An Optokinetic Nystagmus Detection Method for Use With Young Children

Sangi, M., Thompson, B., & Turuwhenua, J. (2015). An Optokinetic Nystagmus Detection Method for Use With Young Children. IEEE Journal of Translational Engineering in Health and Medicine, 3, 1600110. http://doi.org/10.1109/JTEHM.2015.2410286 ©IEEEThe detection of vision problems in early childhood can prevent neurodevelopmental disorders such as amblyopia. However, accurate clinical assessment of visual function in young children is challenging. optokinetic nystagmus (OKN) is a reflexive sawtooth motion of the eye that occurs in response to drifting stimuli, that may allow for objective measurement of visual function in young children if appropriate child-friendly eye tracking techniques are available. In this paper, we present offline tools to detect the presence and direction of the optokinetic reflex in children using consumer grade video equipment. Our methods are tested on video footage of children (N = 5 children and 20 trials) taken as they freely observed visual stimuli that induced horizontal OKN. Using results from an experienced observer as a baseline, we found the sensitivity and specificity of our OKN detection method to be 89.13% and 98.54%, respectively, across all trials. Our OKN detection results also compared well (85%) with results obtained from a clinically trained assessor. In conclusion, our results suggest that OKN presence and direction can be measured objectively in children using consumer grade equipment, and readily implementable algorithms

Applications of ray tracing to a pseudophakic eye model

The calculation of IOL power using keratometry is adversely affected by recent corneal reshaping surgeries. This thesis investigates the application of ray tracing and general anterior corneal surface modeling, for the purpose of improving ophthalmic measurements and in particular, the estimation of IOL power. A new algorithm (based on a multi-step approach) for the recovery of the corneal height using videokeratography is presented. The method ensures a cubic recovery with continuous curvature; skew rays are treated in post-processing. The RMS height error is measured for three simulated (with two skewed) cornea. The total errors are 6.2 x 10⁻⁴ mm ignoring the skew ray error, and 1.7 x 10⁻⁴ mm accounting for it. The individual height errors are submicron in the latter case. The algorithm gives average errors of 2.5 x 10⁻⁴ mm for a set of calibration balls. The completion time is 2.3 s over all cases, using a standard desktop PC. A new method for the recovery of the internal ocular radii of curvature is investigated. The method is used to recover the posterior corneal radii (PII) and the anterior lens radii (PIII) given several anterior cornea models (PI) in simulation. The recovered surface powers are no more than 0.1 D(PII) and 0.006 D(PIII) in error of the true surface powers. A framework is then presented for modeling the effect of lens decenter and tilt on perceived image quality. The SQRI image quality metric is determined for a range of lens tilt and lens decenter values. These are compared with the statistical moments of the spot diagrams. The SQRI shows asymmetric degradation (with tilt for a particular decenter value) of imaging for a plane displaced -0.1 mm from best focus. For a plane displaced +0.1 mm from best focus, the SQRI is symmetric and improves regardless of the sign of tilt. The statistical moments suggest that skew does not necessarily imply poor imaging. Finally, the modeling methods developed are tested on two clinically measured eyes. Minimizing the spot size, predicts the spectacle prescription to 0.0 D(OS) and 0.1 D(OD) of the mean spherical equivalent. Adding prescribed lenses to the model eye, estimates best focus to 0.03 mm and 0.02 mm of the retinal plane; consistent with better than 6/6 VA measured for OS/OD. A VisTech VCTS 6500 contrast sensitivity chart is used to verify the eye model. A 75% match with theory is found for OS, a 50% match is found for OD

Forward ray tracing for image projection prediction and surface reconstruction in the evaluation of corneal topography systems

A forward ray tracing (FRT) model is presented to determine the exact image projection in a general corneal topography system. Consequently, the skew ray error in Placido-based topography is demonstrated. A quantitative analysis comparing FRT-based algorithms and Placido-based algorithms in reconstructing the front surface of the cornea shows that arc step algorithms are more sensitive to noise (imprecise). Furthermore, they are less accurate in determining corneal aberrations particularly the quadrafoil aberration. On the other hand, FRT-based algorithms are more accurate and more precise showing that point to point corneal topography is superior compared to its Placido-based counterpart

Automatic nystagmus detection and quantification in long-term continuous eye-movement data

Symptoms of dizziness or imbalance are frequently reported by people over 65. Dizziness is usually episodic and can have many causes, making diagnosis problematic. When it is due to inner-ear malfunctions, it is usually accompanied by abnormal eye-movements called nystagmus. The CAVA (Continuous Ambulatory Vestibular Assessment) device has been developed to provide continuous monitoring of eye-movements to gain insight into the physiological parameters present during a dizziness attack. In this paper, we describe novel algorithms for detecting short periods of artificially induced nystagmus from the long-term eye movement data collected by the CAVA device. In a blinded trial involving 17 healthy subjects, each participant induced nystagmus artificially on up to eight occasions by watching a short video on a VR headset. Our algorithms detected these short periods with an accuracy of 98.77%. Additionally, data relating to vestibular induced nystagmus was collected, analysed and then compared to a conventional technique for assessing nystagmus during caloric testing. The results show that a range of nystagmus can be identified and quantified using computational methods applied to long-term eye-movement data captured by the CAVA device

Diagnosing dry eye with dynamic-area high-speed videokeratoscopy

Dry eye syndrome is one of the most commonly reported eye health conditions. Dynamic-area highspeed videokeratoscopy (DA-HSV) represents a promising alternative to the most invasive clinical methods for the assessment of the tear film surface quality (TFSQ), particularly as Placido-disk videokeratoscopy is both relatively inexpensive and widely used for corneal topography assessment. Hence, improving this technique to diagnose dry eye is of clinical significance and the aim of this work. First, a novel ray-tracing model is proposed that simulates the formation of a Placido image. This model shows the relationship between tear film topography changes and the obtained Placido image and serves as a benchmark for the assessment of indicators of the ring’s regularity. Further, a novel block-feature TFSQ indicator is proposed for detecting dry eye from a series of DA-HSV measurements. The results of the new indicator evaluated on data from a retrospective clinical study, which contains 22 normal and 12 dry eyes, have shown a substantial improvement of the proposed technique to discriminate dry eye from normal tear film subjects. The best discrimination was obtained under suppressed blinking conditions. In conclusion,this work highlights the potential of the DA-HSV as a clinical tool to diagnose dry eye syndrome