- Department of Neurosurgery, University of California at Los Angeles, Los Angeles, California, USA
Correspondence Address:
Alessandra Gorgulho
Department of Neurosurgery, University of California at Los Angeles, Los Angeles, California, USA
DOI:10.4103/2152-7806.91606
Copyright: © 2012 Gorgulho A. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.How to cite this article: Gorgulho A. Radiation mechanisms of pain control in classical trigeminal neuralgia. Surg Neurol Int 14-Jan-2012;3:
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Abstract
Classical trigeminal neuralgia is a chronic pain condition that was clinically recognized centuries ago. Nevertheless, the pathological mechanism(s) involved in the development of classical trigeminal neuralgia is still largely based on the theory of peripheral versus central nervous system origin. Limitations of both hypotheses are discussed. Evidence of radiation effects in the electrical conduction of peripheral nerves is reviewed. Results of experimental studies using modern and current radiosurgery techniques and doses are also brought to discussion in an attempt to elucidate the radiation mechanisms involved in the conduction block of excessive sensory information triggering pain attacks. Clinical features and prognostic factors associated with pain control, recurrence, and facial numbness in patients submitted to surgical procedures for classical trigeminal neuralgia are discussed in the context of the features related to the pathogenesis of this condition. Studies focusing on the electrophysiology properties of partially demyelinated trigeminal nerves submitted to radiosurgery are vital to truly advance our current knowledge in the field.
Keywords: Demyelination, pain control, pathogenesis, radiosurgery, trigeminal neuralgia
INTRODUCTION
Different mechanisms that ultimately modify pain sensory information within the trigeminal pathway may account for the variable response in patients treated with the available surgical procedures for trigeminal neuralgia (TN). In general, the five most common procedures available for the treatment of classical TN are successful to the order of at least 90% pain relief.[
Radiosurgery directed to the REZ in the affected side, similar to the other destructive techniques used to treat TN, leads to control of the pain in a high percentage of the cases.[
DEFINITIONS
Classical TN is the term coined by the last classification of the International Headache Society[
Secondary TN is defined by the presence of structural damage to the trigeminal system such as a demyelination, tumor invasion/compression, giant aneurysm, arteriovenous malformation (AVM), and herpes zoster infection. If the pain features mimic classical TN, it is classified as typical. If constant, aching and/or burning pain is present, it is classified as atypical.[
PATHOGENESIS OF CLASSICAL TRIGEMINAL NEURALGIA
Aretaeus de Cappadocia was the first to attempt to describe TN. In 1773, John Fothergill provided an accurate clinical description of this painful syndrome. Even though it is a recognized condition for centuries, there is no consensus about the pathological mechanism(s) leading to the most common neuralgia observed. All theories proposed to date are susceptible to criticism since flaws exist when it comes to explain all the features of TN.
Peripheral origin of trigeminal pain
TN has been identified as a peripheral neuropathy since the neurovascular conflict theory was proposed by Peter Jannetta in 1967.[
The root entry zone (REZ) of the trigeminal nerve lies about 2–3 mm away from the surface of the pons. It is characterized by the transition between the central myelin produced by the oligodendrocytes and the peripheral myelin produced by the Schwann cells. The latter is known to be significantly more resistant to injuries, including repetitive vascular pulsation at the myelin sheet, which provides insulation of the trigeminal nerve. Damage to the most sensitive portion of the myelin (central myelin) is the most accepted source for classical TN.[
Figure 1
(a) Axial magnetic resonance imaging (MRI) showing the exquisite visualization of the trigeminal nerve provided by the fast imaging employing steady-state acquisition (FIESTA) or constructive interference in steady state sequence (CISS). The trigeminal nerve can be visualized since the exit in the lateral portion of the pons until the division into roots (VQ, V2, V3) inside the Gasserian ganglion. (b) Axial slice, FIESTA MRI showing a typical radiosurgery plan performed at University of California at Los Angeles. The prescription dose is 90 Gy delivered at the root entry zone. The isocenter is positioned with the 50% isodoseline tangent to the pons
Unfortunately, animal models used to study the physiopathology of TN are not convincingly representative of all key features defining this condition.[
Some of the animal models inducing trauma in the trigeminal roots showed data that would actually support the peripheral origin of trigeminal pain. Generation of extra action potentials to orthodromic stimuli was observed in 23% of the nerves submitted to suture lesions at 3 weeks postoperatively, but not at 1 or 6 weeks postoperatively.[
Since the neurovascular conflict theory is not irrefutable in all circumstances, other theories came about aiming to conciliate the neurovascular principle with other possible pathophysiological factors.
