Corneal Sensitivity & Nerve Testing

The ocular surface and the nervous system constantly communicate. When the tear film becomes unstable or the ocular surface is stressed, corneal sensory nerves detect the disturbance. Those signals can contribute to discomfort, burning, stinging, pain, reflex tearing and blinking.

But nerve function itself can also change. Sensitivity may become reduced, increased, or otherwise altered. TFOS DEWS III therefore recognizes neurosensory abnormalities as one of the etiological factors involved in dry eye disease.

Neural Dysfunction & Altered Sensation

Corneal sensitivity describes how the cornea responds to sensory stimulation. In simple terms: how strongly does the cornea detect a stimulus?

This can be assessed clinically by gently stimulating the corneal surface and observing or asking about the response. The goal is not simply to determine whether the patient can “feel something.” The clinician is looking for whether sensation appears normal, reduced, absent, or sometimes unusually sensitive.

The simplest clinical assessment can be surprisingly basic. A clinician may gently approach the cornea with a very fine material and compare the patient’s response between different areas or between the two eyes.

More standardized instruments are called esthesiometers. These deliver a controlled stimulus to the cornea and allow sensitivity to be measured more systematically. Some devices use physical contact. Others use controlled air or other non-contact stimuli.

The principle is the same: apply a known stimulus and determine how strongly the cornea responds.

Reduced sensitivity means the cornea responds less than expected to stimulation. This can occur when the corneal sensory system has been affected. Possible associations include:

• •previous corneal surgery
• •herpetic eye disease
• •diabetes
• •long-term contact-lens wear
• •certain medications
• •corneal disease or injury
• •neurotrophic keratopathy

Reduced sensitivity can also occur in some people with dry eye. But reduced corneal sensitivity is not one specific diagnosis. The reason for the reduced sensation still needs to be investigated.

Because symptoms depend partly on the nervous system’s ability to detect what is happening.

Imagine two eyes with similar ocular-surface damage. One has normal sensory function. The other has significantly reduced corneal sensation. The second person may actually feel less, even though the ocular surface is not healthier.

This creates an important diagnostic lesson: fewer symptoms do not always mean less disease. Sometimes the eye is sending a weaker sensory signal.

Yes. This is one possible explanation for a signs–symptoms mismatch. A patient may have tear-film instability, surface staining or other objective abnormalities while reporting relatively little discomfort.

Reduced corneal sensitivity is one possible contributor. But it is not the only possible explanation. So we should not reverse the logic and say: “You have few symptoms, therefore your corneal nerves must be damaged.” That conclusion would also go too far.

Neurotrophic keratopathy is a condition in which impaired corneal innervation leads to reduced corneal sensation and compromises the cornea’s ability to maintain and repair itself.

This can become clinically important because the cornea may develop significant epithelial damage while the patient experiences surprisingly little pain. In more advanced disease, persistent epithelial defects or ulcers can occur.

This is very different from simply having mild dry-eye symptoms. TFOS DEWS III emphasizes corneal sensitivity testing when neurotrophic keratopathy is suspected.

Yes. Nerve dysfunction does not always mean less sensation. The sensory system can also become abnormally responsive. Some patients experience intense burning, pain, light sensitivity, wind sensitivity or discomfort that seems disproportionate to what can be seen during a conventional ocular-surface examination. This raises another possibility: abnormal pain processing.

No. This is extremely important.

The phrase “pain without stain” is sometimes used when severe ocular pain occurs despite relatively little visible ocular-surface staining. That pattern can raise suspicion for a neuropathic component. But it is not itself a diagnosis of neuropathic corneal pain.

Dry-eye signs fluctuate. Different tests measure different aspects of the ocular surface. Some abnormalities are not visible with routine examination. And severe symptoms can have multiple contributors.

So symptoms greater than signs does not automatically mean neuropathic pain. It is a clue that may justify looking further.

Neuropathic pain occurs when there is abnormal function or damage within the somatosensory nervous system itself. Instead of pain simply reflecting ongoing tissue stress, the sensory system can begin generating, amplifying or maintaining pain abnormally. This can involve peripheral nerves, central nervous-system processing, or both.

Neuropathic corneal pain can coexist with dry eye. The two are not mutually exclusive. A patient may therefore have genuine tear-film or ocular-surface disease and an altered pain-processing system.

Not reliably. This is another important limitation.

People with neuropathic corneal pain have been reported to show increased sensitivity, reduced sensitivity, and sometimes measurements that do not clearly distinguish them from other patients. TFOS DEWS III therefore notes that conventional corneal esthesiometry has limited usefulness for diagnosing neuropathic corneal pain, because sensitivity findings show poor diagnostic correlation.

