Method Article

Corneal Sensitivity Testing Procedure for Ophthalmologic and Optometric Patients

DOI:

10.3791/66597

⸱

August 2nd, 2024

 , 
1Bascom Palmer Eye Institute, 2Miami Veterans Administration Medical Center, 3Northeastern State University Oklahoma College of Optometry

* These authors contributed equally

In This Article

Summary

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Corneal sensitivity testing provides insight into the health of the corneal nerves and helps diagnose ocular surface diseases. We present a concise protocol to qualitatively assess corneal sensitivity that can be readily used by eye care providers across clinical settings.

Abstract

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The cornea is the most densely innervated structure in the human body, making it one of the most sensitive tissues. Changes in corneal nerve sensitivity can be observed in several ocular surface diseases. Nerve sensitivity may be increased, as is often seen in patients with a neuropathic component to ocular pain, or decreased, as is seen in patients with neurotrophic keratitis. Corneal sensitivity testing involves assessing a patient's reaction to brief corneal stimulation, yields insight into the health of the corneal nerves, and provides diagnostic value for evaluating the health of the nerves and the interplay with the ocular surface. Currently, there is limited published guidance on how to conduct corneal sensitivity testing in a clinical setting. This article presents a protocol for testing corneal sensitivity using easy-to-use, low-cost materials that are readily accessible to eye care providers (either a cotton swab, a piece of dental floss, or a finely tapered tissue). This protocol allows for qualitative assessment of corneal sensitivity in which responses to corneal stimulation are rated from 0 (no response) to 3 (hypersensitive response). This test can be performed quickly (in approximately 30 s). Given its diagnostic value and accessibility, corneal sensitivity testing should be included as part of the standard eye examination for any patient undergoing an ocular surface examination.

Introduction

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The main functions of the cornea are to protect the contents of the eye and to focus light on the retina1. The cornea is the most densely innervated structure in the human body, with 7000 nerve receptors per mm2, and, consequently, is one of its most sensitive tissues2,3. The corneal nerves originate from the ophthalmic branch of the trigeminal nerves and play a key role in maintaining corneal homeostasis and integrity by mediating protective reflexes such as blinking and tear production, providing trophic support to the ocular surface, and stimulating wound healing by releasing neuromediators1,4,5,6.

Damage with subsequent dysfunction of corneal nerves (hypersensitivity or hyposensitivity) can contribute to ocular surface diseases3,7,8. In fact, neurosensory abnormalities have been recognized as potential contributors to dry eye disease symptoms and signs and were incorporated into the 2017 Tear Film and Ocular Surface definition of dry eye disease: "a multifactorial disease of the ocular surface characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiological roles6,9,10." Additionally, injury or dysfunction anywhere along the trigeminal nerve pathway can lead to neurotrophic keratitis (NK)11, a degenerative condition of the cornea; its stages include epithelial keratopathy, ulceration, and perforation, which may result in subsequent vision loss3.

Ocular surface pain can be categorized as nociceptive or neuropathic in origin8. In neuropathic ocular surface pain, nerves become hypersensitive because of the effects of a lesion or disease of the somatosensory pathway, which is often caused by maladaptive healing after trauma or surgery8. Abnormalities in corneal nerves have also been reported in other eye diseases, including glaucoma, thyroid eye disease, keratoconus, diabetic keratopathy, and Fuch's endothelial dystrophy12,13,14,15, and these findings are reproducible in animal models6,16,17. Notably, nerve abnormalities are not always identified as a component of eye disease, and a neurotrophic or neuropathic component to pain is often missed, underscoring the need for more diagnostic procedures to assess for the presence of nerve abnormalities18. Since ocular surface diseases may involve or induce corneal nerve dysfunction, a concise technique to assess corneal nerve function provides considerable diagnostic value.

Corneal sensitivity testing assesses a patient's reaction to brief corneal stimulation, providing functional insight into the status of the corneal nerves (absent, reduced, normal, or increased sensitivity)13,19. For example, in patients with neurotrophic keratitis, studies have found positive relationships between corneal sensitivity and parameters of corneal nerve innervation as assessed using in vivo confocal microscopy20,21, including corneal nerve fiber length (R2 = 0.2951, P = 0.0016)21. A positive correlation between corneal sensitivity and corneal nerve density has also been observed in herpes simplex keratitis (r = 0.55, P < 0.001) and dry eye disease (r = 0.644; P = 0.045)22,23. However, abnormal corneal innervation does not always correlate with aberrant corneal sensitivity13.

