Corneal Sensitivity Test

Key Points at a Glance

Key Points of This Test

• Corneal esthesiometry evaluates the function of the first branch of the trigeminal nerve (ophthalmic nerve).
• The most commonly used qualitative method in clinical practice is the cotton swab method, and the quantitative method is the Cochet-Bonnet esthesiometer.
• Normal values for the Cochet-Bonnet type are 50–60 mm; 45 mm or less indicates decreased corneal sensitivity.
• Do not use topical anesthetic eye drops before the test.
• Decreased corneal sensitivity is an essential finding for the diagnosis of neurotrophic keratitis.
• The cornea has the highest density of sensory nerves in the body, estimated to be 300–400 times that of the skin.

1. What is Corneal Esthesiometry?

Corneal esthesiometry is a test that measures corneal sensitivity (mainly tactile sensation). It evaluates the function of the ophthalmic nerve, the first branch of the fifth cranial nerve (trigeminal nerve). The cornea has the highest density of sensory nerves in the body, with a density 300–400 times that of the skin and 40 times that of the oral mucosa.

Corneal sensitivity plays an important role in triggering tear secretion, maintaining corneal epithelial homeostasis, and signaling pathological conditions of the cornea. When corneal sensitivity decreases, protective reflexes are lost, and corneal epithelial damage tends to progress.

History

The first esthesiometer was reported by von Frey in 1894. It used horse hairs of different lengths. In 1932, Francheschetti improved this model. In 1956, Boberg-Ans reported a device using a single nylon filament of constant diameter but variable length.

Cochet and Bonnet further improved this model and developed an esthesiometer using a nylon filament with a diameter of 0.12 mm. It is currently the most widely used quantitative esthesiometer.

3. Causes and Differential Diagnosis of Decreased Corneal Sensitivity

Decreased corneal sensitivity (corneal hypoesthesia) can result from any cause that damages the corneal nerves or the trigeminal nerve.

Category	Typical Causes
Infectious diseases	Corneal herpes (HSV/VZV)
Systemic diseases	Diabetes mellitus, multiple sclerosis
Post-surgery	Corneal transplant, LASIK, cataract surgery
Contact lenses	Hypoesthesia due to long-term wear
Drug-induced	Beta-blockers, NSAIDs, diclofenac
Intracranial lesions	Acoustic neuroma, aneurysm
Congenital	Congenital corneal anesthesia

Herpes simplex keratitis is one of the most common causes of decreased corneal sensation. In diabetes, corneal sensation decreases in correlation with the severity of retinopathy, and it is known that the morphology of corneal nerves changes in correlation with renal function.

NSAID eye drops can also cause decreased corneal sensation and should not be overlooked.

Raj et al. (2022) reported a case of neurotrophic keratitis in a patient after corneal transplantation who continued to use bromfenac eye drops incorrectly ²⁾. The neurotrophic effect due to the analgesic action of NSAIDs and activation of matrix metalloproteinases are speculated as mechanisms ²⁾.

Congenital corneal anesthesia is an extremely rare condition.

Gelzinis et al. (2022) reported a pediatric case with congenital corneal anesthesia due to bilateral trigeminal nerve hypoplasia ⁵⁾. Sensory function assessment using a Cochet-Bonnet esthesiometer and morphological evaluation of the trigeminal nerve by MRI were useful for diagnosis ⁵⁾.

Signs that should raise suspicion of decreased corneal sensation

• If there is corneal epithelial damage but the patient does not complain of pain, suspect decreased corneal sensation.
• For unilateral persistent epithelial defects or corneal erosions that do not respond to treatment, perform a corneal sensation test.
• Decreased blink rate and reduced tear secretion are also signs of decreased corneal sensation.

Q What happens when corneal sensation decreases?

A

When corneal sensation decreases, the protective reflexes of the eye (blinking, tear secretion) are weakened, making corneal epithelial damage more likely to progress. This is called neurotrophic keratitis. Early stages show punctate keratopathy, progression leads to persistent epithelial defects, and severe cases can result in corneal ulceration or perforation. Because pain is less noticeable, detection is often delayed, making regular corneal sensation testing important.

4. Classification and procedures of examination methods

Corneal sensation testing includes qualitative methods and quantitative methods. Do not use topical anesthetic eye drops before testing. Always check whether anesthetic eye drops have been used in other tests beforehand.

