Endoscopic criteria of offending vessel in neurovascular compression syndrome

Introduction Neurovascular compression syndrome ( NVCS) is a disease caused by presence of contact between a vascular loop in the cerebellopontine angle ( CPA) and one of the cranial nerves. Normally, the CPA is characterized by presence of many vascular and neural structures, which are normally in contact with each other without causing a problem; however, sometimes this contact causes a problem to the patient and becomes symptomatic leading to the so-called NVCS, the symptom of which depends on the compressed cranial nerve and the compressing vessel is called an off ending vessel [1]. Vascular compression syndrome of the cranial nerves, fi rst suggested in 1934 by Dandy [2] and popularized by Jannetta in the 1970s [3], are gaining acceptance with the improvement in MRI assessment and the success of endoscope-assisted microvascular decompression (MVD) procedures; they are commonly described in trigeminal neuralgia and hemifacial spasm, but other disorders such as glossopharyngeal neuralgia and disabling positional vertigo (DPV) can also be treated successfully by MVD of the respective cranial nerves. It has also been shown, although in small series, that some cases of disabling tinnitus can be alleviated by MVD [4].


Introduction
Neurovascular compression syndrome ( NVCS) is a disease caused by presence of contact between a vascular loop in the cerebellopontine angle ( CPA) and one of the cranial nerves. Normally, the CPA is characterized by presence of many vascular and neural structures, which are normally in contact with each other without causing a problem; however, sometimes this contact causes a problem to the patient and becomes symptomatic leading to the so-called NVCS, the symptom of which depends on the compressed cranial nerve and the compressing vessel is called an off ending vessel [1]. Vascular compression syndrome of the cranial nerves, fi rst suggested in 1934 by Dandy [2] and popularized by Jannetta in the 1970s [3], are gaining acceptance with the improvement in MRI assessment and the success of endoscope-assisted microvascular decompression (MVD) procedures; they are commonly described in trigeminal neuralgia and hemifacial spasm, but other disorders such as glossopharyngeal neuralgia and disabling positional vertigo (DPV) can also be treated successfully by MVD of the respective cranial nerves. It has also been shown, although in small series, that some cases of disabling tinnitus can be alleviated by MVD [4].
Operative endoscopic anatomy ( Fig. 1) is clarifi ed through retrosigmoid approach using endoscope; the acousticofacial nerve bundle (level II) is the reference level, crossing the middle of the CPA, giving two separate endoscope-assisted surgical areas. Superiorly, the trigeminal area (level I) is concerned with the treatment of trigeminal neuralgia. Inferiorly, the lower cranial nerve area (level III) is concerned with the treatment of hemifacial spasm and glossopharyngeal neuralgia. Auditory nerve with cross confl icts with tinnitus or DPV requires alternative endoscopic control from above and below the acousticofacial nerve bundle, and fi nally (level IV) an inferior extension of the CPA in which the lower medulla is readily apparent [5].

Endoscopic criteria of offending vessel in neurovascular compression syndrome
Sherif Elaini a , Jaques Magnan b , Arnaud Deveze b

Objectives
The aim of this work was to detect the anatomical relationship criteria of the offending vessel with respect to the cranial nerve in neurovascular compression syndrome using the endoscopicassisted minimally invasive retrosigmoid approach for microvascular decompression (MVD); these criteria help the surgeon to de ne the actual con icting vessel during surgery for better results.

Materials and method
Between 1994 and 2009, we have performed 782 cases of MVD surgeries using endoscopicassisted minimally invasive retrosigmoid approach (453 MVD surgeries for hemifacial spasm, 269 for trigeminal neuralgia, 56 for tinnitus and vertigo, and four decompressions for glossopharyngeal neuralgia). During these surgeries we recorded the anatomical relationships between the offending vessel and the affected cranial nerve to detect the endoscopic criteria of the offending vessel.

Results
There are certain criteria for the anatomical relationship between the offending vessel and affected cranial nerve; these criteria include common features such as a perpendicular contact between the vascular loop and the cranial nerve along two different perpendicular planes at its root exit zone, distortion of the nerve course, and distortion or compression of adjacent neural structures mainly brain stem. Other criteria include the vascular loop causing impression of the nerve or encircling the nerve causing reduction of its diameter. In all cases, we used the MIRA, also described as 'keyhole approach'; MIRA has been performed by combining the use of an endoscope along with microscope to achieve a safer, more reliable approach to CPA surgery and detecting the confl icting vessel and its manner of contact with the aff ected cranial nerve.

