BLOG: Neurotrophic keratopathy is way more common than we THINK
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Most eye care professionals don’t think a lot about corneal sensation. We assume it is similar for most patients, and yet that couldn’t be further from the truth.
With the approval of cenegermin in 2018, we have started to identify reduced corneal sensitivity more commonly, but typically we only test it in patients with very diseased corneas. A recent study we performed, however, shocked me as to how common this condition is in patients we all see every day.
Thanks to Bill Trattler’s 2017 PHACO study, we’ve known for several years that dry eye disease (DED) occurs in about two-thirds of cataract patients, and about two-thirds of those have relatively few symptoms. But how many of those dry eye sufferers presenting for cataract consultation actually have neurotrophic keratopathy (NK)?
The Mackie classification system for NK designates stage 1 disease as punctate epithelial defects with reduced sensation. My guess was this occurred in the range of 10% to 20% of cataract patients with dry eye, so we set up a study to find the actual incidence.
In the study, titled “The THINK Study: Testing Hypoesthesia and the Incidence of Neurotrophic Keratopathy in Cataract Patients with Dry Eye,” now published in Clinical Ophthalmology, we identified patients presenting for cataract evaluation who also had dry eye, defined as a reduced tear breakup time and the presence of corneal staining. In these patients, before the application of any anesthetic drops, we tested corneal sensation quantitatively using a Cochet-Bonnet esthesiometer. This is the traditional instrument with a calibrated, extendable filament in which the examiner determines sensitivity by touching the surface of the eye with the fully extended filament, then gradually shortens the filament and touches the cornea again until it is first perceived by the patient. Because the filament exerts more force on the cornea when it is shorter, lower values mean less sensitivity. Based on several previous studies, the accepted cutoff for reduced sensitivity is 4 cm or shorter.
We found that 58% of patients had reduced sensitivity with a measurement of 4 cm or less. The 95% confidence interval (meaning we are 95% confident the true incidence is in this range) was 39% to 75%. Also, 42% of patients detected the filament at 3.5 cm or less, meaning they had even more profound loss of sensation.
What does this mean? Clearly, NK is more common among cataract patients than we previously thought. We also know that we need to treat DED before performing cataract biometry, based on a 2020 study published by my group. What’s not clear is whether we need to treat this large subset of patients with reduced sensation differently from other cataract patients with DED.
Certainly, cenegermin is not for all these patients. Its use is appropriate for some cases of NK, but its cost means we need to explore the role of other treatments of stage 1 disease as a first step.
We also need to investigate the incidence of NK in DED patients who are younger, with typically higher corneal sensitivity. Younger working people probably use electronics more than their older counterparts, and this may drive a loss of corneal sensation with its own implications for treatment.
Finding these patients in our practices by rigorously testing sensitivity presents an operational challenge: Most of us do not own a Cochet-Bonnet esthesiometer or other objective instrument for sensitivity testing, and many doctors are not eager to add one more unreimbursed diagnostic test. Also, most patients receive topical anesthetic for a pressure test before seeing the doctor, so the typical workflow would need to change. Some commercially available noncontact quantitative esthesiometers now exist and may have a future role in our cataract workups.
The THINK study presents an unquestionably important finding — that more than half of cataract patients with DED have NK, and the proper treatment of this condition will be important before embarking on cataract surgery, a procedure that will redefine a patient’s vision for the rest of his or her life.
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For more information:
John A. Hovanesian, MD, FACS, an ophthalmologist specializing in cataract, refractive and corneal surgery at Harvard Eye Associates in Laguna Hills, California, can be reached at drhovanesian@harvardeye.com.
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