Alterations in corneal sensitivity have been reported in dry eye disease (including both Sjögren's dry eye disease and diabetes mellitus-related dry eye disease); ocular neuropathic pain; neurotrophic keratitis; Fuch's endothelial dystrophy; and ocular treatments for glaucoma including topical drops, laser trabeculoplasty, slow coagulation transscleral cyclophotocoagulation, and micropulse ciliary body ablation3,8,12,24,25,26,27,28,29. Additionally, short-term hypoesthesia can be secondary to refractive surgery30. Reduced or absent corneal sensitivity is a hallmark of neurotrophic keratitis and is key to its diagnosis3,11,31. Reduced corneal sensitivity often presents with low tear production and epithelial disruption, and increased sensitivity can signal ocular neuropathic pain, although neuropathic mechanisms can contribute to pain even in individuals with reduced or normal corneal sensitivity9,32.

Corneal sensitivity can be assessed using either qualitative or quantitative methods, although quantitative methods are primarily limited to research settings2,8,11,31,33,34,35. Quantitative assessments are made using either the Cochet-Bonnet esthesiometer or Belmonte's gas esthesiometer; a new non-contact esthesiometer, the Corneal Esthesiometer Brill, was recently registered by both the European Medicines Agency (EMA) and the US Food and Drug Administration (FDA) for corneal sensitivity testing36,37,38,39. The limitations associated with the Cochet-Bonnet and Belmonte's gas esthesiometers, including the cost and the challenge of maintaining sterility, render their use infrequent in clinical practice3. Qualitative methods can be easily performed by eye care providers or physician extenders, as they are low cost, readily available, and require little training and time8,33,37. Furthermore, there is insufficient published guidance on how to conduct corneal sensitivity testing and how corneal sensitivity scores as part of a clinical workup may inform diagnosis. Here, we detail a protocol for corneal sensitivity testing that is cost-effective, easy to comprehend, accessible, and can be readily adopted by eye care providers across clinical settings.

Protocol

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This study was approved by the institutional review board of the University of Miami, and the methods adhered to the tenets of the Declaration of Helsinki. All patients signed an informed consent form before participation. The reagents and the equipment used in the study are listed in the Table of Materials.

1. Pre-procedural preparation

  1. Perform corneal sensitivity testing on patients who are suspected of having ocular surface disease with alterations in the corneal nerves (Table 1)11,31,40,41,42.
  2. Ensure that testing is conducted before administering any eye drops, particularly anesthetic eye drops, as anesthetic eye drops decrease corneal sensitivity43 for a period (that varies by the type of eye drop used).
  3. Ensure that contact lenses are removed prior to testing.

2. Testing procedure

NOTE: Figure 1 depicts the overview of the testing procedure.

  1. Ask the patient to sit in a comfortable position on a chair.
  2. Use a cotton wisp (from a sterile cotton swab); unwaxed, unflavored dental floss; or a finely tapered tip of tissue as a stimulus for qualitative assessment of corneal sensitivity11,33 (Figure 1, step 2).
    NOTE: Keep the testing stimulus in each lane for easy access.
  3. Instruct the patient to fixate on a target, such as a point or letter of a test chart that is straight ahead44 (Figure 1, step 3).
  4. If the patient presents with one suspected affected eye, test the unaffected cornea first.
    NOTE: If a bilateral condition is suspected, first test the less affected cornea.
  5. Ask the patient to report when they feel the stimulus touch their cornea (Figure 1, step 4).
  6. Approach the patient's lateral side to reduce false positive responses caused by the patient reacting to the stimulus prior to contact44,45.
  7. Gently bring the stimulus in contact with the center of the cornea and observe the blink reflex, being careful not to touch the eyelids or lashes prior to touching the cornea (Figure 1, step 5).
  8. Record the corneal sensitivity of each eye using a 0-3 scale (Figure 1, step 6): Absent (0), reduced (1), normal (2), or increased/hypersensitive (3) response to stimulus.
  9. To perform a thorough examination, test all quadrants of the cornea3,11 in clockwise order using a new stimulus (Figure 1, step 5).
    NOTE: A single measurement from the center of the cornea is generally sufficient to assess corneal sensitivity. Some patients may not tolerate repeated testing and may request that the provider stop after the center of the cornea is evaluated. However, if atypical sensation is present elsewhere in the cornea, testing only central sensation may lead to an inaccurate diagnosis.
  10. To test the limbus, ask the patient to fixate on a target above the horizontal plane44.
  11. Repeat the corneal testing procedure in the contralateral eye using an unused cotton wisp, dental floss, or tissue.

