The disconnect between signs and symptoms in dry eye disease is an intriguing clinical problem. Although we know quite a lot about the etiology of dry eye, the ideal structure of the tear film and how to diagnose and treat dry eye disease, there are still some patients who do not respond to treatment or who present complaining of severe symptoms that seem to be poorly correlated with clinical signs. Complaints may include a burning or stabbing sensation or a feeling of pressure in the eyes.
Many clinicians find these patients—and the chair time they consume—frustrating. But before dismissing them as “crazy,” it may help to consider whether they could be suffering from a different form of dry eye that is difficult to detect by traditional methods.
In 2009, Perry Rosenthal and colleagues observed what they presumed to be corneal neuralgia, which they termed “pain without stain” because of the paucity of corneal staining or other clinical signs.1 They speculated that the pain symptoms could be related to a neuropathic disorder, rather than to qualitative or quantitative tear film factors.
Later work by Rosenthal further elucidated how the nociceptive system—critical to vision when functioning properly, because it monitors and restores the optical tear film—could become dysfunctional.2 When this happens, the corneal nerves, the central trigeminal sensory network, and/or the pain organising centres in the brain escalate the eye’s natural ‘alarm system’, sending pain signals that are out of proportion to the physical insult (if indeed there is any injury at all).
Neuropathic dry eye pain may be triggered or exacerbated by ocular surgery, systemic conditions or even psychological distress.
The dry eye community has been increasingly interested in neuropathic dry eye. In fact, the new DEWS II definition of dry eye explicitly includes “neurosensory abnormalities” in the list of etiological factors in dry eye.3
A DEWS II subcommittee report on pain and sensation notes that long-term inflammation can alter neuron excitability, connectivity and impulse firing and that disturbances in ocular sensory pathways may ultimately lead to neuropathic pain.4 However, as yet, there are no dry eye therapies that specifically target neuropathic pain.