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Microvascular Decompression; Abnormalities at the Trigeminal Nerve Root Entry Zone

A steadily increasing number of neurosurgeons are so favorably impressed with the results of microvascular decompression (MVD) of the trigeminal root entry zone at the pons in the treatment of trigeminal neuralgia that they recommend this for all but medically infirm or older patients when medical management becomes unsatisfactory. The principal uncertainty in the minds of some of us is the extent of the pressure on the trigeminal rootlets required to provoke the syndrome. In some cases causative pressure by one or more arteries exists. Jannetta described "neurovascular compression" at the root entry zone in 395 (96 percent) of 411 patients with trigeminal neuralgia. Zorman and Wilson stated that "veins and arterial loops not in direct contact with the root were presumed to be causative if they were in close proximity to an anatomically deformed nerve root because retracting the cerebellum could displace the pons and trigeminal root away from an offending artery or vein. They found causative extrinsic lesions in 79 percent of 125 patients. Their results after MVD were about the same whether they treated presumed venous or arterial compression or, finding neither, did a partial sensory rhizotomy (PSR). Breeze and Ignelzi likewise considered the finding of a vessel "near" the nerve in 37 percent of their 52 procedures as indicative of the cause of the pain.

In a 1989 review of 252 patients with trigeminal neuralgia treated between 1969 and 1985, Wilson's statements seem to reflect a striking change in his criteria for determining treatment of significant compression of nerve by blood vessel. When the nerve root is "distorted" by an artery and/or a vein, he does an MVD by coagulating and dividing veins, and by dissecting arterial loops from the nerve root. This displacement is maintained by a small sculpted prosthesis of sponge Teflon inserted between the root and the vessel. One drop of cyanoacrylate cement is used to secure the prosthesis to the pia or nerve root. Patients in whom vascular contact was found without deformity of the nerve root underwent both displacement of the vessel (MVD) and PSR consisting of section of the inferior one-half to two-thirds of the portio major. Patients in whom no vascular contact was found underwent PSR only. Using these criteria he performed MVD only in 166 (66 percent) of the cases, MVD and PSR in 56 (22 percent), and PSR only in 30 (12 percent). Seven local tumors and two arteriovenous malformations (AVMs) were included in the group undergoing MVD and no PSR. In 1984, Wilson published a review of 118 of the earliest cases of this series and reported that 92 (78 percent) underwent MVD only while 26 (22 percent) had a PSR; none had both procedures. In this earlier series the therapeutic results in the two groups were indistinguishable. In the same year, however, Piatt and Wilkins described a drop in the success rate of MVD from 83 percent to 62 percent when the artery and nerve were merely in contact. Wilson was apparently impressed by this observation, because in his later publication he added PSR to MVD in the management of this arterial contact group. The resultant percentages in the "excellent" and "good" categories were slightly better in these patients than in those who had only one of the two procedures. In these latter two groups the outcomes were about the same. With respect to complications, Wilson achieved a major triumph, with no deaths and no lasting deficit of any significant function in any patient. In seeking to match his surgical judgment and dexterity by his mental agility, Wilson argued that posterior fossa exploration should be the first­line therapy, stating that even successful prolonged "medical treatment may ultimately reduce the efficacy of surgery." He and Alksne have drawn attention to their finding that any proximal ablative procedure, including any percutaneous tactic in the middle cranial fossa, decreases the likelihood of success of a subsequent MVD. But, as Jannetta (1985) asks, "are these patients just less amenable to any therapy?" It is obvious that ease of controlling the paroxysms of trigeminal neuralgia varies enormously from case to case. In Wilson's series a second posterior fossa procedure was performed in 20 patients with persistent or recurrent pain. In no patient had the prosthesis shifted so it was left in place. He logically did a PSR in 18 of the 20 patients, 7 of whom had undergone a PSR at the first operation, achieving an excellent or good result in 85 percent. It seem that Wilson had general shift to requiring more reasons for doing a PSR that he is less confident that extrinsic vascular compression causes most of the cases of "essential" trigeminal neuralgia.

Wilson also pointed out that his' 'excellent" results were in patients who had a shorter duration of symptoms (average 7.4 years) than those in the "good," "recurrent," and "poor" categories; accordingly, in his view we should operate early "before the nerve is irreversibly damaged." However, his "good" category's average duration of symptoms of 10.5 years is much longer than the worse "recurrent" and "poor" categories. This argument for early operation is made the flimsier by his finding that the younger the patient at the onset of the symptoms, the worse the result of MVD. Hence in those 20 who failed to maintain benefit from sustained adequate decompression as well as in the 30 who never had any evidence of extrinsic compression, totalling 20 percent of the 252 cases, such compression did not cause their pain.

