Introduction


The availability of increasingly powerful computers at affordable prices has led to a proliferation of computers in almost every working environment, the Ophthalmic/ Optometric consulting room included. Until recently, the application of computers in ophthalmology/ optometry was limited primarily to administrative tasks such as patient management, word-processing, internet access etc. However, computers are starting to play an increasingly important role as clinical tools. Many clinical instruments are now computer-controlled and computers are being used increasingly to store and analyse clinical images.


The Binocular Vision Analyser is a tool designed to be used by orthoptists, optometrists, ophthalmologists and other eye care professionals, for assessing ocular motility. It was first developed in the early 1990s and at this time it required a specially adapted computer. However, with advances in computer technology it has now been possible to create a program that can be used on a standard PC.


Background


When investigating strabismus or heterophoria, it is important to establish if the angle of deviation is constant or varies with the direction of gaze. If the deviation is constant, it is said to be concomitant. If it changes with the direction of gaze it is said to be incomitant.


Concomitant deviations are relatively common. They are usually associated with hypermetropia or the anomalous placement of one or more of the extraocular muscles. Various treatment options are available including refractive or prismatic management, exercises or surgery.


Incomitant deviations are usually caused by a functional anomaly of one or more of the extra-ocular muscles or their associated neurology. This may be a result of a defective muscle or mechanical interference in the orbit (myogenic), or a consequence of a lesion in the nerves supplying the musculature (neurogenic).


Patients with long-standing incomitant deviations are often asymptomatic due to the development of a series of sensory and motor adaptations. The presence of diplopia usually suggests that the incomitancy is of recent origin and indicates a disturbance to some component of the oculomotor system. This may have been caused by trauma or may indicate the presence of an intra-cranial tumour, aneurysm or haemorrhage. It is vital that these cases are distinguished from long-standing incomitancies so that these patients may be referred for a full neurological examination at the earliest opportunity.


Therefore, the priority when conducting an oculomotor investigation is to establish whether the deviation is concomitant or incomitant. If incomitant, it is important to establish which muscle(s) is /are affected and if the incomitancy is long-standing or recent.


Gross incomitancies can be detected by a simple motility test. However, it is often difficult to detect subtle incomitancies using this method and it requires a great deal of skill to analyse the nature of an incomitant deviation.


A detailed analysis of the oculomotor fields can be obtained using either a Hess or Lees screen. These instruments dissociate the eyes and allow the size of any deviation to be measured in various directions of gaze.



Both tests are simple to perform and provide valuable diagnostic information. However, the equipment is bulky and expensive and testing is time-consuming, tedious and requires supervision by skilled personnel. Furthermore, recording and quantification is entirely manual and prone to errors.


The Binocular Vision Analyser is a computer program which is designed to run on any computer operating under Windows 7 or later, with a 19’’ (or larger) colour monitor.


When the program is run for the first time, the user is required to calibrate the size of the screen and to enter the preferred viewing distance (usually 25-50 cm).


The patient wears red and green goggles and is positioned in front of the computer screen at the appropriate distance.  The room lights are extinguished and a red and a blue circle are displayed on the screen (the right eye sees the red circle and the left the blue). Initially the red circle is placed in the top left of the screen and the patient is instructed to move the blue circle using the mouse until it appears to be centred on the red circle.


As the eyes are dissociated, any deviation in this direction of gaze will result in a misalignment of the circles. This is repeated for either 9 or 25 directions of gaze (depending on the option selected). The colour of the circles is then reversed and measurements repeated with the left eye fixating. The nine point test takes approximately 4 minutes to complete.


Results are then displayed in the conventional format on the screen. A number of analytical tools can then be applied to the data to help the clinician establish a diagnosis.


Multiple plots can be superimposed to assess longitudinal changes and the exact amplitude of any deviation can be displayed at any point on the chart.


The charts can be printed or exported to other Windows programs for inclusion in a referral report or for further analysis. The program also incorporates a database to allow records to be stored and retrieved.


The program has been compared with a conventional Hess screen on a group of 30 patients attending the Orthoptics Department at Moorfields Eye Hospital (Thomson et al.,1990). Subjects were aged between 6 and 81 years (mean 49) and presented with a variety of concomitant and incomitant deviations. Each patient was tested using a conventional Hess screen and the PC Hess Screen. After the tests were completed, each patient was asked which test they preferred. Of the 29 respondents, 19 preferred using the PC Hess Screen, seven preferred the conventional Hess Screen and 3 had no preference.


In general, the shapes of the oculomotor fields obtained using the two instruments were similar, but the PC Hess Screen tended to give a slightly “eso” (converged) displacement of the plots compared to the conventional Hess Screen. This was presumably attributable to the shorter viewing distance used by the PC Hess Screen in this study (25cm as compared to 50cm).


Clinically, the exact amplitude of the deviation in each direction of gaze is of less importance than the overall shape of the oculomotor fields. To determine the relative diagnostic value of the data obtained using the two instruments, two orthoptists and two ophthalmologists were asked to make a diagnosis from the charts obtained using both instruments. The charts for the two instruments were presented in identical formats and the 60 charts were inspected in a random order.  A third ophthalmologist, with the aid of the patients’ records, was asked to classify the diagnosis as “incorrect”, “partially correct” or “correct”. The results of this analysis are shown graphically below. Members of the panel were marginally better at making a correct diagnosis using charts obtained using the conventional Hess test than the PC Hess Screen although the difference was small.



An important characteristic of any clinical test is its test-retest reliability. To compare the test-retest reliability of the Binocular Vision Analyser and the conventional Hess Screen, a single subject with a stable long-standing superior oblique palsy was tested using both instruments on five successive days. The standard deviation for each point was found to be less than 1 degree for each instrument showing that both instruments provide very repeatable results.


The Binocular Vision Analyser offers a number of advantages over the conventional Hess Screen and the Lees screen. The program uses a standard PC, avoiding the need to purchase expensive and bulky equipment. Computer-control minimizes the supervision required and allows patients to self-pace the examination. Recording errors are eliminated and the data collected can be readily quantified, analysed, displayed in a variety of formats, printed, incorporated into referral reports or archived in a database. The extent of the oculomotor fields that can be assessed is determined by the size of the monitor and the viewing distance. In order to cover the same angle as a conventional Hess screen, the Binocular Vision Analyse has to use a slightly shorter viewing distance. This tends to produce a relatively convergent displacement of the charts but does not greatly diminish their diagnostic value or their test-retest reliability.


The Binocular Vision Analyser provides the clinician with a powerful new tool for analysing ocular motility, without the need for adding any more equipment to an increasingly crowded consulting room.


Reference

Thomson, W.D., Desai, N. and Russell-Eggitt. (1990) A new system for the

measurement of ocular motility using a personal computer. Ophthal. Physiol. Opt. Vol 10, 137-143.