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MICROVASCULAR DECOMPRESSION IN TRIGEMINAL NEURALGIA.

Dandy is credited with developing the posterior fossa approach to the dorsal root of the trigeminal nerve for the purpose of sectioning the nerve as a treatment for trigeminal neuralgia. The "Dandy approach" is well known to neurosurgeons, as are Dandy's observations of the presence of vascular abnormalities and tumors around the dorsal root in a high percentage of patients with trigeminal neuralgia. The incidence of observed abnormalities increased as Dandy's experience grew. Less well known are his anatomic descriptions of the so-called accessory sensory fascicles, which preceded observations of anatomic sequestration and reorganization in the dorsal root as it approaches the brain stem. Dandy operated on hundreds of patients via the posterior fossa route, with low morbidity and a rate of pain relief at least equal to that obtained with middle fossa sectioning, the most widely accepted operative treatment for trigeminal neuralgia at the time. Subsequently, Gardner operated on a small series of patients with recurrent trigeminal neuralgia using a posterior fossa approach. He found vascular compression of the dorsal root in 7 of 18 cases. Neither of these brilliant surgical investigators had the benefit of magnification techniques available to contemporary neurosurgeons. Knowledge gained with the use of the operating binocular microscope in the cerebellopontine angle has shown that trigeminal neuralgia is almost always caused by vascular compression of the root entry zone of the trigeminal nerve. A microsurgical operation was developed for the definitive treatment of this painful affliction. The operative technique has evolved gradually since 1966, and the concept of vascular compression syndromes has been investigated in all of the lower cranial nerves.

Indications for Operation

Microvascular decompression is indicated in patients who have trigeminal neuralgia intractable to medical therapy and for whom anesthesia does not pose an excessive risk. Elderly patients are eligible if healthy and active. Prior destructive procedures are not a contraindication to operation, but true anesthesia dolorosa will not be relieved by microvascular decompression. A series reported by Barba and Alksne showed that patients who have recurrent trigeminal neuralgia following a destructive procedure have only a 43 percent rate of pain relief after microvascular decompression. Selective sectioning of the portio major may be indicated in addition to the microvascular decompression in this group of patients. However, the results obtained in such patients have been almost as good as in virgin cases. Most of patients have failed on drug therapy or had toxic reactions. Several have not been given carbamazepine because of the fear of side effects and toxic reactions by the referring physician and/or the patients.

Preoperative Evaluation

Magnetic resonance imaging (MRI) without and with contrast enhancement is performed to evaluate the presence of ectatic arteries and to rule out extra-axial cerebellopontine angle tumors and arteriovenous malformations. Computed tomography was used in most cases before we began using MRI- again, without and with contrast enhancement. Most patients coming with a prior study in hand. In these patients, a repeat study or the other type of imaging study is performed only if needed. Ectatic arteries that are present are not necessarily the cause of the trigeminal neuralgia. Tumors usually cause trigeminal neuralgia by vascular compression of the nerve. Small tumors, especially meningiomas, may be missed with even the best imaging studies. Otologic testing, including pure tone and speech discrimination testing as well as assessment of brain stem auditory evoked potentials are part of preoperative investigation.. Trigeminal evoked potentials and electromyography of the temporal is and masseter muscles are frequently obtained. An internal medicine consultation is obtained on older patients. Other routine and special studies and consultations are obtained as needed.

Anesthetic Management and Monitoring

The patient is placed on the operating table with the head at the foot of the table. This allows the surgeon enough knee room when operating in the sitting position. The patient is anesthetized with sodium thiopental, intubated, and maintained with nitrous oxide and oxygen. Intravenous fentanyl is given intermittently. Paralyzing agents are used as needed. Intraoperative monitoring of brain stem auditory evoked potentials is essential in decreasing the incidence of hearing loss due to errors of retraction. Doppler ultrasound monitoring over the sternum is instituted after the patient is placed in the lateral position. The Doppler device is used primarily as a convenient auditory pulse monitor for the surgeon, as air embolism is practically nonexistent in most series of patients operated upon in the lateral position.

