Understanding neuropathic pain
Even as research into THC delivery to the ocular surface continues, we have also been seeking to gain a better understanding of neuropathic pain and the characteristics of patients who may suffer from this form of dry eye. To do this, we in the dry eye clinic have been working closely with experts from the University of Cologne’s pain clinic.
We retrospectively evaluated 52 patients seen in our dry eye clinic who fell into the ‘pain without stain’ category. These subjects had normal Schirmer’s, no corneal staining and no signs of blepharitis, but they had OSDI scores >40, in the ‘severe’ symptom range (median score 77).
In addition to a very thorough history and examination in our clinic, many of the patients also underwent a full pain inventory and Hospital Anxiety and Depression Score (HADS) questionnaire. Comorbidities included depression (n=9), chronic pain syndrome (9), anxiety disorders (4), and prior eye surgery (17).7 Other researchers have also recently shown that severe dry eye pain is correlated with antidepressant use but not with corneal staining.8
For ophthalmologists, these are exactly the sort of patients who are mystifying at best, and often get dismissed as people whose symptoms are “just” psychological. To the pain experts we consulted, however, it was very clear that these were typical pain patients, with the same types of concomitant psychosomatic conditions they often see among patients who suffer from chronic headache or back pain.
A typical patient in our study, for example, might have fibromyalgia and rheumatoid arthritis, with an OSDI score of 65 but no clinical signs of dry eye. Upon questioning, she might have first noticed the dry eye pain following LASIK or around the time of a particularly stressful life event, such as the death of a close family member.
In some cases, the patients had no response to topical anaesthesia or to systemic pain medications, which was quite interesting, since it indicates there is something very unusual going on with regards to their pain response. It seems that these individuals may be predisposed to systemic neuropathic pain, of which ocular pain is but one manifestation.
These insights have changed the way I practice in a number of ways. For example, I have started using the HADS questionnaire more frequently for patients with severe symptoms. I ask broader questions about fatigue, anxiety and depression as part of my history taking.
Recognising that ocular surgery can trigger an underlying pain syndrome, I am more thoughtful about evaluating postoperative patients with unexplained symptoms to determine whether they might be most appropriately managed as a pain patient. As medical doctors, it is our responsibility to see the whole patient and, when appropriate, help them to seek treatment for systemic autoimmune disorders, psychosomatic illnesses and other pain syndromes.