Retrosigmoid craniotomy
Th e MIRA in our department is performed as follows: For the area to be shaved, only 2-3 cm behind the ear is suffi cient. Body position should be face up and to protect the face, the forehead of the collateral side should be fastened with a gel sheet, without a twopoint bracing of the skull.
Design the so-called Frankfurter line, which is an extension toward the back of the line joining the outer canthus to the superior border of the external auditory canal, and a line along the posterior margin on the mastoid. A keyhole 2 cm in diameter is drawn below and backward from the point where two lines cross. This is the position of the craniotomy.
For skin incision, an arc 6-8 cm in length is drawn with the convex side toward the back at a position one fi nger's width in back of the oblique line by passing the backside of the keyhole.
Th e keyhole designed in item 2 is burred. Ensure that the emissary vein comes into the center and then bur the hole with the vein as a marker.
When the operation reaches the posterior fossa dura and posterior margin of the sigmoid sinus, use the microscope to make the incision of the dura matter. With use of a surgical knife, incise in a U-shape, and, while sucking the cerebrospinal fl uid, wait for spontaneous cerebellar retraction. Place neurosurgical cotton, 1.5 cm wide by 5 cm long, on the cerebellum and gently press the middle part with the suction pipe toward the back and open the posterior cistern.
For monitoring facial nerve function, the NIM response (Medtronic USA Inc., Minneapolis, Minnesota, USA) is used.
Th e objective of this study was to describe the anatomical criteria of the off ending vessel (endoscopic anatomical relationship), which causes NVCS, through analysis of 782 cases during endoscopic MVD through keyhole minimally invasive retrosigmoid approach ( MIRA): 453 MVD for hemifacial spasm, 269 for trigeminal neuralgia, 56 for tinnitus and vertigo, and four decompressions for glossopharyngeal neuralgia.

Materials and methods
Our study was built on a retrospective manner of analysis of 782 cases of neurovascular decompression surgeries to detect the endoscopic intraoperative criteria of off ending vessel, which produces symptoms when in contact with a cranial nerve (produce the so-called NVCS). Th e measured criteria are the anatomical relationship between the off ending vessel and the aff ected cranial nerve, as normally the CPA is a closed space characterized by presence of many vascular and neural structures that are normally in contact with each other; when the vascular structure comes in contact with the cranial nerve in a special anatomical manner, it may become hazardous and produce symptoms. Th ese anatomical relationships have been recorded and mentioned as off ending criteria of the off ending vessel.
Th e MIRA was fi rst described by senior author, J. Magnan, and his teachers Bremond and Garcin in 1974 [6,7]. We have been performing operations since 1990 using the combined endoscopic and microsurgical technique.  For the vestibule-cochlear nerve, the single presence of a neurovascular contact is not suffi cient; the presence of one or more of the common criteria is mandatory to diagnose an off ending vessel. Another important fi nding is the presence of an imprint in the vestibulocochlear nerve and reduction in its diameter by vascular loop encircling the nerve (Figs. 7 and 8), and fi nally the presence of an arterial loop in the internal auditory canal may cause tinnitus o r DPV (Fig. 9).
In our series of study, we found the cause for neurovascular compression in trigeminal neuralgia (Table 3): the superior cerebellar artery alone either the main trunk or its two branches in 46.5% of cases, the anterior inferior cerebellar artery (AICA) alone in 2.5% of cases, the vertebral/basilar artery in 2% of cases, vein alone in 11% of cases, and multiple combination in 38% of cases. Th e cause for neurovascular compression in hemifacial spasm (Table 4) was: the posterior inferior cerebellar artery (PICA) alone in 37.2% of cases, vertebral artery (VA) alone in 11.6% of cases, PICA+VA in 32.4%, AICA alone in 8.1% of cases, vein alone in 1.9% of cases, and multiple combinations in 8.8% of cases.

Endoscopic surgery in cerebellopontine angle
Th e endoscopes used are mainly the following four: 6 cm in length, 4 mm in diameter, 0°; 6 cm in length, 4 mm in diameter, 30°; 11 cm length, 4 mm (and 2.7 mm) diameter, and 30° angle. We use these endoscopes according to the situation, but mainly the one of 11 cm length and 30° angle is used. When the operator is not familiar with endoscopic procedure, we recommend using only 6 cm length, which is safer and prevents injury. Th e endoscope and the optica l fi ber cable used are completel y sterilized by autoclaving. By covering the camera cable and the camer a adaptor with sterilized polyethylene cover, endoscopes can be easily exchanged under clean handling conditions.

Results
We did analyses of the anatomical relationship of the off ending vessel to the aff ected cranial nerves, during MVD using endoscope-assisted MIRA in vascular compression syndrome in 782 cases (453 hemifacial spasm, 269 trigeminal neuralgia, 56 for tinnitus and vertigo, and four glossopharyngeal neuralgia) from 1994 to 2008.
To confi rm the diagnosis of an off ending vessel, the presence of single neurovascular contact is not suffi cient. Several criteria are required, with some variability depending on the involved cranial nerves.
Th e common criteria include: (a) A perpendicular contact between the vascular loop and the cranial nerve along two diff erent perpendicular planes at its root exit zone (REZ), (b) Distortion of the nerve course, and (c) And distortion or compression of adjacent neural structures mainly brain stem.
Besides the common criteria of the off ending vessel, we found that a vulnerable site of contact varies from one cranial nerve to another, meaning that each nerve has special site in its course, which becomes more vulnerable.
For the trigeminal nerve, we found that it is vulnerable in all parts of its course (Table 1): at root entry zone in 129 cases (48%) (Fig. 2), at the cisternal course of the nerve in 107 cases (40%) (Fig. 3), and at the exit of the nerve at the entrance of Meckel's cave in 67 cases (25%) (Fig. 4).
For the facial nerve, we found that it is vulnerable mostly at the REZ in 430 cases (95%) and at the porus in 23 cases (5%) associated with one or more of the general criteria mentioned before ( Table 2). Th e REZ    AICA, anterior inferior cerebellar artery; PICA, posterior inferior cerbellar artery; VA, vertebral artery.
Superior cerebellar artery (SCA) in contact with trigeminal nerve (V) at root exit zone in a perpendicular plane .

