37 Principles of Trigeminal Neuralgia

Abstract

Trigeminal neuropathic pain (TNP) is an umbrella term that encompasses pain syndromes that consist primarily of paresthesias (shooting pains), dysesthesias (constant burning pain) or a mixture of them. Surgical treatment is either by ablation or decompression. Ablative options are reserved for those patients with a predominance of paresthesias. Microvascular decompression is an option when there is an MRI demonstrated vascular association. Initial clinical benefit is comparable in appropriately selected patients with ablation and decompression. Benefit is less when decompression is done in patients with a predominance of dysesthesias. Neurosurgeons who treated TNP should be able to offer multiple options for treatment based on presentation, health and age.

Generations of physicians have described the stabbing, electric shock pains of trigeminal neuralgia but were frustrated by their inability to treat them. In fact, such shocks are the earliest signs and most treatable symptoms of a progressive neuropathic pain syndrome. Its later, more difficult to treat, manifestations consist of constant, burning, tingling pain that may also be associated with sensory loss.

Recent descriptions of neuropathic facial pain subdivide it into multiple entities. TN1 describes the “classic” elements of intermittent stabbing facial pain triggered by anything that elicits facial sensory input-talking, chewing, or touching one’s face on the side of the pain. TN2 describes the entity in which there is a component of constant pain, best described as burning, tingling, or prickling. Other categories are used for pain that is a result of surgical injury, or the effect of a sclerotic plaque in the trigeminal pathway seen in multiple sclerosis.

A more basic understanding of trigeminal neuropathic pain is to describe it as dominated by paresthesias, by dysesthesias or by a mixture of the two.

Neuropathic pain describes that pain seen in the absence of detectable or ongoing tissue damage and is often located where there is a sensory deficit. Paresthesias are the paroxysmal, shooting, stabbing components to neuropathic pain. Dysesthesias are unfamiliar, unpleasant burning, often constant elements to the pain. An extensive descriptive vocabulary emerges from having patients complete a McGill pain questionnaire. It is validated for trigeminal neuralgia, multimodality, quantifiable and consists of twenty subcategories of terms that indicate progressively more severe discomfort. It can differentiate intertwined nociceptive from neuropathic pain elements.

Peter Jannetta’s discovery, published first in 1967 as a case report, that trigeminal neuralgia is caused by a compressive vascular association of the trigeminal nerve with a loop of the superior cerebellar artery has defined the modern surgical treatment of trigeminal neuropathic pain.

Magnetic resonance imaging of the trigeminal nerve using thin cut FIESTA or CISS techniques should be done when considering the diagnosis, if possible, and allows accurate visualization of venous or arterial vascular association. This should eliminate any need for surgical exploration of the nerve. The only surgical exceptions are re-operations for recurrent pain in which imaging of the nerve is difficult.

Once the diagnosis is made, the first line of treatment consists of an adequate trial of anticonvulsants. Anticonvulsants work by slowing electrical conduction to the site of the “short circuit” in the nerve. Pathologic analysis of micro biopsies taken beneath the site of vascular compression definitively have shown dysmyelinization consistent with repeated injury to large myelinated fibers with failed efforts at healing leading to approximation of “naked” axons. Anticonvulsant drugs must be titrated slowly upwards, stopping when pain is adequately controlled or when symptoms of excessive dose are evident. Such symptoms included lethargy, difficulty with balance or other cognitive issues. Medical treatment has failed if these symptoms become dominant or if pain persists in the face of such symptoms becoming bothersome. Carbamazepine (Tegretol) is approved for the treatment of neuropathic pain. Other anticonvulsants used include gabapentin, oxcarbaxepine, Dilantin and extended release versions of these drugs.

The principles of neuropathic facial pain surgical treatment are:

1. 
   

   Pain consisting primarily of paresthesias can be treated by microvascular decompression or a proven ablative technique.

   
   
2. 
   

   Pain that consists primarily of constant dysesthesias should not be treated by an ablative technique.

   
   
3. 
   

   Pain associated with multiple sclerosis should be treated by an ablative technique if there is a sclerotic plaque within the trigeminal pathway, but may be unrelated to multiple sclerosis if the disease is not active and there is a visible vascular compression.

   
   
4. 
   

   Accepted, often used and well-documented ablative surgical treatments include: radiofrequency thermal rhizotomy, glycerol rhizotomy, gamma knife radiosurgery, LINAC or cyberknife radiosurgery and balloon compression rhizotomy. Peripheral neurectomies of the supraorbital or infraorbital branches of the trigeminal nerve are occasionally performed as secondary efforts.

   
   
5. 
   

   Temporizing percutaneous efforts with Botox, nerve blocks and acupuncture are unproven.

Studies indicate that the best quality of life occurs after successful microvascular decompression and next, after balloon compression. Microvascular decompression is indicated if MR imaging shows a vascular association and the patient is a surgical candidate. When MR imaging cannot be done, a CT cisternogram can provide adequate imaging of the nerve. There is no age limit to the surgery, however anticoagulant drugs must be discontinuable for a variable period of time in the pre and perioperative period. MVD in older patients is technically easier than in younger patients, because of the larger size of the cerebellar cisterns, though the risk of stroke is higher.