Operative Technique

A three-point fixation headholder is applied to the head, using local anesthetic infiltration in the pin sites before tightening the pins. The patient is then turned to the contralateral lateral decubitus position, and the connections of the headholder to the table are loosely joined. A soft blanket roll is placed under the axilla. The patient is taped and strapped across the hip to the table to allow safe lateral rotation of the table if needed for intracranial exposure. The neck is placed in traction with slight flexion and ipsilateral rotation, and the connectors of the headholder are tightened. It is important that the head not be brought forward away from the surgeon and that there be no ipsilateral flexion. The ipsilateral shoulder is taped in a caudal direction and forward as necessary. The end result should be that the surgeon has free access to the ipsilateral occipital boss

Incision and Craniectomy

The hair on the ipsilateral side behind the ear is shaved. The area is prepared for operation. The incision is marked by placing a finger behind the mastoid eminence and constructing a line behind the finger that generally parallels the hair­line, extending caudally for 3 to 6 cm from the level of the top of the pinna (the incision should be longer in obese or heavily muscled patients). The incision is carried down to the bone rostrally and through the galea aponeurotica caudally. Bipolar electrocoagulation is used for hemostasis as needed. The upper part of the incision is separated from the bone with a large periosteal elevator. A small self-retaining angulated or straight Weitlaner retractor with or without posts is put into place and tightened. Further dissection is carried down to the occipital plate using bovie cutting current and periosteal elevators. The Weitlaner retractor is tightened gradually. Further hemostasis is achieved with electrocoagulation. Emissary veins are sealed with bone wax. The posteromedial mastoid eminence is cleared of soft tissue. The occipital plate is perforated. the dura mater is separated, and a small superolateral retromastoid craniectomy. about 2.5 cm in diameter, is made. Raney posterior fossa rongeurs and Leksell rongeurs are most useful in making this craniectomy. The craniectomy should extend to the sigmoid sinus even if mastoid air cells are entered. The mastoid air cells of dolichocephalic patients usually extend behind the sigmoid sinus.

 The edge of the lateral sinus should be identified in all patients and partly exposed in younger patients. This is not as important in women over the age of 50 and men over the age of 60 because the cerebellum is smaller in these patients. The craniectomy should extend caudally behind the lateral aspect. The bone edges are heavily waxed. If mastoid air cells are entered, they are packed with muscle and bone wax.

Dural Opening and Early Exposure

The dura mater is opened obliquely and incised back to the junction of the lateral and sigmoid sinuses or incised in curvilinear fashion. The resulting dural flaps are sewn back out of the way. The lateral sinus, if exposed, can actually be tented up out of the way, exposing the underside of the tentorium cerebelli and thereby facilitating exposure. In younger patients, the cerebellum may bulge out of the opening. Should this occur, the cerebellum is greatly compressed back intradurally over a rubber dam and cottonoid and then elevated gently off the floor of the posterior fossa. The subarachnoid cisterns over the glossopharyngeal and vagus nerves are opened sharply. A microsurgical retractor with a narrow rectangular or tapered blade is connected by two rods and three connectors to the lower posterior post of the Weitlaner retractor base. Alternatively. a gooseneck retractor may be used, although retraction placement is not quite as precise. A 0.5 x 1.5 in. cottonoid strip is prepared with a slightly larger piece of rubber dam under it. This is placed superolaterally over the cerebellum. The retractor blade is bent proximally, usually to about 30 to 35 degrees (the angle depends on the c0l!figuration of the patient). It is most important that the blade have no curve in it other than the proximal bend: otherwise it will interfere with the surgeon's line of vision

Microsurgical Exposure of the Trigeminal Nerve

At this point. with the operating surgeon sitting on a low stool, the microscope is brought into the field. The tip of the retractor blade is placed high and lateral and the tentorium visualized. The surgeon must be aware that bridging veins extending posteromedially or out over the superior cerebellar surface may be present, especially if the superior petrosal vein is small or absent. If a vein is tom, it is better to treat it at once with bipolar coagulation and division than to accept the bleeding or try to stop it with packing. The junction between the tentorium and the petrous bone is identified, and the cerebellum is retracted gently.

