Nicola Pritchard is a research associate and optometrist in the Anterior Eye Laboratory at the Institute of Health and Biomedical Innovation and the School of Optometry at the Queensland University of Technology, Australia.
New technologies for examination of the anterior eye in contact lens practice don’t appear to have taken a huge leap in the past decade however there a several novel adaptations of existing technology worthy of note. In other areas of health we have self-diagnosis via smartphone or other gadgets adapted as medical devices. In practice and research in vitro and in vivo new adaptive technologies have expanded our capabilities in assessing the anterior eye, in particular corneal and conjunctival confocal microscopy.
Topography and tomography
Corneal shape and biomechanics are evaluated using slit-scanning and Scheimpflug imaging technologies, and when combined with a topographer has the advantage beyond a keratometer of not just measuring the anterior surface curvature but also measuring the elevation of all anterior segment structures and corneal thickness.
Anterior segment optical coherence tomography (OCT) utilises the Scheimflug imaging technology, ultrasound and optical coherence tomography combined and Placido disk technologies to evaluate the cornea and anterior segment by cross-sectional sections of anterior segment used in pre- and post-surgical evaluation (e.g. Visante® OCT from Zeiss). Corneal aberration evaluation assessed for custom ablation (e.g Keratron™ from Optikon) has been reported to have a high level of reproducibility1. Measuring corneal hysteresis (e.g the Ocular Response Analyzer® from Reichert) hasn’t yet found a place in our consulting rooms and research labs, but does appear to be an effective yet expensive way of monitoring IOP in glaucoma patients2.
Ocular wavefront sensors are now used clinically to assess aberrations and project vision loss caused by degradation of the pre-corneal tear film. Traditional tests such as tear film break-up time can also be estimated using wavefront sensors. OCT can provide high resolution (2-10 um) images of the anterior segment and also accurately measure tear film thickness over the cornea and at the upper and lower tear menisci3.
Tear film meniscometry
A relatively new adaption of an established technique with a variety of potential applications is measuring tear film meniscus height with video-meniscometry. Yokoi and colleagues4 have suggested the non-invasive technique of mensicometry will be useful in determining tear turnover, as an indication of dysfunction of the tear meniscus and for punctual plugs. Bandlitz et al.5 demonstrated that the portable digital meniscometer was equivalent to the more expensive method of measuring tear meniscus volume than OCT.
Elipsometry & meibography
In contact lens and routine ophthalmic practice the assessment of the tear film lipid layer and meibiomian gland function is integral in dry eye diagnosis. Elipsometry, a technique used to measure thickness and refractive index of the lipid layer, can achieve measures at a resolution of approximately 100 nm using a modified wavefront sensor combined with placido disc6. This appears to be an advance on qualitative interferometry (colour assessment of the tear film by specular reflection) with cool light illumination systems such as the TearScope® by Keelor or the LipiView® Ocular Surface Interferometer by TearScience®.
A simple, handheld instrument has been invented to evaluate meibomian gland secretions during routine eye examination by applying consistent pressure to the outer skin of the lower eyelid. The Meibomian Gland Evaluator (MGE), attributed to Korb and Blackie7, is used to express liquid from the meibomian gland orifices (visualized through old technology – a slit lamp biomicroscope), the presence of which indicates the meibomian gland is not obstructed. Following diagnosis, a new treatment applying an eyelid warming device (Blephasteam® from Théa) has been shown to be safe and effective at melting meibum. Additionally, application of this technique also appeared to reduce redness in those without meibomian gland dysfunction8.
Laser meibometry, using ultra-high resolution OCT to produce morphological images of the tarsal area, may also have applications in clinical diagnosis of lid pathologies9.
Non-contact corneal aesthesiometry
Corneal sensitivity measurement has progressed from a contact to a non-contact technique – the Cochet-Bonnet aesthesiometer and the cotton wisp test are relatively imprecise compared to non-contact corneal aesthesiometry10, 11. Although non-contact aesthesiometry is well validated and used extensively in a research setting10-18, we must currently build our own devices. Further, until these devices become automated and provide a clinical measure instantaneously, their value is limited in a clinical setting.