Central origin of trigeminal pain
In 1756, Nicolas Andre coined the term Tic Douloureux to define TN, which is a direct allusion to the similarities between the pain attacks and a seizure. Episodic recurrence of pain attacks in the absence of neurological deficits reinforced the resemblance with epilepsy.
There are certain features of the trigeminal pain attacks that challenge the validity of a pure peripheral pathophysiological mechanism for the genesis of TN.[
delay between the trigger point stimulation and the trigeminal pain attack; pain attack is self-sustained, its magnitude is larger and outlasts the duration of the starting sensory trigger stimuli; and refractoriness following the pain attack, which consists of absolutely no response to stimulation or to a milder pain attack.
Moreover, demyelination by itself is not enough to generate the pain attacks. Myelinated axons composing the trigeminal nerve are related to thermal and touch inflow, not pain. Partial myelin damage should generate patches of numbness in the hemiface rather than pain, which is sometimes accompanied by hyperesthesia.
Experimental studies evaluating the effect of carbamazepine, phenytoin, and baclofen injections in the subnucleus oralis of the spinal trigeminal nucleus revealed a dual mechanism of action: facilitation of segmental inhibition and decrease of excitatory stimuli coming from the periphery.[
Neuroplasticity and the hypotheses of TN pathogenesis
A conciliatory theory proposing peripheral and central nervous system events that would ultimately lead to TN has been suggested.[
However, not all experts refute the sole peripheral origin of TN. For instance, the ignition hypothesis[
Pain duration has been identified as a predictor of recurrence in the series of patients treated with MVD, after disease chronicity of at least 7 years.[
Neuroplasticity-induced changes in the spinal nucleus would be expected to lead to other sensory features such as hyperalgesia, allodynia, and burning type of pain. Based on clinical observations, some patients with a long-standing history of TN describe a constant background pain defined sometimes as dull and even burning. Still, the most bothersome symptom is the electrical shock-like pain. Other patients, even when submitted to prior surgical procedures, do not report a constant type of pain and report only typical features of classical TN pain, which would reinforce the peripheral theory of TN pathogenesis. It is definitely more common to notice a clear pain “transformation” in patients submitted to multiple surgical procedures, whereas concomitant development of facial numbness is frequently observed.[
EFFECTS OF RADIATION ON THE PERIPHERAL NERVES
Experimental work delivering ionizing radiation to peripheral nerves of cold- and warm-blooded animals showed that abolishment of nervous conduction is clearly dependent on the total radiation dose. Several studies reported complete blockade of peripheral nervous conduction following doses of radiation ranging from 1500 to 3000 Gy in cold-blooded animals.[
The literature is controversial in regards to lower doses of radiation. Some studies reported that relatively lower doses of radiation up to 100 Gy did excite the nerve membranes transiently,[
Papers published in 1970s criticized the peripheral nerve choices of authors showing excitability and attributed their “excitability” findings to artifacts generated by injury currents.[
Schwarz and Fox attempted to elucidate the mechanism involved in nerve conduction blockade triggered by ionizing radiation in a series of experiments. After a delay of 800–1000 sec and once a minimum threshold dose of radiation had been delivered to the nerve, there was decrease in the peak sodium current without further decrease in resting membrane potential. In a series of initial experiments, the minimum radiation dose able to trigger the sodium current decrease varied from 60 to 100 Gy. Nevertheless, in subsequent investigations, this phenomenon was not observed when doses below 100 Gy were delivered. Delay in developing conduction block after radiation delivery is not dependent on the dose delivery rate. This led to the currently accepted theory that slow chemical reactions leading to final block of sodium channels would be the mechanism involved in the process. In summary, nervous stimuli block would be the result of indirect destruction of ionic channels, mainly sodium.[
All experiments described above measured the compound action potential of a given peripheral nerve. The final composition of the nerve ultimately determines the excitability profile observed after ionizing radiation delivery, when measuring the entire nerve rather than the axon. Different subtypes of fibers have different thresholds to radiation response. Gerstner[
Based on some of the facts reported above, some conclusions can be drawn:
Peripheral nerves of warm-blooded animals are more sensitive to ionizing radiation than those of cold-blooded animals. Gamma fibers are more radiation sensitive than alpha and beta fibers. The most accepted theory explaining conduction block following ionizing radiation is that it happens through damage of sodium channels.