The retractor is placed superolaterally over the cerebellum. Care is taken not to retract medially any more than is necessary, as this direction of retraction may stretch the auditory and facial nerves. The auditory nerve is identified as early as possible in the dissection. and the arachnoid cisterns are opened rostral to it. The superior petrosal vein. if present. crosses this arachnoid and commonly has an inverted Y configuration. However. it may be short, long. single. multiple. or absent. If absent. the surgeon should be­ware of other large bridging veins in this area. One branch of the vein at a time is coagulated. partly divided, and recoagulated. and a Valsalva manoeuvre is then performed. If there is no bleeding. the vein is again coagulated and divided, and the Valsalva manoeuvre again performed. If bridging veins tear, they usually do so at a dural sinus. They should be coagulated and divided and the distal end packed at the sinus while satisfactory occlusion of the proximal end is ensured. At this point, the sinus end can be coagulated again. and if that is unsuccessful in stopping bleeding. it can be treated with a piece of Gelfoam or Surgicel and pressure with a small cottonoid. This manoeuvre will usually stop the bleeding. If bleeding is not easily controlled, the head of the operating table is raised (reverse Trendelenburg manoeuvre) to the point where the veins begin to collapse. simplifying hemostasis. The surgeon must be aware that the superior petrosal vein or a branch thereof may be the cause of the trigeminal neuralgia. Therefore, the neurovascular relationships of this vein and the trigeminal nerve should be clarified before the vein is coagulated and divided, if at all possible.

Evaluation of the Neurovascular Compression

The arachnoid over the trigeminal nerve is opened sharply. The tip of the retractor blade is placed over the ala of the cerebellum, and the ala, which normally covers the rostrolateral root entry zone of the trigeminal nerve, is elevated posteriorly and caudally. If one is to make valid observations and treat all possible sources of vascular compression, the entire trigeminal nerve must be visualized from the brain stem to the dura. Subtleties abound in the evaluation of neurovascular compression, and the following rules regarding these observations may facilitate appreciation of the pathologic changes:

1. Generally, lower facial pain (V3, V2-3) is caused by blood vessels on the rostroanterior aspect of the root entry zone. V1 pain is caused by a vessel on the caudal aspect of the root entry zone. Isolated V2 pain is caused by a vessel on the side of the nerve, most frequently a vein, which is often quite distal on the nerve. This condition is more common in women.

2. Multiple vascular impingement is common. All vascular compression should be treated.

3. The underside of the ala of the cerebellum must be evaluated in all patients, as a blood vessel located there may cause compression of the root entry zone.

4. The junctional area of central and peripheral myelin extends out a considerable distance in the portio major. Significant vascular compression of the nerve may therefore occur close to or even at Meckel's cave.

5. Small pontine surface veins in the pia may cause or contribute to trigeminal neuralgia.

6. When the patient is in the contralateral lateral decubitus position, the offending blood vessel may not be in direct contact with the trigeminal nerve. This occurs because the brain stem and blood vessels move differentially with changes in position of the patient.

7. Small veins running between fascicles of the nerve may cause or contribute to compression and must be coagulated and divided.

8. Small arterioles or venules may cause or contribute to trigeminal neuralgia. The outside of an arteriolar loop is enough to cause tic.

9. Mobilization of the significant blood vessel or the placement of an implant at its location may be accompanied by significant but brief bradycardia. This is the second best monitor of efficacy- the best being the eyes and brain of the surgeon.

10. Vascular compression of the motor-proprioceptive fascicles of the nerve distal to the pons causes constant pain. usually centered in V 2 and usually burning in character.