Ocular temperature has been measured using infrared thermometers for several years and a number of potential clinical ocular applications have been demonstrated, as well as a potential marker of carotid artery stenosis19. Anterior eye applications include the evaluation of tear film disorders20, 21, inflammatory conditions of the anterior eye, such as anterior uveitis22, hordeolum, and acanthamoeba keratitis23, scleritis and meibomian gland dysfunction22, neurological disorders, such as Horner’s syndrome in which anhydrosis is demonstrated on the forehead of the patient23, and thermal consequences of photorefractive keratectomy24, 25 (Figure 1).
In vivo confocal microscopy
Examination of the cornea has probably seen the most notable advances in technology as in vivo corneal confocal microscopy made its way from research into the clinical realm. Several research labs around the world are now using laser-scanning corneal confocal microscopy (such as the HRT3 with Cornea Rostock Module from Heidelberg), which represents a technological advance on the older, tandem- and slit-scanning devices.
As postulated by Chikama and colleagues in 200826, this technology is the closest we have to an ‘in vivo biopsy”. For the first time the anterior layers of cells can be appreciated at approximately 400-700x magnification with a quick, non-invasive technique, allowing new insights into form and function of the anterior ocular tissues.
At least three epithelial cells layers can be differentiated using confocal microscopy, as well as sub-basal and stromal nerves, stromal keratocytes and endothelium. The resolution of the instruments, however, does not permit the three membranes (Bowmans, Duas, Descemet’s) to be easily appreciated. Quantification of the sub-layer thicknesses may eventuate with confocal microscopy and 3-D imaging27.
Observation of pathology at the level of the membranes using confocal microscopy is a significant advancement to that afforded by the slit-lamp, and has improved our diagnostic capabilities in clinical practice. Confocal microscopy allows appreciation of acanthamoeba cysts28, y-sutures and pigment epithelial cells on anterior lens surface29 and endothelial dystrophy30. In our lab we’ve noted Hudson-Stahli lines appear as highly reflective material at the level of basal cell layer and Bowman’s layer (Figure 2) and dystrophic or degenerative areas in otherwise healthy cornea (Figure 3).
Evaluation of the inflammatory status of the eye has included documentation of the number and type of presumed dendritic cells (possibly Langerhans cells) in the cornea (Figure 4) and conjunctiva (Figure 5)31. Increase in the number of these cells relative to healthy, quiet eye tissue is evident in contact lens wearers32, 33 and other inflammatory conditions34, although their exact morphological identification in vivo has not been confirmed.
The corneal sub-basal nerve plexus has been extensively explored in healthy and pathologic ocular tissue. New characteristics of this extensive plexus have been revealed by mapping together images captured by confocal microscopy to appreciate a large extent of the sub-basal nerves35, 36, and more recently automatic wide-field mapping techniques have significantly reduced the time taken to perform such tasks37, 38.
Corneal nerve morphology has been documented in a reasonable number of healthy eyes36, 39 as well as, for example, the corneal nerve deficits associated with diabetic neuropathy40, vernal keratoconjunctivitis41, herpes keratitis42, keratoconus43-45 and corneal transplantation46.
Stromal keratocytes imaged by confocal microscopy are easily quantifiable, and recently automated detection and cell densities from ultra-high resolution optical coherence tomograms have been performed47. Quantification of stromal nerves using contact29 and non-contact slit, tandem or laser scanning devices48, 49 has also been reported with a limited degree of agreement, suggesting that if the technique is to be used routinely, it requires some further validation.
The ‘in vivo biopsy’ of the conjunctiva is an exciting adventure, and the structural detail is far from the traditional text description of the tarsal papillae, crypts and cellular features50, 51. Figure 6 shows presumed goblet cells in the nasal bulbar conjunctiva and the lid margin or lid-wiper region examined by confocal microscopy may inform new perspectives in dry eye (Figure 7).
