Article Text
Abstract
Aim To establish the effect of acute loss of stereopsis on simulated intraocular surgical performance.
Methods This study was performed using the EYESi ophthalmic surgical simulator. Thirty junior doctors with no previous ophthalmic surgical experience were enrolled and distance visual acuity (Snellen), near visual acuity and stereoacuity (Frisby) were recorded. All participants completed a standard introductory programme on the forceps module to eliminate the learning curve. They then undertook four attempts of level 4 forceps module binocularly and another four monocularly to simulate an acute loss of stereopsis. Total score, odometer movement, corneal area injured, lens area injured and total time taken were recorded.
Results Mean age was 31 years (SD±9). None had amblyopia, with all demonstrating distance visual acuity of 6/6 or better and N6 for near. Mean stereopsis was 35 s of arc (SD±18). Average total score decreased from 60 while operating binocularly to 47 monocularly (p<0.05). Average corneal area injured increased from 0.95 mm2 to 2.30 mm2 (p<0.05), average lens area injured increased from 1.76 mm2 to 3.53 mm2 (p<0.05) and average time taken increased from 69.6 s to 77.4 s (p<0.05).
Conclusion The importance of stereopsis for intraocular surgery is difficult to establish in a live theatre setting without compromising patient safety. Virtual reality simulators provide a safe alternative. This study demonstrates a statistically significant decrease in simulated intraocular surgical performance with acute loss of stereopsis in potential ophthalmic training applicants. Caution is recommend in using these results to advocate stereopsis testing as a screening tool in interviews because some participants performed well despite an absence of stereopsis.
- Anatomy
- child health (paediatrics)
- conjunctiva
- drugs
- electrophysiology
- intraocular surgery
- stereopsis
- trauma
- virtual reality simulation
Statistics from Altmetric.com
- Anatomy
- child health (paediatrics)
- conjunctiva
- drugs
- electrophysiology
- intraocular surgery
- stereopsis
- trauma
- virtual reality simulation
Stereopsis is defined as the perception of depth secondary to binocular retinal disparity within Panum's fusional space.1 Decreased stereopsis can occur as a result of deficient binocular functions during childhood in a range of disorders such as anisometropia, visual deprivation, amblyopia and strabismus. In adults loss of stereopsis can be due to loss of acuity in one eye secondary to trauma or disease.
While stereopsis is a particularly refined form of depth perception, various monocular cues can also provide information on depth including linear perspective, relative size, interposition, aerial perspective and parallax.
As depth perception is not solely dependent on stereopsis, it is an ongoing matter of debate whether stereopsis is a prerequisite for activities requiring high levels of visual skills; for example, flying and microsurgery. In intraocular surgery this is difficult to ascertain in a live theatre setting without compromising patient safety. However, the advent of virtual reality simulators affords an opportunity to study this issue in a safe environment.
One commercially available ophthalmic virtual reality surgical training system on the market is the EYESi produced by VRMagic, Mannheim, Germany (figure 1). Originally designed as a vitreo-retinal surgical training device it now has a dedicated anterior segment training module. It allows repeated measurements of standardised surgical tasks. Feedback is provided in the following main categories: surgeon efficiency, achievement of surgical goal, surgeon error/tissue injury and formative education/feedback.2 The forceps training module has previously demonstrated construct validity,3–5 and we used this to assess the effect of an acute loss of stereopsis on intraocular surgical performance.
Materials and methods
The study participants included 30 junior doctors with no previous ophthalmic microsurgical experience. All were recruited through local announcement of the study on a first-come first-serve basis. Local research and development department approval for the study was obtained and all participants provided written consent. Each participant was also informed that the data collected would not be part of any professional appraisal and would be reported anonymously.
The simulator consists of a mannequin head prop with an electronic eye that pivots and rotates when manipulated by the surgeon. Probes inserted into the electronic eye can virtually emulate different intraocular instruments. A virtual operating microscope complete with zoom/pan/focus foot pedal provides stereoscopic images of the eye and instruments to the surgeon. The simulator is loaded with various modules such as antitremor, forceps, capsulorhexis and phacoemulsification. Each module has different difficulty levels to simulate increasingly complicated tasks.
The anterior segment forceps module requires the surgeon to grasp six objects from the periphery of the anterior chamber and place them in a basket in the centre of the anterior chamber (figure 2). The size, shape and anteroposterior location of the objects within the anterior chamber vary according to the module's difficulty level (ranging from one to four). It is meant to teach surgeons the skills required to grasp the edge of a capsulorhexis flap accurately while keeping the eye centred and avoiding injury to the lens or cornea. We chose to use this module as it has previously shown construct validity. For each attempt the total possible score can vary from 0 to 100. The simulator awards positive points for the percentage of the task completed. It then subtracts from this for reduced efficiency and errors such as excessive time taken, corneal injuries, lens injuries, distance travelled by the instrument within the anterior chamber (odometer), operating without red reflex, non-horizontal insertion of the instrument and out-of-focus interactions.