Technique of Vascular Mobilization and Decompression. The pia-arachnoidal chordae over the trigeminal nerve should be cut sharply. If the superior cerebellar artery (SCA) is causing compression. it will often be found to have bifurcated (occasionally trifurcated) before or at the site of compression. The entry zone of the portio minor must be seen clearly in its entirety. The underside of the cerebellar ala should be inspected. The entire course of the nerve to Meckel's cave must be seen. The arterial loops are mobilized using microsurgical dissectors. not by pinching with forceps. and are brought into a horizontal position over the trigeminal nerve that the offending loops are resting easily in their new attitude rostral to the nerve. Shredded Teflon felt is moistened. rolled into various-sized pledgets. and placed under the artery. The surgeon must make sure ( 1 ) that enough felt is placed to separate the entire artery from the entire nerve: (2) that the felt extends around the nerve and pons so that it cannot slip: (3) that it is thick enough to pad the nerve satisfactorily. and (4) that it does not kink or compress the artery. (These general rules are the same for all arterial decompressions.) Horseshoe-shaped arterial loops should have a tail of the implant fulled through the loop to anchor it in place.

Veins bridging from the brain to the dura are generally easy to separate from the nerve with sharp and blunt dissection. Similarly, the implant is easy to place between the trigeminal nerve and large bridging veins, or the veins may be coagulated and divided instead. Small veins running through the nerve should be coagulated and divided at multiple sites, necessitating manipulation of the nerve. The lowest possible bipolar current should be used to avoid spread of current to the trigeminal nerve. Small veins on and around the nerve that lie in the pia should be coagulated, divided, and excised if possible. These surface veins have a great tendency to recanalize, and at present that is the most common cause of early recurrence of neuralgia in most series. The recurrence occurs most commonly 4 to 6 months postoperatively.

All blood vessels anywhere near the nerve should be separated from it. The retractor is gently released to ensure that relationships are stable and that blood vessels do not kink. Small bits of felt are placed under the ala of the cerebellum if vessels are present. Gelfoam is placed around the vein and implant if necessary to ensure stability. The Valsalva manoeuvre is performed several times to ensure hemostasis in the venous system.

Points of Difficulty

One difficulty arises when the whole nerve cannot be adequately exposed. Usually the problem involves the root entry zone, but occasionally the distal portio major is the site of the problem. The distal nerve is harder to visualize; indeed, the entire exposure is more difficult in dolichocephalic than in brachycephalic patients, as the cranial nerves travel more laterally to their foramina in the latter.

A second point of difficulty is that an elongated arterial loop, especially a loop of the superior cerebellar artery, cannot be easily brought out of the axilla of the nerve into a horizontal position. This manoeuvre can almost always be accomplished by starting distally on the artery (proximally on the nerve) and placing a temporary piece of Teflon felt between the distal arterial loop and the trigeminal nerve while the more proximal parts of the artery are mobilized. More felt is placed distally, and the manoeuvre is repeated as often as necessary. Perforating arterioles of the SCA are always elongated and emerge from the rostral part of the loops. They move as the loops are mobilized.

The next point of difficulty has to do with small veins and arteries, especially the former. Small veins in the pia and arachnoid and arterioles with the outside of a loop compressing the nerve must be treated, as they may be the cause of the trigeminal neuralgia. The veins must be lifted up before they are coagulated so that current is not transmitted to the nerve. The veins tend to recanalize, as noted above. Small arteries are easily decompressed and held away with wisps of shredded Teflon felt.

Closure

The dura mater is closed in watertight fashion. A narrow strip of Gelfoam may be placed under the suture line before sewing if necessary. A piece of Gelfoam is placed over the dura, and a methylmethacrylate cranioplasty is performed. Use of a cranioplasty has decreased the incidence of chronic postoperative headaches ("pseudo-occipital neuralgia") to the vanishing point. Also, patients prefer not to have a soft spot or depressed area. The caudal deeper muscle layers are approximated after the self-retaining retractor is removed. The remainder of the wound is closed, and a dry dressing applied.

