase the incidence of female patients with HFS in the general population compared to YOHFS. In other words, hormonal decrease in post-menopausal patients could be the related with the fact that female prevalence of YOHFS patients is lower than the general population.

The results of MVD for HFS in the general population were similar to those for YOHFS. In a meta-analysis by Holste et al. [19], 39 studies and 6,249 patients were meta-analyzed with an overall cure rate of 90.5% at a mean follow-up of 1.25 years. This is similar to the outcomes of YOHFS studies. Therefore, we could infer that MVD is an effective treatment in young patients as well as in the general population.

Jo et al. [20] reported that 7 out of 331 patients (2.1%) experienced intraoperative BAEP changes and 2 patients had permanent hearing loss after MVD (0.6%). In our study, 11 patients had intraoperative BAEP changes (7.3%). However, none of the patients experienced hearing loss after surgery, regardless of BAEP improvement during surgery. One patient who had transient hearing loss without intraoperative BAEP changes fully recovered 3 days after surgery. There were more frequent BAEP changes in YOHFS than in previous studies, which might be related to the fact that the brain volume of younger patients is larger than that in older patients, and the CSF space is relatively small [21]. Therefore, more retraction of the cerebellum might be needed in young patients, and consequently, BAEP changes can be more frequently found. Nevertheless, nerve fibers might be more durable in younger patients and no hearing loss was found in younger patients, even though there were more BAEP changes. This might be related with the previous studies which showed that neuroplasticity of young population is more active compared with older population [22,23].

The incidence of postoperative facial palsy can be explained in the same fashion. According to a systematic review of 5,685 patients by Miller and Miller [24], permanent postoperative facial palsy occurred in 0.9% of patients after MVD. However, the young patients in our group showed no permanent facial palsy and showed only transient facial palsy. Although hearing loss or facial palsy after MVD is infrequent, the results of this study may be biased; further studies are needed to confirm these hypotheses.

The offending vessels and the type of compression in YOHFS were similar with the general population. As we previously reported that AICA (53.2%) was the most common offender and the PICA (30.9%) as the second most common offending vessel [5]. For type of compression, YOHFS as well as previous reported studies showed dominant ratio of arachnoid type.

The limitations of our study are that it included data from only patients with YOHFS and lacked data from patients in the general population with HFS. The comparative study of our group and the general population relies on a comparison with previous reports. Therefore, statistical analysis between patients with YOHFS and those in the general population with HFS should be performed to confirm our conclusions.

The pathophysiology of HFS remains unclear. However, the pathophysiology of YOHFS seems to be similar to HFS in the general population, considering that there was no significant difference between YOHFS and the general population. The clinical outcomes of MVD for YOHFS were satisfactory. In addition, complications such as facial palsy and hearing loss are relatively rare. Therefore, MVD should be considered as the treatment of choice for YOHFS.