Raising the bar in corneal sensitivity assessment

Fig 1.a: the SDZ Electronics non-contact corneal aesthesiometer (credit: SJ Dean) and b: the NCCA in clinical use

The Tear Film & Ocular Surface Society’s Dry Eye Workshop III (TFOS DEWS III) report defines dry eye disease (DED) as a multifactorial condition in which patients experience ocular symptoms due to a loss of tear film and/or ocular surface homeostasis¹. Key contributing factors are recognised to include not only tear film instability and hyperosmolarity, ocular surface inflammation and damage, but also neurosensory abnormalities.

The cornea’s dense sensory innervation is central to ocular surface homeostasis. With approximately 7,000 nerve terminals per square millimetre, the cornea is 300–600 times more sensitive than skin².

Corneal sensory nerves can be broadly grouped into three receptor types: polymodal nociceptors (60–70%), which respond to thermal, chemical and mechanical stimuli; mechano-nociceptors (20–30%), which respond primarily to mechanical stimulation; and cold thermoreceptors (10–15%), which are sensitive to cooling and to changes in tear osmolarity². Together, the nerves regulate tear secretion and blinking, support epithelial integrity and wound healing and protect the ocular surface from environmental stress. Damage or dysfunction can contribute to both the onset and persistence of ocular surface disease¹.

Clinically, corneal nerve dysfunction tends to present in two main, sometimes overlapping, forms. With a reduction in nerve density, and reduced corneal sensitivity in a neurotrophic state, epithelial maintenance and wound healing are compromised. In more advanced cases, this may progress to neurotrophic keratopathy, characterised by epithelial defects or even ulceration, despite relatively few symptoms.

At the other end of the spectrum, abnormal nerve regeneration and altered sensory signalling can lead to nociplastic or neuropathic corneal pain. Affected patients typically report burning, dryness, or ocular pain (allodynia) that appears to the clinician to be disproportionate to the clinical examination findings. Accordingly, DED and related ocular surface disorders can exhibit a range of clinical presentations. This perspective is reflected in the TFOS DEWS III Digest, which reports that both reduced and increased corneal sensitivity can be associated with DED and that variability exists even among patients diagnosed with the same dry eye subtype¹.

Optometry Comment

Dr Ally Xue

Dr Ally Xue is a therapeutic optometrist and postdoctoral research fellow at the University of Auckland’s Ocular Surface Laboratory, with a special interest in light-based therapies for dry eye.

Professor Jennifer Craig

Professor Jennifer P. Craig heads the research team at the Ocular Surface Laboratory within the Department of Ophthalmology at the University of Auckland.

Optometry Comment

Let’s get phygital

This article explores 'phygital' technology, combining physical and digital elements, and its application in optometry, from virtual try-on eyewear to

25 Jun 2026

Optometry Comment

This makes assessment of corneal nerve structure and function increasingly meaningful as an indicator of ocular surface health. Advances in imaging and sensory testing now offer researchers standardised and objective ways to evaluate these parameters. Where available, structural assessment of corneal nerves can be performed using in vivo confocal microscopy (IVCM), which offers high-resolution images of the subbasal nerve plexus, allowing detailed assessment of nerve density, tortuosity, morphology and alterations associated with ocular surface disease¹. Recent advances in IVCM methodology, termed functional-IVCM, can even allow dynamic immune-cell tracking, which is helping shed more light on the pathophysiology of ocular surface disease³.

Clinically, functional assessment of corneal nerves is commonly performed using aesthesiometry, which assesses corneal sensitivity thresholds, either by contact or non‑contact methods. The Cochet-Bonnet aesthesiometer is considered the standard contact device, using a retractable fine nylon filament of adjustable length pressed against the cornea to elicit a sensory response, with the requirement of a shorter filament to elicit a response corresponding to reduced corneal sensitivity⁴. The KeraSense is a more recent disposable option demonstrating good agreement with the Cochet-Bonnet⁵. While these methods are simple, portable and inexpensive, they primarily measure mechanical sensitivity, offer only a limited stimulus range and require direct contact with the ocular surface, which may influence results and may be unsuitable in certain disease states.