Using confocal microscopy to observe cells in conjunctival vessels has been used as an indicator of sub-clinical inflammation – the less the cells roll (or the more they stick), the more inflamed the eye is52, and less conjunctival blood flow was observed in contact lens wearers compared to non-contact lens wearers when measured using a modified Heidelberg Retinal Flowmeter53.
A video from our lab shows this phenomenon of rolling and sticking cells (Figure 8).
Tear film thickness and integrity has been explored by in vivo confocal microscopy. The Heidelberg instrument with Tomocap readily reveals dry spots29. More recently using a non-contact objective attached to the Heidelberg instrument, we were unable to image the anterior cellular layers of the cornea but dynamic tear film evaluation was possible49, shown in Figure 9.
In the future, the electronic contact lens promises bionic capabilities for everyone. There are already apps to assist reading urine strips, monitor heart rate and blood glucose level. A group in London is developing a portable eye examination kit to assist in performing visual acuity and contrast sensitivity, colour vision, visual field, lens and retinal imaging in remote populations.
A great deal of cooperation between funders, biotech, clinicians and researchers is required. Bringing established technologies to developing countries will hopefully help to reduce preventable blindness in prone populations. Reducing invasiveness of procedures will also pose a challenge. For example, the equivalence of contact vs. non-contact instrumentation in diagnosis of corneal and conjunctival anomalies and pathology should be an attainable goal, and technological cooperation is necessary to make this happen.
No app currently exists for monitoring the contact lens wearing eye, however relatively sophisticated technology and the ability to monitor parameters such as cell counts and tissue thickness is becoming routine and these new technologies tend to redefine standard of care. Automation is a critical factor if these tools are to be useful in situations where information is needed immediately, such as in clinical practice. It is hoped that automation tasks will push forward rapidly in the near future, enhancing our technological capabilities, hence patients’ well-being in all parts of the globe.
1. Prakash G, et al. Reliability and reproducibility of assessment of corneal epithelial thickness by fourier domain optical coherence tomography. Invest Ophthalmol Vis Sci 2012;53(6): 2580-5.
2. Mollan SP, et al. Accuracy of Goldmann, ocular response analyser, Pascal and TonoPen XL tonometry in keratoconic and normal eyes. Br J Ophthalmol 2008;92(12): 1661-5.
3. Koh S, et al. Simultaneous measurement of tear film dynamics using wavefront sensor and optical coherence tomography. Invest Ophthalmol Vis Sci 2010;51(7): 3441-8.
4. Yokoi N, Komuro A. Non-invasive methods of assessing the tear film. Exp Eye Res 2004;78(3): 399-407.
5. Bandlitz S, et al. Comparison of a new portable digital meniscometer and optical coherence tomography in tear meniscus radius measurement. Acta Ophthalmol 2013; epub Oct. 7, 2013.
6. Kottaiyan R, et al. Integrated multimodal metrology for objective and noninvasive tear evaluation. Ocul Surf 2012;10(1): 43-50.
7. Korb DR, Blackie CA. Case report: a successful LipiFlow treatment of a single case of meibomian gland dysfunction and dropout. Eye Contact Lens 2013;39(3): e1-3.
8. Purslow C. Evaluation of the ocular tolerance of a novel eyelid-warming device used for meibomian gland dysfunction. Cont Lens Anterior Eye 2013;36(5): 226-31.
9. Bizheva K, et al. In vivo volumetric imaging of the human upper eyelid with ultrahigh-resolution optical coherence tomography. J Biomed Opt 2010;15(4): 040508.
10. Murphy PJ, et al. Reliability of the non-contact corneal aesthesiometer and its comparison with the Cochet-Bonnet aesthesiometer. Ophthalmic Physiol Opt 1998;18(6): 532-9.
