Face Pain Info Pages - Part 5

Use of MRI in Diagnosis of Facial Pain

One of the recurrent themes in various discussions on this site concerns the use of MRI (and sometimes other types of medical imaging) to aid doctors in coming to a diagnosis of long-lasting facial pain. The following is edited from a couple of my responses to posters in some of those threads.

TN is not "diagnosed" as such from MRI results alone. The key elements of TN diagnosis are medical history, the physical pattern and "kind" of pain, where in the face it manifests, and under what circumstances. A "positive" indication of nerve compression in an MRI can lend support to a diagnosis of facial neuralgia or a decision to do Microvascular Decompression surgery. But a "negative" indication should not be interpreted as a reason for eliminating MVD as an option. Likewise, millions of people who have nerves compressed by blood vessels have no symptoms at all.

To the best of my knowledge based on 16+ years of research as a layman, there probably is no generally accepted practice standard with regard to when and how an MRI should be performed on patients who present with facial pain attributable to neuropathic causes. However, from what I've read in multiple sources, I believe there is an MRI procedure that would be considered appropriate by most specialists who are strongly familiar with this category of pain and its treatment. This might be called informally "the gold standard of imaging practice."

This procedure is a full-skull series MRI, performed for 0.65-mm, thin-slice resolution. The MRI images are done both with and without a contrast agent. Post-procedure processing of the images is performed to generate a 3-D composite map. There are special magnet weightings considered appropriate for detailed examination of the trigeminal nerve. You might hear this procedure referenced as a "FIESTA" MRI or a "Trigeminal Protocol" MRI.

High-resolution MRI is useful in eliminating or confirming several potential sources of facial pain. These include the following:

• Arterial or Venous Compressions of the Trigeminal or Other Nerves

•  Benign or Malignant Tumor

•  Benign Cyst

•  Aneurysm

•  Arterioveinous Malformation

• Arachnoiditis

• Calcium Deposits

•  Small-Vessel Disease

•  Schwannoma

•  MS Plaques on the Trigeminal Nerve

(and probably more than a few that I've forgotten at this writing)

Some physicians use a CT Scan -- a series of high-resolution X-rays -- to examine for features in soft tissue. Resolution isn't quite as good, but the ability to see through bone and examine X-ray shadows can be helpful in some cases.

Caution must be applied in interpreting and acting upon even high-resolution imagery. It is frequently the case that MRI generates false negatives for relatively small features, such as arterial compressions or MS plaques. The features may be present but are still not seen in medical images. Likewise, diagnosing MS by means of MRI can require multiple procedures over a period of several months to look for changes and track them to patterns of pain response or physical limitation.

There are numerous case reports in the medical literature that relate doctor experience in having found a nerve compression by an artery or vein during an MVD procedure, although the compression did NOT show up in an MRI prior to surgery. Surgical teams led by Dr. Peter Jannetta (who popularized MVD for Type 1 TN) have reported that compressions were found during surgery in all but a very few among more than a thousand Type 1 TN patients treated by MVD. Other sources recently discussed here on Living with TN suggest a false negative rate of at least 10%, and I recall that sources I've seen in other literature reports indicated on the order of 20% or more.

Results in MS and Type 2 (Atypical) TN patients treated by MVD appear to be less definitive but are still suggestive of a basic principle in common with Type 1 TN. In my view and that of at least some neurosurgeons (Dr. Jannetta was at one time among this group, although I don't know his current opinion), an absence of MRI-visible compressions of the trigeminal nerve by blood vessels should NOT be taken as a definitive reason to deny a patient access to MVD surgery. Such an absence simply requires the surgeon to assess possibilities of success based on other factors in the patient's medical history and pain presentation.

MVD is frequently successful in reducing "Typical" TN pain in patients who have a mixture of Typical  and Atypical TN symptoms. It appears to be less frequently successful in reducing pain for patients whose symptoms are dominated by Atypical TN features or complicated by previous surgeries and nerve lesions (scars). However, "less" success is not zero success. My reading is that more research is needed to better discriminate the cases where success rates are higher from those where the surgery is more obviously unlikely to help.

It is also wise to be aware that not all MRI centers are equipped to work at sub-millimeter resolutions, or to do the post-procedure image processing. Some insurance companies also refuse to pay for high resolution, claiming (in my view, incorrectly) that the additional expense of the processing does not generate value in better diagnosis or treatment results. The last time I heard, a sub-millimeter MRI performed with and without contrast could cost over $3,000 at many medical centers in the US.

So (in my personal view as an informed layman) we may need to throw out the term "definitively" in just about everything we know or think we know about the disorder we are dealing with. The spectrum of symptoms and sensitivities in this patient population is simply too large to generalize reliably. MRI is probably the best technique that doctors have at their disposal. But some of the effects that generate pain in the 5th cranial nerve may be at the level of individual nerve fibers rather than in the nerve as a whole. MRI is not up to the task of diagnosis at that scale.

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Face Pain Info Pages - Part 6

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