Each participant had measurements taken of distance visual acuity using a Snellen chart at 3 m monocularly and binocularly (readings converted to 6 m for analysis). Near vision was measured using a standard Times new roman faculty of ophthalmologists reading chart. Stereopsis was measured using the Frisby test. All participants received a standardised orientation to the simulator with eight practice sessions binocularly (one attempt level 1 forceps module, one attempt level 2 forceps module and six attempts level 4 forceps module) to eliminate the learning curve. Scores for these were not recorded. Main data collection then commenced. All participants completed a further four attempts of the level 4 forceps module binocularly using the dominant hand. This was followed by another four attempts using the dominant hand but with one eye occluded to simulate an acute absence of stereopsis. The parameters recorded for each attempt were total score, total time, corneal area injured (mm2), lens area injured (mm2) and odometer measurement (mm).
Results
Tests of normality (histograms and Q–Q plots) were applied to determine data distribution. Parametric testing (paired t test) was used for normally distributed data and non-parametric testing (Wilcoxon signed rank test) for skewed data. Multiple comparison corrections were not required for our sample size. A p value less than 0.05 was considered statistically significant. All data were analysed using SPSS and the results are presented as means±SD (table 1).
The mean age was 31 years (SD±9). None had amblyopia, with all demonstrating distance visual acuity 6/6 or better and N6 for near. Only one participant had a difference of one line in distance acuity between the two eyes. The mean stereopsis was 35 s of arc (SD±18). Following the standardised orientation all participants' scores reached a plateau level whereby further attempts binocularly did not produce a continuous improvement in performance.
The average total score showed a statistically significant decrease from 60 while operating binocularly to 47 with occlusion of one eye (p<0.05). The average corneal area injured increased from 0.95 mm2 to 2.30 mm2 (p<0.05), the average lens area injured increased from 1.76 mm2 to 3.53 mm2 (p<0.05), and the average time taken increased from 69.6 s to 77.4 s (p<0.05). The average odometer measurements did not show a statistically significant difference (202 mm binocularly and 203 mm with one eye occluded). Looking at individual participant scores, several isolated exceptions were noted in which the measured parameters showed improvement despite monocularity (table 2).
Discussion
From an evolutionary standpoint it is easy to understand the need for two eyes on either side of the head where each has a completely different field of view and moves independently. However, in some species the eyes have migrated forward such that in human beings the visual fields almost overlap thus largely negating this purpose.
Therefore, it is postulated that the major advantage of having two forward-facing eyes is the ability to perform stereoscopic depth discrimination. Stereoacuity values of approximately 30–40 s of arc are generally regarded as normal. Stereopsis is thought to be present from a distance of 500 m and improves with closer distances, until limited by accommodation.6
While stereopsis is the only direct measurement of depth perception, various other monocular cues can provide an indirect measurement.7 ‘Relative size’ describes the process of retinal image size allowing us to judge distance based on our past and present experience and familiarity with similar objects; for example, as a car drives away, the retinal image becomes smaller and smaller. We interpret this as the car getting further and further away. ‘Interposition’ occurs when there is object overlapping. The overlapped object is considered further away. ‘Linear perspective’ is when as an object subtends a smaller and smaller angle, it is interpreted as being further away; for example, parallel lines converge with increasing distance such as roads, railway lines and electric wires. ‘Aerial perspective’ is when the relative colours of objects give us indications of their distance. Due to the scattering of blue light in the atmosphere, distant objects appear bluer. Thus distant mountains appear blue. The contrasts of objects also provide clues to their distance. When the scattering of light blurs the outlines of an object, it is perceived as distant. Similarly, highlights and shadows can provide information about an object's dimensions and depth. Another phenomenon is ‘monocular movement parallax’. When we move our head from side to side, objects at different distances move at different relative speeds. Closer objects move ‘against’ the direction of head movement and further objects move ‘with’ the direction of head movement.
