First digital retinal imaging, then OCT – the American Academy of Optometry conference has often been the place to see the latest instrumentation making an impact in optometry in the US before it takes off back here in the UK. This year saw a number of newly launched or evolved instruments on display in the exhibition arena (Figure 1) that I feel are worth looking out for.
Screening for glaucoma
I first reported on the Reichert ORA back in 2005. This non-contact tonometer was of interest because, as well as measuring IOP, it also assessed corneal hysteresis (CH). We are familiar with the corneal thickness influencing tonometer measurements and pachymetry is increasingly used by optometrists to see if a higher IOP value might be related to a higher than average corneal thickness (CCT). But what about when thickness is equal – might there be differing resistance to the incoming force of the tonometer related to the specific viscoelastic properties of the cornea. If one is able to measure the force needed to first flatten the cornea and then, having achieved a concave surface, the force on reduction at which the flattening is again achieved, one finds a difference between the two forces – the hysteresis.
[CaptionComponent="1807"][CaptionComponent="1808"]The two applanations take place within approximately 20 milliseconds, a time sufficiently short to ensure that ocular pulse effects or eye position do not change during the measurement process. Lower values for CH have been found to be independently predictive of visual field defect progression, and in fact two to three times more so than IOP alone or CCT measurement. A measurement called corneal-compensated intraocular pressure (IOPcc) is obtained from the difference between the two applanation pressures using the formula P2-kP1, where P1 and P2 are the ?rst and second applanation pressures, respectively, and k is a constant. As the difference between P1 and P2 is related to the corneal biomechanical properties, the value of IOPcc is supposed to represent a measure of intraocular pressure that is free of the corneal in?uence.1,2,3,4
The recently launched Reichert ORA G3 (Figure 2) attracted much interest as it has improved CH measurement capability and is now able to also assess the IOPcc. As Dr David A. Taylor (senior product manager, Advanced Diagnostics and Tonometry, Reichert Technologies, pictured) told me: ‘IOPcc is closer to the true pressure and more indicative of glaucoma damage than actual Goldmann readings. IOPcc has been proven to be more accurate than GAT and, more importantly, more associated with actual glaucoma risk. IOPcc measurement should be a home run for us.’ Optician is due to trial one of the first G3 units in the UK in the coming months – look out for our findings and fuller discussion.
I was also interested in a new desktop electrophysiology unit, the EvokeDx (from Konan Medical, Figure 3). It is well known that visual fields assessment suffers from both its subjective nature and also the fact that a significant (30 to 40 per cent) amount of ganglion cell death occurs before we detect repeatable field loss.
[CaptionComponent="1809"]No more ‘swimming hat’ electrode arrays, the EvokeDX is an all-in-one design with a touchscreen user interface and integrated gaze tracker that provides quantitative, objective measures of visual function through visual-evoked potentials (VEPs) and electroretinograms (ERGs).
The patented test strategies are designed to isolate and test various visual pathways and visual functions;5 Fourier-transform frequency analysis methods assess data and this allows the EvokeDx to assess glaucomatous damage. According to the release, this is strategy with a classification accuracy of 89 per cent to 94 per cent in clinical trials.6 As the EvokeDx has just received CE approval, expect to hear about this again soon.
Screening for AMD
Much has been written about macular pigment as a way of implying macular health and maculopathy risk, but most commercially available instruments measuring this suffer from repeatability concerns and a lack of further focused clinical strategy in interpreting results.
I was interested to see the new AdaptDx (from maculogix, Figure 4). Earlier in the year Optician published a paper on how rod function is inhibited in early lipofuscin accumulation (way before aggregate drusen are visible) resulting in a reduced dark adaptation or photorecovery.7 The AdaptDx, again recently CE approved, provides a non-invasive objective measurement of dark adaptation function and so may predict, at a very early stage, diseases such as AMD, retinitis pigmentosa and other retinal conditions.8 Several research posters at conference confirmed the units potential as an indicator of retinal health and, again, I look forward to trialing the unit soon.