11. Teson M, 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(6): 3154-60.
12. Golebiowski B, et al. Understanding the stimulus of an air-jet aesthesiometer: computerised modelling and subjective interpretation. Ophthalmic Physiol Opt 2013;33(2): 104-13.
13. Pritchard N, et al. Corneal sensitivity is related to established measures of diabetic peripheral neuropathy. Clin Exp Optom 2012;95(3): 355-61.
14. Chen J, Feng Y, Simpson TL. Human corneal adaptation to mechanical, cooling, and chemical stimuli. Invest Ophthalmol Vis Sci 2010;51(2): 876-81.
15. Situ P, Simpson TL. Interaction of corneal nociceptive stimulation and lacrimal secretion. Invest Ophthalmol Vis Sci 2010;51(11): 5640-5.
16. Situ P, Simpson TL, Fonn D. Eccentric variation of corneal sensitivity to pneumatic stimulation at different temperatures and with CO2. Exp Eye Res 2007;85(3): 400-5.
17. Acosta MC, et al. Sensations evoked by selective mechanical, chemical, and thermal stimulation of the conjunctiva and cornea. Invest Ophthalmol Vis Sci 2001;42(9): 2063-7.
18. Belmonte C, et al. Measurement of corneal sensitivity to mechanical and chemical stimulation with a CO2 esthesiometer. Invest Ophthalmol Vis Sci 1999;40(2): 513-9.
19. Morgan PB, et al. Ocular temperature in carotid artery stenosis. Optom Vis Sci 1999;76(12): 850-4.
20. Craig JP, et al. The role of tear physiology in ocular surface temperature. Eye (Lond) 2000;14(Pt 4): 635-41.
21. Morgan PB, Tullo AB, Efron N. Infrared thermography of the tear film in dry eye. Eye (Lond) 1995;9 (Pt 5): 615-8.
22. Kawali A. Thermography in ocular inflammation. Indian J Rad Imag 2013;23(3): 281-283.
23. Morgan PB, et al. Potential applications of ocular thermography. Optom Vis Sci 1993;70(7): 568-76.
24. Betney S, et al. Corneal temperature changes during photorefractive keratectomy. Cornea 1997;16(2): 158-61.
25. Maldonado-Codina C, Morgan PB, Efron N. Thermal consequences of photorefractive keratectomy. Cornea 2001;20(5): 509-15.
26. Chikama T, et al. In vivo biopsy by laser confocal microscopy for evaluation of traumatic recurrent corneal erosion. Mol Vis 2008;14: 2333-9.
27. Petroll WM, et al. Quantitative 3-dimensional corneal imaging in vivo using a modified HRT-RCM confocal microscope. Cornea 2013;32(4): e36-43.
28. Parmar DN, et al. Tandem scanning confocal corneal microscopy in the diagnosis of suspected acanthamoeba keratitis. Ophthalmology 2006;113(4): 538-47.
29. Guthoff R, Baudouin C, Stave J. Atlas of confocal laser scanning in vivo microscopy in ophthalmology principles and applications in diagnostic and therapeutic ophthalmology. 2006, Berlin; New York: Springer. xii, 200 p.
30. Patel SV, McLarenJW. In vivo confocal microscopy of Fuchs endothelial dystrophy before and after endothelial keratoplasty. JAMA Ophthalmol 2013;131(5): 611-8.
31. Zhivov A, et al. In vivo confocal microscopic evaluation of Langerhans cell density and distribution in the normal human corneal epithelium. Graefes Arch Clin Exp Ophthalmol 2005;243(10): 1056-61.
32. Zhivov A, et al. In vivo confocal microscopic evaluation of Langerhans cell density and distribution in the corneal epithelium of healthy volunteers and contact lens wearers. Cornea 2007;26(1): 47-54.
33. Efron N, Al-Dossari M, Pritchard N. Confocal microscopy of the bulbar conjunctiva in contact lens wear. Cornea 2010;29(1): 43-52.