One particular study found decrease in the sodium peak current with radiation doses used for TN radiosurgery. We must take into account the fact that the human trigeminal nerve has a different fiber composition in comparison to the sciatic nerve, which was extensively used in cold-blooded animal experiments. The difference in fiber composition of the trigeminal nerve could potentially predispose to conduction block at a lower radiation dose threshold. Demyelinated areas, acting as autonomous hyperexcitable pacemakers, may have a different density of membrane receptors in comparison to the intact segments of the nerve. This could imply more sensibility to radiation-induced electrical conduction block. It may also explain the clinical results observed with radiosurgery for TN using doses significantly lower than the ones required to block nerve conduction in intact peripheral nerve experiments. Histology data on trigeminal nerves submitted to radiosurgery are rare, but they suggest that radiation causes partial block of nerve conduction rather than complete nerve knockout. Obviously, physiological studies evaluating trigeminal nerves treated with radiosurgery are necessary to corroborate this hypothesis.
Genetics may play a role in the susceptibility of response to radiation among different individuals, accounting for the differences in the time required to experience pain relief after radiosurgery treatment. Very few patients experience pain cessation within days of radiation delivery. In average, patients present response within 4–6 weeks post-radiosurgery.[
PARALLEL BETWEEN ANIMAL EXPERIMENTATION AND HUMAN FINDINGS
Even though far from ideal, findings from radiation experiments in peripheral nerves of different species can be extrapolated to parallel the radiation effects on the human trigeminal nerve. These approximations are the best we are able to gather at the moment.
Radiosurgery to the nerve – Pathological examination of experimental data
Our experimental data with 90 Gy radiation delivered to the peripheral nerve of the swine,[
Identical histological findings have been observed in the trigeminal nerves of baboons.[
Radiosurgery devices and doses used in clinical practice for the treatment of TN were applied in both studies. There are, however, important limitations. The irradiated nerves were intact. Unfortunately, data on radiosurgery effects in partially damaged nerves are not available. In swine, clinical outcome measures used to confirm accurate radiation delivery in the dorsal root ganglion were numbness and limb paresis. Patients with classical TN do not necessarily present with facial numbness after radiosurgery, and motor dysfunction of the masseter is not noticed. Moreover, pain control is achieved prior to the development of facial numbness. Obviously, neurophysiological studies of the irradiated nerves are the key to elucidate radiation mechanisms involved in pain control.
Radiosurgery to the nerve – Pathological examination of clinical data
Foy et al.[
Szeifert et al.[
RADIATION EFFECTS AND PAIN CONTROL
Radiofrequency rhizotomy, balloon compression, partial section of the nerve, and glycerol injection cause an abrupt disruption of the sensory transmission. The injury elicited by radiation implies a process leading to sub-total damage and partial block of sensory information overtime. Animal experiments established that the ability of radiation to completely block nervous conduction is dose dependent. It was also shown that there is a minimal time latency required to elicit conduction block independent of the dose rate and is conditional upon the delivery of a minimal total radiation dose. The minimal radiation effective dose varies according to the level of myelination of the nerve fiber. Radiation effect on myelinated fibers is likely due to inhibition of autonomous pacemakers that generate recurrent action potentials, which spreads to myelinated and non-myelinated axons by ephaptic and after-discharge potentiating mechanisms.