Qualitative method (cotton wisp test)

Equipment: A thin wisp pulled from the tip of a cotton swab.

Method: Compare sensation in all 4 quadrants of both eyes

Recording: Three levels: normal, reduced, absent

Advantages: Easily available and simple

Quantitative method (Cochet-Bonnet type)

Instrument: Retractable nylon monofilament

Method: Shorten from 60 mm in 5 mm steps to measure threshold

Recording: Minimum detectable length (mm)

Advantages: High reproducibility and quantitative assessment

Qualitative testing methods

This is the most commonly used method in clinical practice. A wisp of cotton from a cotton-tipped applicator is used to compare corneal sensation between both eyes. Approach from the patient’s side and test all 4 quadrants of the cornea. Record sensation at each site as “normal,” “reduced,” or “absent.” Dental floss can also be used.

Epitropoulos et al. (2022) reported a retrospective case series of 4 patients with stage 1 neurotrophic keratitis, in which qualitative assessment using dental tape recorded corneal sensation in the central and peripheral regions³⁾.

If a Cochet-Bonnet esthesiometer is not available, a cotton ball can be moistened, squeezed well, twisted into a thin wisp, and touched to the cornea to obtain information about reduced sensation.

Raj et al. (2022) assessed corneal sensation using a cotton-tipped applicator during the COVID-19 pandemic when the Cochet-Bonnet esthesiometer was unavailable²⁾.

Cochet-Bonnet corneal esthesiometer

This is the most widely used device for quantitative assessment. It consists of a 0.12 mm diameter nylon filament that can be extended or retracted via a dial, changing the pressure applied to the cornea. Shorter lengths produce higher pressure. The Cochet-Bonnet score ranges from 5 mm to 60 mm, with lower values indicating more severe hypoesthesia⁶⁾.

Procedure:

1. Set the nylon filament to the longest length (60 mm).
2. Check that the tip is not bent, and disinfect it by wiping with an alcohol swab.
3. Instruct the patient to fixate on a distant point. It is easier to perform the test with the chin rest of a slit lamp microscope.
4. Touch the tip of the nylon filament to the central cornea as perpendicularly as possible to the corneal surface.
5. If no contact is felt, shorten the filament by 5 mm and repeat the same motion until the patient feels it.
6. Usually, the average of three measurements is taken as the corneal sensitivity value.
7. After the test, disinfect the tip of the nylon filament with an alcohol swab.

• Normal value: 50–60 mm
• Decreased sensitivity: 45 mm or less

Corneal sensitivity is highest in the central area and decreases toward the periphery, so comparisons should be made at the same location.

Kymionis et al. (2025) confirmed a marked decrease in corneal sensitivity using the Cochet-Bonnet esthesiometer in the diagnosis of herpes simplex virus neurotrophic keratitis, and used it to confirm the clinical diagnosis¹⁾.

Other Quantitative Methods

For research purposes or complex cases, the following methods may be used.

• Non-contact air puff method: Evaluates corneal sensitivity non-invasively using an air jet. Unlike the Cochet-Bonnet type, it does not cause measurement errors due to contact.
• Chemical stimulation (capsaicin): Evaluates corneal sensation to chemical stimuli
• Thermal stimulation (carbon dioxide laser): Evaluates corneal sensation to thermal stimuli

Corneal nerve assessment using confocal microscopy

In vivo confocal microscopy is a non-invasive method to observe the corneal subepithelial nerve plexus. It is useful as a morphological assessment that complements quantitative evaluation of corneal sensation.

Gouvea et al. (2021) confirmed a decrease in corneal subepithelial nerve plexus density using confocal microscopy in a case of neurotrophic keratitis associated with lattice corneal dystrophy, and observed improvement in nerve plexus density after cenegermin treatment⁴⁾.

Q Is the corneal sensitivity test painful?

A

In the Cochet-Bonnet esthesiometer test, the tip of the nylon thread only lightly touches the cornea, so there is no strong pain. However, since the test must be performed without anesthesia, you may feel a slight discomfort at the moment of contact. The test itself takes only a few minutes.

6. Sensory innervation of the cornea

The sensory innervation of the cornea is provided by the nasociliary nerve, a branch of the first division of the trigeminal nerve (ophthalmic nerve). The long ciliary nerves branch from the nasociliary nerve, penetrate the sclera, and reach the cornea.

