Corneal Nerve Sensitivity Declines Progressively in Adults With the Eye Condition Aniridia
Congenital aniridia is rare. It is caused in most cases by mutations in the PAX6 gene, which plays an essential role in the development of eye structures, and its most visible feature - the total or partial absence of the iris - gives the condition its name. But aniridia is not simply a cosmetic anomaly. Those affected commonly experience focusing problems, extreme light sensitivity, and a range of complications that can worsen over time, including progressive corneal disease that can threaten vision.
Previous studies had already established that the density of nerve fibers in the corneas of adult patients with aniridia is reduced. What no one had rigorously examined was whether the nerves that remain are functioning normally - whether they can detect stimuli, whether they respond appropriately to threats, and whether they maintain the protective tissue functions that healthy corneal innervation provides. A clinical study led by the Ocular Neurobiology Group at the Institute for Neurosciences in Spain, published in the journal Cornea, now addresses that gap directly.
Testing Nerves, Not Just Counting Them
Professor Maria Carmen Acosta, who led the study, framed the research challenge clearly: fewer nerves in the cornea was established, but whether those nerves worked properly remained unknown. The team evaluated a group of aniridia patients that included both children and adults, comparing them against individuals without the condition. The geographic and clinical scope of the study was made possible through collaboration with ophthalmologist Nora Szentmary at Semmelweis University in Hungary, whose specialized expertise in aniridia enabled the recruitment of patients across different age groups - an essential requirement for studying how nerve function changes over time.
The researchers used carefully calibrated air pulses to measure corneal sensitivity to mechanical stimuli, and measured cold sensitivity separately. They also assessed tear production under baseline conditions and after triggering the lacrimal reflex using CO2 microstimulation delivered via the i-Onion device, an instrument developed from a patent held by the research group. This combination allowed them to assess both sensory perception and the downstream protective response that healthy corneal nerves should activate.
A Pattern of Progressive Decline
The findings traced a clear developmental arc. Children with aniridia showed corneal sensitivity very similar to that of healthy controls - the inherited nerve deficit had not yet translated into significant functional impairment. Adults with the condition told a different story. They required substantially stronger stimuli to perceive contact with the cornea and showed impaired ability to discriminate between different stimulus intensities.
Equally significant was what happened to tear production. Baseline tear output in aniridia patients was comparable to that of people without the condition. But the ability to amplify tear secretion in response to a stimulus that should trigger the lacrimal reflex was reduced. This matters because the lacrimal reflex is one of the eye's primary defense mechanisms. When something contacts or irritates the cornea, healthy nerves signal the lacrimal gland to flood the eye surface with tears, diluting irritants, flushing debris, and distributing protective proteins. If that signal weakens, the defense fails.
Professor Juana Gallar, head of the Ocular Neurobiology Group, noted that sensory nerves do more than detect sensation. They actively maintain and regenerate corneal tissue. When innervation deteriorates, the cornea loses its capacity to repair minor injuries - leading to persistent lesions, loss of transparency, and chronic pain. The study's data suggest this trophic function is also compromised in adult aniridia patients as their nerve sensitivity declines.
What Remains to Be Learned
The study is limited by the rarity of the condition, which constrained the sample size available for analysis. As a rare disease study involving both children and adults, the cohort could not be large, and the cross-sectional design - comparing different individuals across ages rather than following the same individuals over time - means the progressive pattern is inferred rather than directly observed. Longitudinal follow-up of children with aniridia through adulthood would provide stronger evidence for the progressive nature of the decline.
The next research phase will examine nerve function in a mouse model carrying a PAX6 mutation, allowing the cellular mechanisms behind the progressive deterioration to be studied at a level of detail that is not possible in clinical patients. Understanding those mechanisms is necessary for designing interventions that might slow the process before vision-threatening complications develop.
For patients and clinicians, the practical implication of the current findings is that corneal nerve assessment should be considered a component of aniridia monitoring, not merely as a predictor of discomfort but as an indicator of the eye's capacity for self-protection and repair.