The French neurosurgeon Sindou and his colleagues are the only ones who have compared the cerebellopontine (CP) angle findings at operation between patients with trigeminal neuralgia and those with facial pain due to neoplasm. Among 150 patients in the former group they found a vasculoneural "conflit" in 132 (88 percent) of them. A further 14 patients (9 percent) had some other obvious lesion. However, they found a vasculoneural "conflit" in not a single one of 52 patients in whom they were doing a trigeminal rhizotomy for pain due to tumor located away from the sensory root.

Sindou and colleagues have thoughtfully attempted to determine whether the pain-relieving feature of "microvascular decompression" is really the removal of a compressive lesion-the vein or pulsating artery- or is merely another form of compression-a "neocompression." Does an interposed substance-muscle or other material-between the nerve and vessel simply shift from a vascular to another form of constant compression of the rootlets, analogous to the temporary frank rootlet compression of the open or percutaneous operations in the middle cranial fossa. Accordingly they studied decompression by two relevant differing methods. In two clinically similar groups, each of 60 patients with trigeminal neuralgia, the first group had the artery dissected away from the nerve and maintained in that position by the interposition of foreign material (Dacron) and at times some periosteum as well between the artery and nerve. In the second group the offending vessels were dislodged without any material touching the nerve. The artery was held away from the nerve by Teflon strips, at times with a 7 x 10 mm piece of Dacron also. A follow-up at 1 year revealed a recurrence rate of 10 percent in the first group and 4.5 percent in the second. They concluded that a "true decompression" rather than a "neocompression" was the more effective type of operation.

Electron microscopic study of the rootlets distorted by the blood vessel and of other specimens maximally removed from the site surely is the most direct way to secure relevant data as to the cause of the paroxysms.

At the other end of the scale are the observations of Adams et al., who saw "Indentation, grooving, or distortion of the nerve by a vascular structure" in only 6 of 57 patients explored in the posterior fossa. Similarly Rovit noted such finds in only 2 of 25 patients.

Postmortem studies to determine the neurovascular relationships in patients without facial pain are about as divergent as the in vivo observations on those with trigeminal neuralgia. The four publications to evaluate are those of Hardy and Rhoton (1978), Haines et al. (1980), Mehta et al. (1981), and Klun and Prestor (1986 ). The first group saw arterial contacts in 52 percent of 50 CP angles, "often" with arterial compression. In contrast were the observations of Mehta et al., who saw arterial contacts with the trigeminal root in only 13 percent of 60 CP angles, and no distortion of nerve by artery. The following were the figures in non-tic cadavers of Haines et al. in 40 CP angles: 25 percent arterial contact, 23 percent venous contact, and 10 percent compression. The corresponding figures of Klun and Prestor in 130 CP angles were 23 percent, 4 percent, and 7 percent.

In view of the far higher percentages in the cadavers of contacts, arterial or venous, than of compression recorded by all four groups, it clearly makes sense to correlate the percentages achieving pain relief with the various types of neurovascular relationship. The first ones to do this. Piatt and Wilkins, found that when an artery distorted the root or was wedged between the root and the pons, as seen in 37 cases, MVD realized an expected cure rate of 83 percent. This dropped to 62 percent of 31 cases when the artery to rootlet relation was one only of contact. A further drop to 42 percent occurred in those with merely venous contact or distortion, or "no abnormality." Their highest percentage of excellent and good results ensued in their 21 patients in this last status on whom they did a PSR.

Comparison of Results of Minimally Destructive Operations in the Middle or Posterior Fossa

There is good evidence that there is some other causative factor in the majority of cases not related to the anatomy of the root entry zone at the pons or the nearby root beyond that zone. These begin with the results of the various largely nondestructive operations in the middle cranial fossa carried out in the fifties and sixties in an effort to avoid the deafferentation dysesthesia (anesthesia dolorosa) of trigeminal root section. A variety of procedures were performed. Taarnhoj stayed with his proposal of "decompression" by incising the length of the dural roof of the trigeminal cistern. Shelden et al. noted that their original concept of "decompressing" the peripheral second and third divisions at the foramina rotundum and ovale gave about the same results as the Taarnhoj operation. They decided that the feature common to the two manoeuvres was probably modest trauma to the trigeminal pathways; hence, they changed the decompression to "compression," rubbing vigorously the ganglion and rootlets with a blunt instrument after opening the dura over them. Several minor variations on these concepts were proposed by other neurosurgeons. Thus Gardner "frees the sensory rootlets from their dural sleeve, gently touches them with a cotton pledget, and sprays them forcefully with a Ringer's solution." Other modifications of local manipulation by six other neurosurgeons (summarized by White and Sweet) all shared the feature of cutting no rootlets. Over 1600 cases thus treated had been reported by 1969.