Figure 2
Superior cerebellar artery (SCA) in contact with the cisternal part of the trigeminal nerve (V). MP: motor part, DV: dandy vein.

Figure 3
Aberrant vein (AV) in contact at the exit of the trigeminal nerve at the entrance of Meckel's cave . V is Latin number (5) 5th cranial nerve.

Figure 4
Posterior inferior cerebellar artery (PICA) in contact with the facial nerve (VII) at the root exit zone in perpendicular plane causing distortion of the course of the nerve . V VII are Latin numbers 5 7.

Figure 5
Posterior inferior cerebellar artery (PICA) in contact with the facial nerve (VII) at the root exit zone (REZ) in perpendicular plane compressing the brain stem (PONS) .

Figure 6
Subarcuate artery crossing the vestibulocochlear nerve causing an impression on the nerve . responsible for the painful phenomenon. Th e MVD surgery for trigeminal neuralgia was performed for the fi rst time in 1959 by Gardner et al. [8,9], but it was popularized by Jannetta et al. [10,11] after late 1960s, who is now considered the father of 'MVD surgery'. Th e clinical syndrome of hemifacial spasm was fi rst described by Gowers [12] in 1888, distinguishing it from other motor abnormalities of the face. As early as 1909, Hunt [13] explained the hemifacial spasm by a 'refl ex' theory where the trigeminal nerve acted as activating factor. In 1912, Sicard and Bollack [14] proposed a supranuclear origin. In 1952, Wartenberg [15] proposed hyperactivity at the nuclear level. Alajouanine and Th urel [16] proposed the presence of a lesion along the nerve itself whether in the CPA or in the interapetrous segment. Gardner and Sava [17] in 1960 published the presence of a vascular loop as a cause of surgically reversible hemifacial spasm. Jannetta et al. [18,19] confi rmed and developed in the 1970s the concept of the neurovascular confl ict as a cause for hemifacial spasm.
Sirikei et al. [20]  When there are multiple vessels running closely to the nerve, the real off ending vessel causing compression of the nerve should be accurately diagnosed. If that is not possible, all the confl icting vessels (even vein) should be decompressed.
Trigeminal neuralgia was fi rst described by Dandy [2] in 1934, as compression syndrome of the trigeminal nerve, from his operative fi ndings made on 215 patients who underwent surgery in the CPA; he described the presence of the superior cerebellar artery in contact with the trigeminal nerve in 45% of his cases an d hypothesized that the vascular compression may be Loop of anterior inferior cerebellar artery (AICA) encircles the vestibulocochlear nerve (VIII) causing an impression of the nerve and reduction of its diameter .

Figure 8
Loop of anterior inferior cerebellar artery (AICA) at the internal auditory canal (IAC) between the facial (VII) and the vestibulocochlearar nerves (VIII) .

Figure 9
visualized along two diff erent perpendicular planes, and distortion of the nerve or other neural structures; for the facial nerve the location of the confl ict at the REZ of the nerve and distortion of the brain stem at this level are additional criteria. For the trigeminal nerve, reduction of the diameter is an additional criterion [1].
Th e operating microscope, despite its qualities of brightness, magnifi cation, and depth of fi eld, provides only axial view to the operator. Yet, the site of the confl ict is to be found most often against the brain stem at the emergence of cranial nerves, which then requires a mechanical retraction of the cerebellum and other neural structures. According to the landmark paper of Janetta et al. [21], the pioneer of MVD 'turning the corner is the most dangerous stage of the operation and must be executed with patience and the utmost care', and most complications can be attributed to cerebellar retraction. Th e intraoperative endoscopy, thanks to its panoramic vision and angulated view, allows a complete exploration of the CPA without retraction of the cerebellum or brain stem and leaves no hidden areas to be missed [22,23]. It is therefore the procedure of choice to locate the site of confl ict without signifi cant cerebellar retraction, positively identify the vessels responsible as well as their path, and defi ne the bes t maneuver to decompress the artery.

Conclusion
Vascular compression syndrome is characterized by presence of contact between a vascular structure and a cranial nerve leading to manifestations according to the compressed nerve. Th e endoscopic-assisted MIRA is used successfully to correct this condition; intraoperative endoscopy has helped very much in detecting the off ending vessel even in hidden areas in CPA. Th ere are certain criteria for the anatomical relationship between the off ending vessel and aff ected cranial nerve; these criteria include common features such as a perpendicular contact between the vascular loop and the cranial nerve along two diff erent perpendicular planes at its REZ, distortion of the nerve course, and distortion or compression of adjacent neural structures mainly brain stem. Other criteria include the vascular loop causing impression of the nerve or encircling the nerve causing reduction of its diameter .