Given the presence of monocular cues and the distance limitations of stereopsis we can understand why individuals perform without problems in everyday life even when this high form of depth perception is lacking. In a study looking at the influence of depth perception on automobile driving performance, the researchers found that stereopsis had a positive effect only in dynamic situations at intermediate distances; for example, driving through a slalom course while other routine driving tasks were unaffected.8 However, there are specific benefits afforded by stereopsis, which while not of consequence in daily life might be of advantage in more delicate visual tasks. Studies have indicated that the development and use of compensatory cues for depth perception in people with weak stereopsis are insufficient to deal with interception tasks (like catching a tennis ball) under high temporal constraints (ball moving at high speed) and that this disadvantage cannot be fully attenuated by specific and intensive training.9 10 Similarly, high grade stereovision has been found to be essential for skilled precision grasping. Reduced stereopsis results in inaccurate grasp point selection and greater reliance on non-visual (somesthetic) information while proceeding from object contact to grip stability.11
Therefore, the importance of stereopsis in professions requiring a high level of visual skill remains debatable. Aircraft pilots need excellent vision but the value of stereopsis has yet to be clarified. In a study of the attrition rate during US Air Force pilot training, absence of stereopsis was not found to be a significant factor. The authors suggested that stereopsis does not correlate with flying ability and in most situations monocular cues suffice.12 Nonetheless, candidates with deficient stereopsis are not deemed eligible for military pilot training.
In surgery the answer remains just as elusive. Barry et al13 conducted a prospective study that compared patients with strabismus with age-matched controls in performance on a simulated (non-virtual reality) validated surgical training module (involving peg transfer tasks). They found that patients with strabismus performed less well. However, there was significant overlap between groups and several patients with strabismus performed better than the mean of the control group. Grober et al14 built on this and used artificial monocular conditions to assess the effect of absent stereoacuity on microsurgical performance of a suturing task in surgical trainees. They did not find a significant correlation and concluded that the predictive validity of stereopsis and microsurgical performance remained unclear. More recently, Sachdeva and Traboulsi15 compared the performance of patients with deficient stereoacuity with that of those with normal stereoacuity in microsurgical tasks on the EYESi simulator. Individuals with normal stereoacuity performed better but again there were several exceptions in which patients with impaired stereoacuity actually performed as well as, or better than, controls. While that study is similar to ours, it used patients with no ocular anatomical knowledge and with an age range of 10–64 years. This is important in the context of stereopsis testing while selecting ophthalmic trainees. Our study group more accurately represents candidates likely to apply for ophthalmic training programmes and the results appear to agree with the findings of Sachdeva and Traboulsi.15 At first glance the two studies would indicate that impaired stereopsis leads to decreased ocular surgical proficiency, but in both there were isolated parameters in which participants with deficient or absent stereopsis performed better than their counterparts. While not statistically significant, this does suggest that the use of stereopsis testing as a screening tool for future ocular surgeons might inaccurately exclude a minority of potentially good surgeons.
Anecdotally, it would appear that some surgeons with deficient stereopsis are able to maintain a safe surgical practice. Others feel comfortable performing surgery without microscopes; for example, strabismus surgery and oculoplastic surgery, but feel less confident while performing microscopic surgery.
Given the complex nature of the issue and conflicting evidence, it does not appear reasonable for the ophthalmic profession to use stereopsis testing as a means of trainee selection. A more reasonable approach would be to advise potential applicants to undertake stereopsis testing at their local orthoptic departments before applying. Those with deficient stereopsis should then have a counselling session with their local college tutor in which they should be made aware of the current evidence. They should then be offered the chance to undertake simulated surgery on a regional virtual reality simulator, and decide themselves whether they feel confident pursuing a microsurgical career with the knowledge that inability to operate will halt progression through an ophthalmic training programme.
One limitation of our study is that it has only evaluated the effect of an acute loss of stereopsis on intraocular surgical performance. It is often postulated that people with long-standing stereodeficiency may be more adapted to various monocular cues and thus might perform even better. Although two of the studies mentioned above13 15 used participants with long-standing stereodeficiency and found results similar to ours, these were not representative of potential ophthalmic training applicants. A further study utilising the methods described by us but using a cohort of junior doctors with long-standing stereodeficiency would be required to evaluate this aspect further.
Conclusion
The importance of stereopsis for intraocular surgery is difficult to ascertain in a live theatre setting without compromising patient safety. Virtual reality simulators provide a safe alternative. Our study demonstrates a statistically significant decrease in simulated intraocular surgical performance with acute loss of stereopsis in potential ophthalmic training applicants. However, we would recommend caution in using these results to advocate stereopsis testing as a screening tool in interviews because a small percentage of our participants performed well despite an absence of stereopsis.
References
Footnotes
Competing interests None.
Ethics approval Local research and development department approval for the study was obtained.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement Complete data are available on request from Salman Waqar: salmanwqr{at}gmail.com.