TABLE-1 Results of Minor Manipulations in the Middle Fossa


Number of Patients

Initial Failure+ Significant Recurrence (%)

Duration of Follow-up (Years)

Bohm & Hojeberg





Gardner & Miklos









Up to 8





Up to 6










Up to 10





Up to 20


Avg >12






Several reports describe the great majority of recurrences as taking place within the first year or two both for the middle fossa manipulations and for MVD in the posterior fossa. Although after both types of operation there may be a period of some days to a week before the neuralgic pains subside, they did so after the middle fossa procedures in all but 3.8% reported by 14 surgeons. After MVD, initial sustained failure occurred in 6.7% in eight reports.

Dahle et al. in their 57 cases found that the procedures failed to give complete relief in 44 percent of their patients (average follow­up only 3.1 years. Their results included one fatal cardiac infarction at 1 week; recoveries from a hemiparesis and partial bulbar palsy at 3 months, and from bacterial meningitis in two cases; and two permanent sequelae- a patient with severe hemifacial sensory loss and another with unilateral deafness and disabling dysesthesia. That these surgeons should continue with MVD operations in the face of the data on percutaneous operations is interesting.

Results of Retrogasserian Glycerol Injections

The procedure, pioneered by Häkanson, of injecting glycerol into the fluid of the tiny trigeminal cistern has found favour in many quarters because of the minimal numbers of patients developing dysesthesias and showing any lasting significant trigeminal sensory loss after such injections. Once the small quantities of glycerol injected leave the cistern they are diluted by the much larger volumes of CSF in the CP and cerebellomedullary cisterns. Hence, this toxic effect must be confined to that portion of the rootlets in the middle cranial fossa. Several authorities consider glycerol as their first choice when the medical management of trigeminal neuralgia has failed. Although the percentages of early and late failures exceed those of either of the two open operations discussed, the modest degree of sensory loss may be comparable. There is essentially no mechanical manipulation of the rootlets involved and this may account for the less favourable results. As with the other procedures, the longer the follow-up the higher the recurrence rate. De la Porte et al. state only that the rate of pain relief was lower than that with thermocoagulation or percutaneous compression.

A point to be noted, the tendency of the glycerol to strike V1 fibers more often than those in the second and third divisions, has been much more quantitatively shown by Bergenheim, Hariz, and Laitinen. They measured the thresholds for facial pain perception in milliamperes at six standard sites (using the ISSAL 1412 apparatus), delivering a constant current to bipolar electrodes. The day after the glycerol instillation into the trigeminal cistern in 57 patients the threshold current for perception of any sensation and the higher threshold for pain were increased as compared with preoperative thresholds. They say this increase "was especially marked and well localized for those with pain in the first branch." (This loss need not extend to corneal anesthesia.) The absence of any sensory loss and minor sensory loss in the remainder characterized the first 100 patients of Häkanson and  preservation of much or all sensation was the finding of most, but not all, of the 22 rapporteurs. There is essentially no mechanical manipulation of the rootlets or increased pressure against them and this may account for the less favourable results. As with the other procedures, the longer the follow-up the higher the recurrence rate. However, the fact that the majority have long-lasting relief associated with minimal sensory loss adds another large group of patients to those whose relief cannot be attributed to any change in neurovascular relations at the trigeminal root entry zone into the pons.

Conclusion upon Comparison of Open Operations

The similarity between the general results in the two groups of open operations is striking. The likelihood of modification by middle fossa manipulations of anatomic relationships at the pontine trigeminal root entry zone is so small that the hypothesis that local pressure there causes the pain is for the great majority of cases untenable by virtue of this set of facts alone. Adams, in a commendable detailed analysis of many factors bearing on the hypothesis, has adduced an impressive array of considerations leading to the same conclusion, none of which alone seems as telling a refutation of the hypothesis as the results of minimally destructive middle fossa operations. However, taken in toto they add up to a major challenge to, but do not disprove, the concept of vascular compression as the most common cause of trigeminal neuralgia. Scepticism as to the validity of this concept comes as a bitter disappointment to many neurosurgeons. The micro­neurosurgery involved in "vascular decompression" is such a delightful exercise that it saddens to recommend great reduction in its usage because its mortality and morbidity cannot be reduced to the levels of the percutaneous operations.