3. Recording the corneal sensitivity

  1. When the patient does not react to the stimulus and endorses not feeling the stimulus, put the score as "Absent (0)".
  2. When the patient reports a minor sensation in their eye, which may or may not cause them to blink, assign the score as "Reduced (1)". If using a cotton swab or tissue paper, apply slightly more pressure to touch a greater diameter of the cornea before the patient responds.
  3. When the patient blinks when the stimulus touches the cornea or exhibits a mild reflex (e.g., gently pulls away) and indicates that they felt the stimulus, assign the score as "Normal (2)".
  4. When the patient exhibits a hypersensitive response, reports extreme discomfort or pain, strongly pulls away from the stimulus, or even jumps out of their chair, assign the score as "Increased/hypersensitive (3)".
    NOTE: Upon subsequent analysis, if the patient is found to have an unaffected eye, the response of the unaffected eye should then be used as a comparator when testing the affected eye.

4. Post-procedural steps

  1. Dispose of the cotton wisp, dental floss, or tissue on completion of the test.
  2. Ensure there are no infections or discomfort in the eyes.
  3. Return for follow-up examinations as per the ophthalmologist's instructions.

Results

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This article aims to provide a detailed protocol for corneal sensitivity testing that is cost-effective, easy to comprehend, accessible, and can be readily adopted by eye care providers across clinical settings. The absence of corneal sensitivity results, or with no response from the patient, indicates that they did not feel the stimulus and is scored as 0. Reduced sensitivity manifests as a minimal observable response from the patient (although they may sometimes blink), in which they report barely feeling the stimulus and is scored as 1. Normal sensitivity is indicated by the patient acknowledging feeling the stimulus, blinking, and/or showing a mild reflex to pull away, and is scored as 2. Increased sensitivity manifests as a dramatic response to minor stimulation (e.g., the patient jumps up and/or indicates that the stimulus was painful or uncomfortable) and is scored as 3. Assessment of corneal sensitivity could lead to further assessments and potential diagnoses8,12,13,31.

Representative results of patient case reports of clinical presentation, relevant medical history, corneal sensitivity testing, and subsequent diagnoses are presented in Table 2; patients presented with medical histories and symptoms associated with ocular surface disease. Overall, two patients were diagnosed with neurotrophic keratitis after examination and corneal sensitivity testing. Of note, these two patients also had a history of diabetes mellitus or herpes simplex infection, which are key etiologies for neurotrophic keratitis46. Both patients presented with irritation and redness and received a score of 0 or 1 in the affected eye. Patients with a corneal sensitivity score of 2 (normal) were subsequently diagnosed with dry eye disease; patients with a corneal sensitivity score of 3 (hypersensitivity) were either diagnosed with dry eye disease or neuropathic corneal pain. Representative images of NK stages are presented in Figure 2.

Corneal sensitivity testing diagram; steps and tools for evaluation; includes cotton wisp, sensitivity scale.
Figure 1: Overview of the corneal sensitivity testing procedure. Please click here to view a larger version of this figure.

Stages of neurotrophic keratopathy: photos showing progression from superficial to corneal ulcer.
Figure 2: Stages of neurotrophic keratitis (NK). Representative images of fluorescein stain revealing superficial punctate keratopathy (stage 1 NK), persistent epithelial defect (stage 2 NK), and corneal ulcer (stage 3 NK) in individual patients with NK. Please click here to view a larger version of this figure.

Ocular disease symptoms table; corneal sensitivity indicators; neurotrophic keratitis, dry eye.
Table 1: Factors supporting the decision to perform corneal sensitivity testing by ocular surface disease. Signs and symptoms supporting corneal sensitivity testing in patients with neurotrophic keratitis, neuropathic ocular pain, and dry eye disease. Please click here to download this Table.

Corneal sensitivity chart; clinical data table for eye disease analysis including demographics and symptoms.
Table 2: Summary of outcomes using qualitative corneal sensitivity testing. Representative results of patient case reports of clinical presentation, relevant medical history, corneal sensitivity testing, and subsequent diagnoses. Please click here to download this Table.

Discussion

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This article described a procedure for performing corneal sensitivity testing using a qualitative method that is effective in assessing absent, reduced, normal, or increased corneal sensitivity. This procedure can be readily performed by eye care providers or physician extenders across clinical settings as it is cost-effective and accessible, requires minimal training and preparation, and can be performed quickly (approximately 30 s)8,33,37. The results are consistent across patients based on the authors' experiences47, and the procedure can be easily incorporated into the eye care provider's workflow to support a clinical workup in patients with ocular diseases that affect the sensory corneal nerves.