[CaptionComponent="1810"]Colour Vision
Much research in the US into acquired colour vision defects has been driven by the military. High ‘g-force’ impact on retinal blood flow causes colour shifts and this is one reason why specific tests for early acquired loss have been developed. The Rabin Cone Contrast Test is one such and is now available as a commercial unit (from Innova Systems, Figure 5). The unit uses a series of coloured letters on a high resolution electronic display and the patient indicates when each is first seen and identified correctly.9 This provides a test sensitive enough to detect even early maculopathy changes affecting cone function.10
[CaptionComponent="1811"]A very user-friendly tablet-based colour vision screening app was also being promoted – the ColorDx (again from Konan Medical). This looks similar to an Ishihara style pseudoisochromatic test, though with far greater flexibility of target setting than its paper counterpart. Again, look out for a future review once I have given it a road test.
Imaging
There were two imaging systems that caught my eye, in among the now familiar OCTs and fundus cameras. The Volk Pictor has been improved significantly and claims to have ironed out many of the focusing difficulties and final image clarity loss found with earlier models (Figure 6 shows the Volk Pictor Plus in action, with in-built fixation and higher resolution image capture). Perhaps even more interesting, the team I met was about to travel to the UK to see if this might be approved as the first portable unit for diabetic screening. Watch this space.
[CaptionComponent="1812"]Scleral lenses are enjoying a major renaissance both sides of the channel and Visionary Optics were boasting of the exceptional 22mm field and 360-degree scleral coverage of their sMap 3D topographer (Figure 7).
[CaptionComponent="1813"]The full scleral coverage is achieved by a clever ‘image stitching’ software program that seamlessly blends data from three positions of vertical gaze by the patient. This allows accurate first-time ordering of scleral lenses – a long way from the instillation of liquid rubber onto the eye!
And Finally…
I was surprised by the number of hand-held autorefractor units on display. Because the US, like the UK, requires the refraction prescription to be issued by an optometrist, these were mainly aimed as supplementary to subjective refraction but also seemed to be gaining in popularity in developing country liaison projects, as well as for domiciliary care use.
Pick of the bunch was the nifty SVOne (Smart Vision Labs, Figure 8), which is effectively a Hartmann-Shack aberrometer that fits neatly over an iPhone and boasts excellent repeatability and accuracy11.
[CaptionComponent="1814"]Prize for the most left-field innovation has to go to the BioDOptix. This is an allograft taken from amniotic tissue which may be applied over areas recently having tissue removed, such as corneal ulcers, pterygia and so on, to significantly reduce regrowth. Not sure if this will be available to UK optometrists in my lifetime.
References
1 Medeiros FA, et al, Corneal hysteresis as a risk factor for glaucoma progression: a prospective longitudinal study. Ophthalmology. 2013 Aug;120(8):1533-40.
2 Ehrlich JR, Radcliffe NM, Shimmyo M. Goldmann applanation tonometry compared with corneal-compensated intraocular pressure in the evaluation of primary open-angle Glaucoma. BMC Ophthalmol. 2012 Sep 25;12:52.
3 De Moraes CV, et al. Lower corneal hysteresis is associated with more rapid glaucomatous visual field progression. J Glaucoma. 2012 Apr-May;21(4):209-13.
4 Deol M, Taylor D, Radcliffe NM. Corneal hysteresis and its relevance to glaucoma. Review, Ophthalmology, Volume 26, Number 2, March 2015
5 Zemon, V, Gordon, J, Welch, J (1988) Asymmetries in ON and OFF visual pathways of humans revealed using contrastevoked cortical potentials, Visual Neuroscience, 1, 145-150.
6 Zemon V, et al, Novel electrophysiological instrument for rapid and objective assessment of magnocellular deficits associated with glaucoma, Documenta Ophthalmologia, 2008 117:233-243.
7 Elena Rodrigo-Diaz, et al. http://www.opticianonline.net/continuing-education/c40494-rods-early-age-related-macular-degeneration/
8 : Jackson GR, et al. Diagnostic sensitivity and specificity of dark adaptometry for detection of age-related macular degeneration. Invest Ophthalmol Vis Sci. 2014;55:1427–1431.
9 Rabin J. Quantification of color vision with cone contrast sensitivity. Vis Neurosci. 2004 May-Jun;21(3):483-5.
10 Chacon A, et al. Quantification of color vision using a tablet display. Aerosp Med Hum Perform. 2015 Jan;86(1):56-8.
11 Yaopeng Zhou, Ph.D and Jordan Kassalow. Preliminary Evaluation of SVOne Autorefractor for Low Order Refractive Errors. https://smartvisionlabs.com/downloads/SVL_Kassalow_Dec2014.pdf.