Other Evidence of the Salutary Effect of Manipulation Near the Nerve

Parkinson recently described a 60-year-old patient at whose middle fossa operation in 1940 he assisted another neurosurgeon. The trigeminal nerve was never identified in the 4-h operation, at which much packing and coagulating occurred. Sensation in the face remained' 'perfect." The patient never suffered any more trigeminal pain for the 35 years of follow-up. Other modest manoeuvres near the trigeminal pathway may stop the severe paroxysms-mysterious though this mechanism may be.

Relief after Minor Extracranial Manipulation

Entering an artery subcranially (presumably the internal carotid artery) instead of the foramen ovale and stopping the procedure, which could possibly lead to a small embolus breaking off from the puncture site within the lumen and causing occlusion of a significant intracerebral artery. Such subcranial arterial puncture occurred in several patients. Remissions from 1 month to over a year occurred in 60% of them. The prompt appearance of a large hematoma on the cheek had led to stopping the procedure before entry into the foramen ovale, led to permanent recovery from the neuralgia in several reports.

Relief after Minimal Intracranial Manipulation

Relief after such procedures has occurred in three of Sweet W.H. cases. He reports the following:

1. A 45-year-old man with second-division trigeminal neuralgia for 4 years. In February 1967, a pair of electrodes in a plastic cuff was slipped easily around the second division of the trigeminal nerve just behind the foramen rotundum. The objective was to test the efficacy of chronic electrical stimulation of the trigger zones that were confined to this division. He was not stimulated at operation, but awoke pain-free and remained so for 8 years before recurrence developed at the same site. (This pain was then not altered by electrical stimulation.)

2. A 48-year-old man who had had third-division trigeminal neuralgia for 6 years, during which there had been no drug-free remission. An atypical feature was hypalgesia of the second and third trigeminal, vagoglossopharyngeal, and upper cervical nerve zones. Although this cleared after a normal pneumoencephalogram, I elected to expose the CP angle. In August 1975, an unusually easy exposure of the fourth and fifth nerve rootlets aided by the absence of any obscuring veins permitted minimal cerebellar retraction to disclose the single large straight superior cerebellar artery nearer the fourth than the fifth rootlets. No instrument or cottonoid was ever less than a centimetre from the trigeminal rootlets. The rootlets IV through XI and their relations to vessels were also normal. The wound was closed with the intention of making a radiofrequency lesion. This was never needed, because the patient remained pain-free until death from a myocardial infarction 10 years later.

3. A 77-year-old woman with typical first- and second-division trigeminal neuralgia for 20 years had had her longest remission 2 years earlier for about 2 months. Placement of a 20-gauge electrode through the foramen ovale to a point 8 mm behind the dorsum sellae and 5 mm above the horizontal plane of the sellar floor was obviously above the trigeminal rootlets as shown also by failure of electrical stimulation to evoke a response. Inas­much as the cerebrospinal fluid from the needle electrode continued somewhat bloody after 13 ml, the procedure was terminated. There was no trigeminal sensory loss, or other sequel, but complete pain relief persisted for 5.5 months. On one other occasion in which I terminated a procedure without a lesion because of intracranial arterial bleeding, there were no sequelae but no relief of pain.

Varma and Frazer observed vascular "contacts" with the nerve in the CP angle in 13 of 20 patients with trigeminal neuralgia. Of the 7 without such contact, 3 had multiple sclerosis. They wrapped the trigeminal nerve with muscle in 19 of the patients. Only in a 20th patient with multiple sclerosis did they cut the nerve, with complete relief. All 19, except the 2 with multiple sclerosis, were also completely relieved during follow-ups from 4 to 30 months.

Nugent and Rovit seek to stop their lesions upon achieving "minimal hypalgesia or analgesia" in the trigger zones. Their recurrence rates are about the same as those of the rest groups. Hypalgesia is much less likely to be accompanied by dysesthesias, and attainment of this level of sensory loss may well suffice for many of the patients.


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