As corneal sensitivity testing can be uncomfortable for the patient, it is important to test patients for whom corneal sensitivity testing would provide insight into the diagnosis of ocular surface diseases. We recommend performing corneal sensitivity testing in patients for whom there is a suspicion of ocular surface disease characterized by alterations in the corneal nerves, such as recalcitrant dry eye disease (including Sjögren's dry eye), ocular pain with neuropathic etiology, and neurotrophic keratitis. The suspicion of ocular surface disease is influenced by the patient's history and signs and symptoms8,31,48.

It is essential that corneal sensitivity testing be performed prior to administration of any eye drops, particularly anesthetic eye drops, to obtain accurate results11. Other factors to consider before assessing corneal sensitivity include that corneal sensitivity is greatest in the central cornea, that sensitivity decreases with older age, that the cornea is more sensitive in the periphery of elderly patients, and that corneal sensitivity is not affected by iris color49. Varying sensitivity with age could require baseline adjustment of normal sensitivity. An additional consideration during testing arises if the suspected disease is focally located (e.g., in the superior or inferior quadrant of the eye). In this case, the testing procedure should be modified to start in the affected eye in the area of suspected disease to minimize patient discomfort and to limit testing bias. Furthermore, patients may experience a transient loss in corneal sensitivity after some surgical procedures, such as cataract surgery or LASIK, and this should be considered when evaluating corneal sensitivity results30.

Corneal sensitivity scores inform the diagnosis of eye diseases characterized by alterations in the corneal nerves. For neurotrophic keratitis, reduced or absent corneal sensitivity is a key component of diagnosis11,31. Alterations in corneal sensitivity have been reported in dry eye disease and ocular neuropathic pain, among others3,8,12,24,25,26,27. However, corneal sensitivity alone is not sufficient to diagnose eye disease, and corneal sensitivity testing should be performed in conjunction with other tests to confirm the diagnosis. Notably, patients with a corneal sensitivity score within the normal range may receive a diagnosis of dry eye disease.

Limitations of the method include the qualitative nature of the assessment and the fact that corneal sensitivity testing indirectly infers the health of the corneal nerves. Alternative quantitative methods to assess corneal sensitivity are available; however, these methods have limitations, including high cost, challenges maintaining sterility, limited adoption into practice outside of a research setting, and increased time required to train personnel and conduct testing8,33,37. Quantitative methods are useful in a research setting for obtaining precise measurements of corneal sensitivity, but qualitative methods are more accessible and are generally sufficient in the clinic as part of a total clinical evaluation of a patient (e.g., medical history, clinical observations, etc.). Further studies are needed to assess the repeatability and reproducibility of qualitative corneal sensitivity testing methods and their level of agreement with quantitative methods.

In conclusion, this procedure for qualitative corneal sensitivity testing can be readily performed by eye care providers across clinical settings. Given its diagnostic value, corneal sensitivity testing should be included as part of the standard ocular surface examination for appropriate patients.

Disclosures

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AG is or has been a consultant for Alcon, Allergan, Dompé farmaceutici S.p.A., EyeCool Technology, Novaliq GmbH, Novartis, and Tarsus Pharmaceuticals. NL is or has been a consultant for Alcon, Allergan, Bausch & Lomb, Bruder, Dompé farmaceutici S.p.A., Ellex, EyePromise, Horizon, iCare, Innova Systems, LKC Pharma, Lumenis, NIDEK USA, Novartis, OCuSOFT, Orasis Pharmaceuticals, Reichert Technologies, RVL Pharmaceuticals, Sight Sciences, Sun Pharmaceuticals Industries Ltd, Tarsus Pharmaceuticals, Thea Pharma, Viatris, and Visionix USA.

Acknowledgements

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Writing and editorial assistance was provided under the direction of the authors by MedThink SciCom and funded by Dompé US, Inc.

Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Cotton-tipped applicatorsCardinal Health5823043Sterile plastic shaft applicators
Cotton-tipped applicatorsAMD Ritmed1191634Autoclave-ready nonsterile applicators
Cotton-tipped applicatorsDukai Corporation7166142Sterile wood shaft cotton-tipped applicators
KimwipesKimberly Clark Professional1030492Kimwipes tissue wipes
Unwaxed dental flossAcclean1552880Spool of dental floss
Unwaxed dental flossSunstar71270496 individual packages of floss
Unwaxed dental flossProcter & Gamble1090833Single-use packets of floss

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Corneal SensitivityCorneal Sensitivity TestingOcular Surface DiseaseQualitative AssessmentBlink ReflexNeurotrophic KeratitisNeuropathic Ocular PainEye ExaminationCotton Swab TestClinical Protocol

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