Corneal nerves first run as myelinated fibers within the stroma, lose their myelin sheath when penetrating Bowman’s layer, and become unmyelinated fibers. They form a rich nerve plexus (subepithelial nerve plexus) beneath the corneal epithelium.

Corneal sensation has the following characteristics.

• It is most sensitive in the central area and decreases toward the periphery. However, in elderly individuals, the periphery may be more sensitive in some cases.
• It decreases with age.
• Decreased sensation has been reported in type 1 and type 2 diabetes.
• It decreases during pregnancy.
• Not affected by iris color
• The temporal limbus is more sensitive than the inferior limbus

Corneal nerves have two functions: sensory function and trophic function. The sensory function transmits pain and foreign body sensation, inducing blink reflex and tear secretion. The trophic function releases neurotransmitters such as nerve growth factor (NGF), substance P, and calcitonin gene-related peptide, maintaining corneal epithelial proliferation, migration, adhesion, and differentiation.

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7. Latest Research and Future Perspectives (Investigational Reports)

For Patients: Please Read Carefully

The following content is currently under investigation or in clinical trials and is not standard treatment available at general hospitals. It is reference information for professionals regarding future medical developments.

Cenegermin and Corneal Sensory Recovery

Cenegermin, a recombinant human nerve growth factor (rhNGF) ophthalmic solution, has received FDA approval for neurotrophic keratitis. In addition to promoting corneal epithelial healing, recovery of corneal sensation has also been reported.

Epitropoulos et al. (2022) administered cenegermin (20 µg/mL, 6 times daily for 8 weeks) to 4 cases of stage 1 neurotrophic keratitis, and all cases showed improvement in corneal sensation and reduction of punctate epithelial erosions³⁾.

Gelzinis et al. (2022) used cenegermin for a corneal ulcer in a child with congenital corneal anesthesia, achieving corneal epithelial healing⁵⁾. However, recovery of corneal sensation was limited in cases of severe congenital sensory impairment⁵⁾.

Corneal Neurotization

Corneal neurotization is a surgical approach that transfers healthy sensory nerves (such as the supraorbital, supratrochlear, and infraorbital nerves) to the cornea to restore corneal sensation⁶⁾. There are two surgical techniques: direct method (direct transfer of donor nerve) and indirect method (transfer via autologous nerve graft)⁶⁾. Recovery of corneal sensation is achieved within a few months postoperatively, and stabilization of the corneal epithelium and improvement in long-term prognosis have been reported⁶⁾.

Anterior Lens Capsule Transplantation

Kymionis et al. (2025) reported a case of persistent epithelial defect due to herpes simplex virus neurotrophic keratitis treated with allogeneic anterior lens capsule transplantation (ALCT), achieving complete epithelialization within 2 weeks¹⁾. The anterior capsule is obtainable during cataract surgery and has the advantage of superior transparency compared to amniotic membrane¹⁾.

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8. References

1. Kymionis GD, Vakalopoulos DG, Chatzea MS, Togka KA, Tsagkogiannis VA, Petrou PC. Allogeneic anterior lens capsule transplantation (ALCT) for the management of 単純ヘルペスウイルス neurotrophic keratitis. Am J Ophthalmol Case Rep. 2025;38:102292.

2. Raj N, Panigrahi A, Alam M, Gupta N. Bromfenac-induced neurotrophic keratitis in a corneal graft. BMJ Case Rep. 2022;15:e249400.

3. Epitropoulos AT, Weiss JL. Topical human recombinant nerve growth factor for stage 1 Neurotrophic Keratitis: Retrospective case series of cenegermin treatment. Am J Ophthalmol Case Rep. 2022;27:101649.

4. Gouvea L, Penatti R, Rocha KM. Neurotrophic keratitis after penetrating keratoplasty for lattice dystrophy. Am J Ophthalmol Case Rep. 2021;22:101058.

5. Gelzinis A, Simonaviciute D, Krucaite A, Buzzonetti L, Dollfus H, Zemaitiene R. Neurotrophic Keratitis Due to Congenital Corneal Anesthesia with Deafness, Hypotonia, Intellectual Disability, Face Abnormality and Metabolic Disorder: A New Syndrome? Medicina. 2022;58:657.

6. Rosenblatt MI, et al. Corneal neurotization for the treatment of neurotrophic keratopathy. Curr Opin Ophthalmol. 2025;36:294-301.