Contact-free assessment

Non‑contact corneal aesthesiometers (NCCAs) address these limitations by delivering controlled pulses of air (Murphy’s NCCA⁶), gases (Belmonte gas aesthesiometer⁷), or liquid to the cornea (Swiss Liquid Jet⁸), to induce mechanical, thermal and chemical stimulation without direct physical contact with a solid filament. While early NCCAs were largely confined to research settings, commercial devices have begun to be available for use in routine ophthalmic practice. The Brill esthesiometer, regularly incorporated into assessments at the Ocular Surface Laboratory (OSL) at the University of Auckland, is one such portable non‑contact air‑puff device that offers rapid, standardised measurements of ocular surface sensitivity⁹.

With growing evidence suggesting that tear hyperosmolarity may preferentially activate cold thermoreceptors in the cornea via the TRPM8 pathway, newer technologies have begun to address this mechanism more directly. Building on an earlier validated model, the SDZ Electronics enhanced NCCA – designed by New Zealand ophthalmologist Dr Simon Dean – incorporates an integrated cooling system, allowing air stimuli to be delivered at either room or a reduced temperature (Fig 1). This may enable more targeted assessment of cold‑sensing corneal nerves that are thought to play a key role in the experience of dry eye symptoms. A recent study on room-temperature measurements from the OSL confirmed this enhanced device provides reliable corneal sensitivity measurements and shows acceptable agreement with the previously validated NCCA, supporting its use in clinical settings¹⁰.

The recent technological advances in corneal aesthesiometry highlight the evolving understanding of the pathogenesis of DED, in which neurosensory dysfunction is now recognised as a contributing factor in the disruption of ocular surface homeostasis. However, there’s still much to learn about the neuroinflammatory nature of dry eye, and the OSL is excited to work within this ever-evolving field by applying emerging technologies in our research to seek to address important knowledge gaps.

In clinical practice, as more accessible devices enter the commercial market, we can expect to see a move away from the cotton wisp corneal sensation check towards more precise corneal nerve assessment, since it is through facilitating accurate diagnosis that treatment selection can be optimised and patients offered advanced dry-eye care with a more targeted and personalised approach.

References

Stapleton F, Argüeso P, Asbell P, et al. TFOS DEWS III: Digest. Am J Ophthalmol. 2025;279:451-553.
Yang AY, Chow J, Liu J. Corneal innervation and sensation: The eye and beyond. Yale J Biol Med. 2018;91:13-21.
Downie LE, Zhang X, Wu M, et al. Redefining the human corneal immune compartment using dynamic intravital imaging. Proc Natl Acad Sci U S A. 2023 Aug;120:e2217795120.
Cochet P, Bonnet R. Corneal esthesiometry. Performance and practical importance. Bull Soc Ophtalmol Fr. 1961;6:541–50.
Giovannetti F, Sacchetti M, Marenco M, et al. New disposable esthesiometer (KeraSense) to improve diagnosis and management of neurotrophic keratitis. Ocul Surf. 2024;32:192-7.
Murphy PJ, Lawrenson JG, Patel S, et al. Reliability of the non-contact corneal aesthesiometer and its comparison with the Cochet-Bonnet aesthesiometer. Ophthalmic Physiol Opt. 1998;18:532–9.
Tesón M, Calonge M, Fernández I, et al. Characterization by Belmonte’s Gas Esthesiometer of Mechanical, Chemical, and Thermal Corneal Sensitivity Thresholds in a Normal Population. Invest Ophthalmol Vis Sci. 2012;53:3154–3160.
Nosch DS, Käser E, Bracher T, et al. Clinical application of the Swiss Liquid Jet Aesthesiometer for corneal sensitivity measurement. Clin Exp Optom. 2024;107:14–22.
Merayo-Lloves J, Gomez Martin C, Lozano-Sanroma J, et al. Assessment and safety of the new esthesiometer BRILL: Comparison with the Cochet-Bonnet Esthesiometer. Eur J Ophthalmol. 2024;34:1036–45.
Xue AL, Britten-Jones AC, Zhuang D, et al. Validation and Comparative Analysis of a Contemporary Non-Contact Corneal Aesthesiometer. J Clin Med. 2026;15(8):3145.