Postoperative Care

In the recovery room, hypertensive episodes are no more common after cranial nerve microvascular decompression than after any other intracranial procedure. The blood pressure should be controlled. The patient stays overnight in the neurosurgical intensive care unit and returns to a standard hospital room the next day. Headache is usually mild and brief if only a small volume of cerebrospinal fluid was removed at operation. Incisional pain is treated with small doses of narcotics.

Patients may go to the bathroom with assistance the evening of operation if they feel so inclined. They may ambulate on the first postoperative day, but should not bend or stoop over for 4 to 5 days. Patients who have been on large doses of carbamazepine preoperatively should be placed on decreasing doses postoperatively to prevent withdrawal symptoms, usually manifested as agitation.

The wound dressing is removed on the third postoperative day, and the patient is instructed to shower and to shampoo daily with mild shampoo. The patient is discharged from the hospital on the fourth or fifth postoperative day, occasionally on the third day. The sutures are removed on the seventh postoperative day.

Postoperative Course, Results, and Complications

The patient may still have trigeminal neuralgia on awakening from the anesthetic or may develop recurrence of pain in the next few days. It should not be as severe as the preoperative pain and should disappear promptly. If the pain is as severe as it was preoperatively, or suddenly recurs with that severity, it may be presumed that a causative blood vessel has been missed or that the decompression was inadequate. Reoperation should be performed early, as it is easier on the surgeon and the patient if done promptly.

Cerebrospinal fluid leaks are uncommon, but if one occurs, a pressure dressing is applied to the wound, and closed lumbar drainage is instituted using the method of McCallum and colleagues.

Postoperative delayed headache, with or without nuchal rigidity or fever, may occur from the fourth to seventh postoperative day. The cerebrospinal fluid typically shows elevated pressure and may show leukocytosis, but no bacteria appear on Gram stain or culture. The fluid may be xanthochromic with an elevated protein value and normal or low-normal sugar content. This syndrome of delayed headache occurs in about 15 percent of patients. It must be differentiated from infection. Lumbar puncture is often curative if the opening CSF pressure is reduced halfway to 100 mmH2O, If symptoms persist, a repeat lumbar puncture is performed and a single 10-mg dose of dexamethasone given. In general one can expect that about one patient in 200 will have recurrent pain by 10 years after the operation.

Barba and Alksne in 1984 observed that permanent relief is obtained in over 90 percent of patients who have not undergone a prior destructive procedure, but in only 43 percent of those who have had such a procedure. The implications of this important observation are not yet clear. Are these patients just less amenable to any therapy? Does the pre-existing, more peripheral lesion create a pathophysiologic situation that makes the tic more difficult to treat? Is the reverse situation also true (that is. for a failed microvascular decompression followed by a destructive operation)? Should selective sectioning of the portio major be performed in addition to the vascular decompression in patients who have had recurrence after a prior destructive procedure'). Results do not support the observation that prior destructive procedures worsen outcome so severely.

The morbidity and mortality rates in patients have decreased significantly owing to increased experience. to the use of the lateral decubitus rather than the sitting position. to auditory monitoring. In Janneta P.J series, more than 1700 microvascular decompressions for trigeminal neuralgia were performed. with two operative deaths. the first in 1971 and the second in 1976. The latter patient is actually doubly reported in a series from another institution. They have had two postoperative deaths after excision of tumors causing trigeminal neuralgia, both in the 1970s. The major current morbidity consists of the delayed headache syndrome. which is self­limited. Major cranial nerve deficits consist of facial numbness in some patients in whom the vascular decompression was particularly difficult and of hearing loss in the ipsilateral ear.

The incidence and prevalence of trigeminal neuralgia may increase as the median age in our population rises. Microneurosurgeons who learn the nuances of microvascular decompression will be able to perform the operation safely and effectively. The data of others, show that microvascular decompression offers the best quality of life, best results, and least true morbidity of any major operative procedure, for trigeminal neuralgia

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