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Microvascular decompression for hypoglossal nerve palsy secondary to vertebral artery compression: A case report and review of the literature

Background: Neurovascular-compression syndrome (NCS) is described as a prominent pathological contact between cranial nerves and vessels. Trigeminal neuralgia, hemifacial spasm, and glossopharyngeal neuralgia are typical clinical entities associated with NCS. On the other hand, the hyoglossal nerve is rarely affected by NCS. Case Description: We present a case of hypoglossal nerve palsy (HNP) secondary to vertebral artery (VA) compression. A 47-year-old man presented to our hospital with a 1-month history of dysarthria and dysphagia. Neurological examination revealed left HNP, with an intact swallowing reflex and no oropharyngeal or palatal weakness. Magnetic resonance imaging (constructive interference in steady state) revealed left hypoglossal nerve compression by the V4 segment of the left atherosclerotic VA. He underwent microvascular decompression (MVD) surgery. Intraoperatively, the VA was compressing the hypoglossal nerve. The left VA was moved and attached to the dura matter using a polytetrafluoroethylene (Teflon®) sheet and fibrin glue. Postoperatively, the patient exhibited gradual recovery of HNP in 3 months without dysfunction of lower cranial nerves. Conclusion: In patients with isolated HNP, vascular compression should be considered as a cause of these symptoms, and subsequent MVD can lead to resolution.


INTRODUCTION
Neurovascular compression syndromes (NCS) occur when cranial nerves are compressed by intracranial vascular loops. Trigeminal neuralgia, hemifacial spasm, and glossopharyngeal neuralgia are typical clinical entities associated with NCS. These diseases usually occur when the root exit zone of the nerves is affected. Microvascular decompression (MVD) surgery is effective and usually results in immediate postoperative improvement of symptoms. The hyoglossal nerve is rarely affected by NCS, but we encountered a case of hypoglossal nerve palsy (HNP) secondary to vertebral artery (VA) compression.

History and examination
A 47-year-old man presented to our hospital with a 1-month history of dysarthria and dysphagia. He did not show swallowing difficulty, but had subtle difficulty in moving his tongue so that he frequently unintendedly bit his tongue. He had a past history of multiple cerebral aneurysms and had a checkup by angiography 6-months prior to the onset. He consumed alcohol occasionally, and had never smoked cigarettes. Neurological examination revealed isolated left HNP [ Figure 1a].

Imaging studies
Magnetic resonance imaging (MRI) (CISS, constructive interference in steady state) revealed left hypoglossal nerve compression by the V4 segment of the left atherosclerotic VA [ Figure 2]. Further investigation revealed that his intracranial arteries were partially dilated and tortuous as a result of atherosclerotic changes. Multiple aneurysms were revealed including a partially thrombosed large left MCA saccular aneurysm and a fusiform aneurysm at the left PICA. The left VA was also dilated and tortuous. The hypoglossal nerve was composed of two bundles, and both of them were running along the left VA in a tortuous pathway to the hypoglossal canal. In contrast, the contralateral hypoglossal nerve was seen linearly and very clearly on the same images. This finding strongly suggested that the hypoglossal nerve was compressed by the left VA [ Figure 3].

Surgical procedure
On the basis of the above diagnosis, he underwent MVD of the left hypoglossal nerve. In a park-bench position, retrosigmoid lateral suboccipital craniotomy enabled us to visualize the glossopharyngeal, vagal, accessory, and hypoglossal nerve together with the VA. Intraoperatively, the VA was compressing the hypoglossal nerve. The hypoglossal nerve was stretched and was extremely thin. The left VA was moved toward the deep side of the surgical field and attached to the dura matter with a polytetrafluoroethylene (Teflon®) sheet and fibrin glue. The left hypoglossal nerve became free from compression after the procedure [ Figure 4a and 4b].

Postoperative course
He was discharged 7 days after the operation without any obvious complication. At a 3-month postoperative follow-up, he exhibited improvement of hypoglossal nerve palsy without lower cranial nerve dysfunction [ Figure 1b].