Pathological specimens of patients submitted to MVD showed that demyelinated areas are observed at the REZ. This is the site where the central myelin is more sensitive to injuries (compression, stretching, radiation). Blood vessels are commonly observed to touch the nerve precisely at the REZ. There are radiosurgery protocols that limit the total dose of radiation to the brainstem surface to 12 Gy,[
Clinical data suggest that pain outcomes tend to be better when the isocenter is positioned more proximally to the brainstem/REZ.[
It would be relevant for radiosurgery practice to determine the threshold dose of radiation to achieve conduction block in demyelinated portions versus intact trigeminal nerve segments. Also, experiments comparing radiation dose effects on partially damaged versus completely intact nerves will provide important information about the radiation mechanisms involved in the blockade of electrical axonal conductivity. These experiments may even contribute to validate the current hypothesis about the genesis of TN.
TECHNICAL ASPECTS OF RADIOSURGERY TREATMENT
Radiosurgery for TN is very challenging. The target has a diameter of 3 mm and the most commonly used collimator has a diameter of 4 mm [
As the current thickness of the targeting MRI and computerized tomography (CT) scans is between 1 and 1.5 mm, the best possible targeting accuracy is of 0.75 mm, based solely on the imaging factor. We also need to consider that the expected accuracy of the stereotactic frame is 1.5 mm. This was shown in a multicentric study for functional neurosurgery that included our own center data[
Delivered doses vary from 80 to 90 Gy, through a 4-mm collimator, which results in prolonged treatment time. Radiosurgery devices used for treatment of this condition include: Gamma unit,[
Total disease duration and periodicity of pain attacks at the time of radiosurgery treatment (multiple daily attacks vs. low frequency pain attacks vs. remission) may also be relevant in the context of radiation effects. Radiosurgery is not delivered to patients under acute pain attacks because of the known latency necessary to trigger pain relief. If radiation controls excessive action potentials generated in the demyelinated areas by inducing changes in the density of the membrane ionic channels, it certainly cannot be used in patients under acute pain attacks who need an immediate method of pain control. Radiation would be expected to best work on patients treated while they experience low frequency of pain attacks. Abnormal re-myelination is considered by some experts to explain the periods of remission observed in the course of the disease. Adding radiation to remyelinated segments of the nerve may potentiate the block and/or generation of excessive sensory information. However, if experimental studies show that demyelinated areas present lower radiation dose threshold for impulse blocking than remyelinated or intact areas of the nerve, it may throw light on the optimal timing to radiosurgery treatment. Concomitantly, careful analysis of these variables in future series treated with radiosurgery may suggest which hypothesis seems to best explain how radiation leads to pain relief..
On the other hand, the potential effect of radiation on the trigeminal nuclei at the brainstem should be considered as a possible additional mechanism of radiation-induced pain control and facial hypoesthesia, noticed in some cases post-radiosurgery. The dose distribution achieved in our radiosurgery plans with the 5-mm collimator shows that only about 2.7 Gy and 0.9 Gy reach the area of the nucleus principalis and spinalis of the trigeminal nerve, respectively. When using the 4-mm collimator, these doses are even smaller. It has been shown by our group that radiosurgery delivered with the 3-mm collimator has the ability to modulate neurotransmitters in the brain.[
CONCLUSION
Based on the review of the experimental literature in parallel with the findings in humans undergoing procedures for TN, it is likely that any procedure tending to decrease the input of information from the trigeminal sensory fields across the junction of the peripheral portion of the trigeminal nerve with the central nervous system trigeminal pathways leads to pain control. Accepting the hypothesis that the transition of peripheral myelin (Schwann cells) to central myelin (oligodendrocytes) is the weak site of the nerve, and is therefore vulnerable to short circuitry formation, it is interesting to observe that the MVD results are, indeed, the most durable with the least disruption of the trigeminal system. It is likely that imprecision of the stereotactic technique with 1-1.5 mm error in a target of 3 mm leads to variation of dose to the REZ, potentially offering the basis to the variability in the results achieved with this method and also justifying observed recurrences.
Experimental data on the effects of focused radiation on the electrophysiology properties of partially demyelinated trigeminal nerves are a major need to truly advance our current knowledge in the field. It should allow further refinement of radiosurgery protocols and, hopefully, improvement in clinical outcomes.
Publication of this manuscript has been made possible by an educational grant from
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