34. Mastropasqua L, et al. Epithelial dendritic cell distribution in normal and inflamed human cornea: in vivo confocal microscopy study. Am J Ophthalmol 2006;142(5): 736-44.
35. Lum E, Golebiowski B, Swarbrick HA. Mapping the corneal sub-basal nerve plexus in orthokeratology lens wear using in vivo laser scanning confocal microscopy. Invest Ophthalmol Vis Sci 2012;53(4): 1803-9.
36. Patel DV, McGhee CN. Mapping of the normal human corneal sub-Basal nerve plexus by in vivo laser scanning confocal microscopy. Invest Ophthalmol Vis Sci 2005;46(12): 4485-8.
37. Edwards K, et al. Wide-field assessment of the human corneal subbasal nerve plexus in diabetic neuropathy using a novel mapping technique. Cornea 2012;31(9): 1078-82.
38. Turuwhenua JT, Patel DV, McGhee CN. Fully automated montaging of laser scanning in vivo confocal microscopy images of the human corneal subbasal nerve plexus. Invest Ophthalmol Vis Sci 2012;53(4): 2235-42.
39. Edwards K, et al. Utility of corneal confocal microscopy for assessing mild diabetic neuropathy: baseline findings of the LANDMark study. Clin Exp Optom 2012;95(3): 348-54.
40. Malik RA, et al. Corneal confocal microscopy: a non-invasive surrogate of nerve fibre damage and repair in diabetic patients. Diabetologia 2003;46(5): 683-8.
41. Leonardi A, et al. Corneal confocal microscopy in patients with vernal keratoconjunctivitis. Ophthalmology 2011;119(3): 509-15.
42. Rosenberg ME, et al. In vivo confocal microscopy after herpes keratitis. Cornea 2002;21(3): 265-9.
43. Ozgurhan EB, et al. Evaluation of corneal microstructure in keratoconus: a confocal microscopy study. Am J Ophthalmol 2013;156(5): 885-893 e2.
44. Patel DV, McGhee CN. Mapping the corneal sub-basal nerve plexus in keratoconus by in vivo laser scanning confocal microscopy. Invest Ophthalmol Vis Sci 2006;47(4): 1348-51.
45. Niederer RL, et al. Laser scanning in vivo confocal microscopy reveals reduced innervation and reduction in cell density in all layers of the keratoconic cornea. Invest Ophthalmol Vis Sci 2008; 49(7): 2964-70.
46. Niederer RL, et al. Corneal innervation and cellular changes after corneal transplantation: an in vivo confocal microscopy study. Invest Ophthalmol Vis Sci 2007;48(2): 621-6.
47. Karimi AH, Wong A, Bizheva K. Automated detection and cell density assessment of keratocytes in the human corneal stroma from ultrahigh resolution optical coherence tomograms. Biomed Opt Express 2011;2(10): 2905-16.
48. Oliveira-Soto L, Efron N. Morphology of corneal nerves using confocal microscopy. Cornea 2001;20(4): 374-84.
49. Pritchard N, Edwards K, Efron N. Non-contact laser-scanning confocal microscopy of the human cornea in vivo. Cont Lens Anterior Eye 2014;37(1):44-8.
50. Hu VH, et al. In vivo confocal microscopy of trachoma in relation to normal tarsal conjunctiva. Ophthalmology 2011;118(4): 747-54.
51. Kobayashi A, Yoshita T, Sugiyama K. In vivo findings of the bulbar/palpebral conjunctiva and presumed meibomian glands by laser scanning confocal microscopy. Cornea 2005;24(8): 985-8.
52. Nguyen TH, et al. In vivo confocal microscopy: increased conjunctival or episcleral leukocyte adhesion in patients who wear contact lenses with lower oxygen permeability (Dk) values. Cornea 2004;23(7): 695-700.
53. Jiang H, et al. Human conjunctival microvasculature assessed with a retinal function imager (RFI). Microvasc Res 2012;85: 134-7.