Phantom Pain and Ocular Pain in Sjögrens Syndrome
As noted earlier, ocular symptoms in patients with SS are out of proportion with objective observations of the ocular surface, largely due to the mysterious perception of “phantom pain.”
The concept of “phantom pain” may arise at the level of the cranial nerve, spinal dorsal root ganglia, and thalamus or brain cortex. As noted above, these processes may be strongly influenced by local release of cytokines by microglial cells.3 This observation correlates with our clinical observation that the pain and discomfort in patients with SS whose disease is longstanding is much less improved following treatment with biologic agents than in patients with early disease.
Phantom pain explains the aberrant, painful ocular sensation that chronic post-Lasik patients or patients with SS experience when they report that their pain is only partially relieved by a topical anesthetic.
Sites for ocular pain have been mapped by functional magnetic resonance imaging to a specific region of the prefrontal cortex (see Figure 3).15
To emphasize, a patient’s complaints of pain derive not only from the pain fiber on the surface of the eye, but also from the part of the brain where that sensation is processed and remembered. Thus, a new frontier for therapy will be the understanding of noxious stimuli from primary somatosensory cortex that involves alpha-delta fibers.16
I began following the simple procedure of Rosenthal et al in our rheumatology clinic by asking patients with SS about their ocular pain level at their routine exams. Not unexpectedly, patients with longstanding disease would report their pain level to be a seven or 8 on a scale of 10. What was surprising was that patients reported their pain level had diminished only slightly, to a 4 or 5 score after instillation of a topical anesthetic into the eye. There is little doubt that the anesthetic was effective, since the corneal reflex (the blink after gentle pressure to cornea) was obliterated indicating loss of the afferent loop of the reflex.
In summary, it appears that a patient with an acute insult to their eye (such as a corneal abrasion) may have significant, although temporary improvement in ocular discomfort with a topical anesthetic. However, the patient with SS who has chronic eye pain does not obtain the same relief from anesthetic. The simplest explanation is that patients with SS develop a phantom pain as the result of the chronic ocular irritation.
This interaction of different central pathways has been postulated by neuroscientists such as Dr. Ramachandran and colleagues at the University of California at San Diego’s Salk Institute to explain the phantom pain felt in an amputated extremity. His hypothesis is complex, but is nicely summarized at Ted.com/talks and in a YouTube video presentation.
Thus, we can make some immediate suggestions to help get on track to treat the benign symptoms by identifying target molecules:
- Use animal models of SS and their normal congenics (such as nonobese diabetic or thrombospondin, knockout mice) in preclinical studies to screen molecules with antiinflammatory, antidepressant, and antinociceptive agents. I predict that the “immune” animal will have a different dose response due to their inflammatory milieu and danger-signal responses.
- Study the trigeminal nerves in SS models as well as the salivary glands. Examine neurotransmitters at each site to identify new drug-target molecules.
- Investigate the corneal surface and lacrimal function in preclinical models of pain and depression, as these may be abnormal and provide new therapeutic targets.
- Involve neurochemists in the design of clinical therapies for SS.
We began our discussion of SS with the observation that local inflammation in the glands leads to a global dysfunction in secretory ability disproportionate to the number of residual acini and ductal cells. Perhaps this is an important starting point to understand the interface between inflammatory and neural/secretory pathways.
In order to make a significant difference in the quality of life for patients with SS, new therapies must not only improve extraglandular manifestations of SS, but also alleviate the symptoms (fatigue, dryness, and pain) that lead to patient disability.
We must first learn to measure the objective variables that reflect the patient’s symptoms. This will require better cooperation between rheumatologists and experts in the field of neural pain circuits. With a more focused effort, the complex interplay among immune, neural, and hypothalamic pathways in SS may finally be unraveled.
The authors gratefully acknowledge helpful discussions with Drs. Perry Rosenthal of Harvard Medical School, Boston, Mass.; Simon Bowman of University Hospitals Birmingham NHS Trust, Birmingham, England; Jules Birnbaum of the Jerome L. Greene Sjögren’s Syndrome Center, Baltimore, Md.; James Cahill of Scripps Memorial Hospital–Ximed, La Jolla, Calif.; and Vilayanur Ramachandran of the University of California at San Diego’s Salk Institute.
Dr. Fox is a rheumatologist and Carla Fox is a registered nurse in the rheumatology clinic at Scripps Memorial Hospital–Ximed in La Jolla, Calif.
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