So an abnormal sensitivity result may be informative. But there is no simple sensitivity number that proves neuropathic corneal pain.

When neuropathic corneal pain is suspected, clinicians may sometimes use a topical anesthetic challenge. A local anesthetic such as proparacaine temporarily blocks sensory input from the ocular surface. Then the patient is asked what happens to the pain.

In simplified terms: if the pain disappears almost completely, this suggests that much of the pain signal may be arising from the peripheral ocular surface and corneal nerves. If the pain remains largely unchanged, this raises suspicion that central pain processing may be playing a substantial role. If the pain improves but does not disappear, both peripheral and central mechanisms may be involved.

But this test is not a perfect switch that cleanly divides every patient into three categories. It is one piece of the clinical assessment.

Another test you may encounter is in vivo confocal microscopy, or IVCM. This is a specialized imaging technique that allows clinicians and researchers to visualize microscopic structures within the living cornea.

Unlike an ordinary slit-lamp examination, IVCM can show the sub-basal corneal nerve plexus. Researchers can examine features such as nerve density, nerve length, tortuosity, branching and other microscopic nerve features. It can also visualize inflammatory cells and other corneal structures.

It can show structural abnormalities of corneal nerves. Studies have found reduced sub-basal nerve density and other nerve changes in different forms of dry eye. IVCM can also provide valuable information when neurotrophic keratopathy is suspected.

But interpreting these images is not simple. The same type of structural nerve change can occur in more than one condition. For example, reduced nerve density has been reported in both dry eye and neuropathic corneal pain. So abnormal-looking nerves do not point to one specific diagnosis.

You may come across the term microneuroma, particularly in discussions about neuropathic corneal pain. Certain nerve structures seen on confocal microscopy have been proposed as possible markers of abnormal nerve regeneration or neuropathic pain. Some studies have found these structures more often in patients with neuropathic corneal pain.

That has made them scientifically interesting. But the evidence is not strong enough to treat a microneuroma as a definitive diagnostic marker. TFOS DEWS III specifically notes that the relationship between microneuromas, nerve beading and corneal nerve pain remains unclear and requires further research.

Not by itself. At present, neuropathic corneal pain remains largely a clinical and exclusionary diagnosis. The clinician considers the patient’s symptoms, the history, possible triggers and risk factors, the ocular-surface examination, the relationship between symptoms and signs, response to topical anesthetic in selected cases, and sometimes specialized nerve imaging.

TFOS DEWS III concludes that more research is needed before confocal microscopy can reliably differentiate the different neural drivers in an individual patient with dry eye.

No. This is another place where we need to separate a finding from an explanation.

Dry eye itself can affect corneal nerves. Corneal nerve abnormalities may contribute to dry-eye disease. Surgery, contact lenses, inflammation, systemic disease and other factors may affect both the ocular surface and the nerves. And neural dysfunction can sometimes persist even after the original ocular-surface trigger has changed.

So when nerve abnormalities are found, the relationship may be complex. The test does not necessarily tell us which problem came first.

No. A simple clinical sensitivity test examines only one aspect of a complicated sensory system. Someone may have a relatively normal response to a basic touch stimulus while still having abnormalities in other sensory pathways or pain processing. Likewise, normal sensitivity does not prove that corneal nerve structure is completely normal.

Because the mismatch itself can contain useful information.

If someone has significant ocular-surface damage but unexpectedly little sensation, reduced corneal sensitivity may be worth investigating. If someone has severe persistent pain with relatively limited conventional ocular-surface findings, altered pain processing may deserve consideration.

But neither mismatch gives us the diagnosis by itself. The correct response is to investigate further — not to assume the explanation.

It can help identify whether corneal sensation appears normal, reduced, absent, or sometimes increased. This can be particularly important when neurotrophic keratopathy is suspected or when the relationship between symptoms and ocular-surface findings seems unusual. Corneal sensitivity testing is especially important in clinical situations where reduced sensation could place the cornea at risk.

In vivo confocal microscopy can provide microscopic information about corneal nerve density, nerve morphology, branching and tortuosity, and other cellular structures within the cornea. It can reveal abnormalities that cannot be seen during a standard eye examination. But it does not turn nerve morphology into a simple diagnosis.

By themselves, corneal sensitivity and nerve tests cannot tell you:

• •whether you have dry eye disease
• •whether your symptoms are “real”
• •whether severe pain is definitely neuropathic
• •whether reduced symptoms mean your ocular surface is healthy
• •whether a nerve abnormality caused your dry eye
• •exactly where within the nervous system a pain problem originates

Those conclusions require the larger clinical picture.