Principles of MVD surgery are:

1. 
   

   Proper positioning is achieved either in a straight lateral position or supine with head rotation in younger more supple patients.

   
   
2. 
   

   Cranial exposure should be to the margins of the juncture of the lateral and sigmoid sinuses. This can be done by use of a neuronavigation system.

   
   
3. 
   

   Intra operative monitoring of auditory brainstem evoked potentials should be used in order to limit risk of hearing loss or injury to the facial nerve.

   
   
4. 
   

   Opening of the arachnoid overlying the trigeminal nerve in order to achieve visualization of the full cisternal portion of the trigeminal nerve since compression may be at any point within the cistern. This may require coagulation and sectioning of the superior petrosal venous complex.

   
   
5. 
   

   Decompression of compressive veins is preferable to coagulation and sectioning because there is risk of cerebellar swelling with sectioning of larger veins and injury to the trigeminal nerve during coagulation (▶ Fig. 37.1).

   
   
6. 
   

   Reoperation should only be undertaken by neurosurgeons experienced in the procedure and, it should be expected that (a) the cerebellar hemisphere will be adherent to the suture line of the previous closure and (b) the Teflon fibers previously inserted will be adherent to the trigeminal nerve leading to a higher risk of nerve injury from the exposure.

   
   
7. 
   

   Younger patients have a higher incidence of constant neuropathic pain, bilateral pain and a venous source of compression.

Pain relief after MVD for patients with predominant paresthesias is about 90%; with predominance of constant, burning dysesthesias, the success drops by 15%. Once successful the incidence of recurrent pain is 15% at 15 years as demonstrated by a Kaplan-Meyer survival curve for pain free days, or about 1%/year to that point.

The incidence of stroke, hearing loss, facial weakness or other cranial nerve injury is in the range of 1–3% in large published series.

The principles of ablative treatment are:

1. 
   

   The goal of treatment is hypesthesia in the region of pain, not anesthesia.

   
   
2. 
   

   Thermal and glycerol rhizotomy and gamma knife radiosurgery (when placing the frame) are performed under brief intravenous sedation. Cyberknife radiosurgery does not require sedation at any point. Balloon compression requires general anesthesia.

   
   
3. 
   

   Ablative treatments target the retrogasserian portion of the trigeminal nerve.

   
   
4. 
   

   Ablative treatments cause partial demyelination of the nerve proximal to the site of “short circuit” except in cases of multiple sclerosis.

   
   
5. 
   

   Balloon compression is the only truly selective cause of injury and is specific for large myelinated fibers that mediate light touch. It preserves small and unmyelinated fibers that mediate pain. It is successful because it reduces the electrical input to the site of short circuit, not because it “stops” pain transmission (▶ Fig. 37.2).

   
   
6. 
   

   Recurrence rates after ablative treatment are in the range of 30% within 3–5 years. This is because the goal of treatment is hypesthesia and this allows re-myelinization to occur with resultant possible pain recurrence.

If the predominant neuropathic pain is constant and MVD and medication have not been successful in alleviating the pain, then several forms of neuromodulation have been used. These are currently off label applications of devices approved for use in humans for other pain syndromes. Options include motor cortex stimulation and, more recently, peripheral branch trigeminal stimulation. Motor cortex stimulation has been repeatedly shown to provide 50–75% likelihood of reducing pain by 50–75%. More recent laboratory studies in rodents have confirmed the hypothesis that motor cortex stimulation works by inhibiting thalamic hyperactivity caused by the trigeminal nerve injury and secondary loss of sensory nerve input. The mechanism for peripheral stimulation effectiveness, its duration and likelihood of benefit has not been adequately studied. With motor cortex stimulation there are a number of long-term studies showing ongoing benefit.

Glossopharyngeal neuralgia may be caused by vascular compression of the IXth and upper Xth nerve fibers by a loop of the posterior inferior cerebellar artery. MVD of these fibers should be considered if there is MRI visualization of a compressive loop. Sectioning of the GPN can cause bothersome dysesthesias and should not be considered an early option. Specialized intraoperative monitoring of vagal motor function is required when operating on the GPN.

In summary, the surgical treatment of neuropathic facial pain requires that the neurosurgeon be able to perform multiple surgical options, as there is no single surgery appropriate to all patients. A clear understanding of the nature of the pain to be treated is essential in the surgical decision making process. ^{1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11}

Related posts:

1 History of Innovation in Stereotaxy/Functional Neurosurgery 9 Cortical and Subcortical Mapping 21 Epilepsy: Temporal Lobectomy with Invasive Monitoring 28 Functional Neurosurgery for Obsessive-Compulsive Disorder 34 Neuromodulation for Neuropathic Pain 39 Neuroprosthetics

Stay updated, free articles. Join our Telegram channel

Join