If your eyes feel chronically dry but have adequate tears do you have dry eye disease? Confused? So are eye doctors – at least we should be. This is what we do know: 1) Although people who suffer from chronic symptoms of dry eye have fewer tears as a group, many have adequate tears: 2) Improving the function of diseased Meibomian glands can ease abnormal dry eye symptoms: 3) Diseased Meibomian glands do not cause them (Galor A and others: Ocular surface parameters in older male veterans. Invest Ophthalmol Vis Sci 2013, 54(2):1426-1433). If this puzzle were presented to an intelligent creature from outer space it would conclude that symptoms is the disease and that tears and Meibomian glands are secondary confounding complications. So you patiently point out that dry eyes cause dry eye symptoms. Its response is likely to be equivalent to a shrug.
Dry eye symptoms are normally triggered by tear evaporation immediately prior to the breakup of the mirror-smooth optical tear layer that if it occurs, instantly reduces our vision to non-functional levels. The development of the powerful sensitive and complex dry eye alarm and optical tear film sustaining system has been central to the survival of the human species.
Alarms that are active when they should be silent are described as false alarms that point to malfunctions in the alarm system. In that context I argue that our use of artificial tears to mitigate dry eye symptoms is equivalent to treatments directed to maintaining the dry eye stimulus below the level that triggers the hypersensitive alarm. In other words it accommodates the defective alarm rather than fixing it by lowering the alarm’s abnormally elevated sensitivity. Have we been confusing association with causation? Can this explain why cures are not even on the horizon?
I think that most of us would agree that it makes better sense to be looking for pathogenetic mechanisms that can explain the development of increased corneal sensitivity to tear evaporation (corneal evaporative hyperalgesia) independently of tear metrics. The latter can also occur as a separate complication that further exacerbates symptoms. I suggest that these observations point to the presence of dysfunctional elements in the trigeminal pain system.
Photo credit: D.Prakoso
Source: Peer Review