DISCUSSION
Stino et al. [11] and Keane et al. [4] reviewed cases of isolated HNP. According to their reports, possible causes of HNP include tumor, trauma, stroke, hysteria, surgery, multiple sclerosis, infection, Guillian-Barre neuropathy, and radiation. HNP in almost one-half of the patients in Keane's investigation was caused by neoplasia, and over one-half of those were malignant. The next most common cause was trauma. The majority of the palsies were associated with other cranial nerves. There were no patients in their reports who showed isolated HNP due to VA compression. In our case, we believe the atherosclerotic VA compressed the hypoglossal nerve and finally caused HNP. To date, there have been only 12 reported cases of HNP secondary to VA compression, including the present case. A summary of these cases is shown in Table 1.
This pathophysiology was first described in English by Rollnik et al. [9] as hypoglossal-vertebral entrapment syndrome. Morini et al. [8] reported two cases of isolated HNP due to VA compression. In their cases, both of the patients were treated conservatively and did not show any improvement of their symptoms. Graham et al. [3] also reported a similar case in 2007, and the patient also did not recover after conservative therapy. Aladdin et al. [1] argued that no effective treatment was found for this phenomenon. In 2011, Cheong et al. [2] presented a 32-year-old man with a 1-month history of dysarthria and difficulty in tongue movement and swallowing. This is the only patient who underwent surgical intervention among previously reported cases. Intraoperatively, VA was compressing the hypoglossal nerve and interposition of the VA and the  hypoglossal nerve was performed using Teflon®. The patient showed improvement of his tongue deviation at 3 months postoperatively. This case strongly suggests that surgical intervention is effective for this pathophysiology.
In reviewing previous MRI findings, most of the patients who had conservative therapy show atherosclerotic or ectatic changes of the VA, which was similar to our case study. Thus, we believe atherosclerotic changes to the VA caused hypoglossal nerve compression, finally resulting in HNP in these patients. The fact that their symptoms appeared gradually supports the theory that slowly ongoing atherosclerotic changes caused hypoglossal nerve compression. In those patients, prognosis is good when treated with MVD surgery. Two cases of MVD surgery, including ours, have shown good recovery of tongue deviation after 3 months. [2] On the contrary, in other patients who were treated conservatively, the symptoms were unchanged or aggravated. In patients with a preoperative diagnosis of HNP with VA compression, decompression via MVD surgery is strongly recommended.
We also found other interesting case reports with similar pathophysiology. Mahadevappa et al. [5] presented a 20-year-old woman with sudden onset dysarthria caused by HNP preceded by a 2-week history of neck pain. MR angiography revealed an ipsilateral dissected VA, which was responsible for the symptom. She was treated with aspirin followed by warfarin. At a 1-month follow-up her tongue deviation had disappeared. Mckeon et al. [6] also reported a 54-year-old man with HNP without any other cranial nerve palsies. He was also reported to have undergone VA dissection, and subsequent warfarin intake resolved his symptoms in 2 weeks. These cases indicate the necessity of careful consideration of VA dissection as a differential diagnosis for patients with isolated HNP. For patients with dissected VAs, a conservative therapy with anti-coagulative drugs is recommended. For diagnosis, we should consider the preceding history of sudden onset headaches or neck pain. MRA, which is essential for diagnosis of HNP due to VA compression, is also useful for screening VA dissection as intimal flap or diameter changes, including occlusion, can be detected. [5] Several cases of HNP caused internal carotid artery dissection have also been reported. [7] According to Mokri et al., [7] 5.2% of cervical ICA dissection patients showed lower cranial nerve palsy. In general, conservative treatment is the first choice.

CONCLUSION
To our knowledge, NCS of the hypoglossal nerve is a rare condition. In patients with isolated HNP, vascular compression should be considered as a cause of the symptoms, and subsequent MVD can lead to resolution of the symptoms. We should carefully differentiate HNP due to VA dissection, which can cause similar symptoms, but does not require surgical intervention. As far as we know, this is the first case report with a thorough literature review of hypoglossal nerve palsy by VA compression.