Oman Journal of Ophthalmology

ORIGINAL ARTICLE
Year
: 2016  |  Volume : 9  |  Issue : 3  |  Page : 135--138

Validity and cost-effectiveness of cone adaptation test as a screening tool to detect retinitis pigmentosa


Rahul Deshpande1, Prajakta Save1, Madan Deshpande1, Mahadev Shegunashi1, Marium Chougule1, Rajiv Khandekar2,  
1 Department of Retina, H. V. Desai Eye Hospital, Pune, Maharashtra, India
2 Department of Research, King Khaled Eye Specialist Hospital, Riyadh, Saudi Arabia

Correspondence Address:
Rajiv Khandekar
Department of Research, King Khanled Eye Specialist Hospital, Aruba Road, King Khaled Eye Specialist Hospital, P. O. B: 7191, Riyadh 11462
Saudi Arabia

Abstract

Background: The cone adaptation test is to detect retinitis pigmentosa (RP) cases confirmed by electroretinogram (ERG). We present the validity and cost-effectiveness of cone adaptation test as a screening tool for detecting RP. Methods: This cross-sectional study was conducted between November 2013 and December 2013. All RP cases diagnosed by ophthalmologists of H. V. Desai Eye Hospital in the last 5 years were participated in this study. The cone adaptation test was done in photopic and scotopic illumination. Failed test means 10 s or more to complete the test under scotopic illumination. A technician who was masked for cone adaptation test finding carried out ERG. Demographics, symptoms, and history of treatment were inquired. Those with flat ERG wave in scotopic condition and corresponding clinical findings were defined as having RP. Sensitivity, specificity, and false-positive and false-negative parameters of validity were estimated. The unit cost of performing test and ERG was calculated. Results: All 32 RP patients (28 male, age median 23.5 ΁ 14.5 years) had a vision more than 6/60 and flat wave in ERG under mesopic/scotopic illumination. Thirty-one participants failed cone adaptation test. The sensitivity was 31/32 Χ 100 = 97%. The specificity was 100%. There was no false-positive case. Consanguinity rate among parents was 43%. The cost of testing one child using DQcone adaptation test kitDQ was 2.5 US $. The unit cost of diagnosing RP using ERG was 10 US $. Conclusion: Cone adaptation is a valid and cost-effective screening tool test for RP. The consanguinity rate among parents of an RP patient was high.



How to cite this article:
Deshpande R, Save P, Deshpande M, Shegunashi M, Chougule M, Khandekar R. Validity and cost-effectiveness of cone adaptation test as a screening tool to detect retinitis pigmentosa.Oman J Ophthalmol 2016;9:135-138


How to cite this URL:
Deshpande R, Save P, Deshpande M, Shegunashi M, Chougule M, Khandekar R. Validity and cost-effectiveness of cone adaptation test as a screening tool to detect retinitis pigmentosa. Oman J Ophthalmol [serial online] 2016 [cited 2019 Sep 17 ];9:135-138
Available from: http://www.ojoonline.org/text.asp?2016/9/3/135/192262


Full Text

 Introduction



Retinitis pigmentosa (RP) causes significant visual impairment. It is one of the pigmentary retinopathies and includes all retinal dystrophies in which photoreceptors are lost and retinal pigments show deposits. The sequence of photoreceptor involvement results in night-vision impairment in the early stages of the disease and visual acuity loss at later stages. [1] In advanced stages, attenuated retinal vessels, intraretinal pigment deposits, and waxy pallor of the optic discs are the diagnostic criteria.

In the preclinical stage, clinical signs are often not visible on retinal examination. The diagnosis of RP, therefore, has to be confirmed by electroretinogram (ERG). Classical ERGs recorded from RP patients with different hereditary types show that cone b-wave implicit time delay is a consistent finding in generalized RP. [2],[3] Genetic tests are also available to confirm the diagnosis of retinal dystrophies including RP. [4] Unfortunately, they are time-consuming, uncomfortable to children, expensive, and often inaccessible to patients in developing countries. Therefore, a simple and acceptable test for children where the diagnosis of RP is crucial could be helpful. LEA Test kit contains a toy game called cone adaptation test. [5] It has been found to be useful for detecting clinical and preclinical stages of RP and other Rod-Cone dystrophies. To the best of our knowledge, we could not find independent validation of this test in literature. Therefore, we undertook a study to validate it and compare its cost-effectiveness among the Indian population to other modalities to confirm RP. We also aimed to study the additive effect of other risk factors in enhancing the validity of cone adaptation test.

 Design and Methods



The Ethical and Research Committee of the institute approved this study. Written consent of the participants was obtained. This study was conducted at H. V. Desai Eye Hospital, Pune, India, between June and December 2013. Ours is a tertiary level eye hospital with subspecialties of medical retina and vitreoretinal surgery as well as low-vision rehabilitation services. Children with eye problem from Pune and other nearby districts of Maharashtra avail services at our institution. The identity of the participants was kept confidential.

This was a cross-sectional type of validity study. The findings of ERG were the gold standard to define a case of RP.

We assumed that based on the magnitude of RP in the USA, there could be 5000 RP cases in the catchment area of our institute. [6] To achieve 95% confidence interval and 10% error margin in sensitivity of detecting RP cases using cone adaptation test, we need 35 cases to be tested by new method and ERG.

Two ophthalmologists and one allied eye care professional experienced in electrophysiology of eye were our field investigators. The ophthalmologists inquired about demographic information such as age, gender, birth order, and number of brothers and sisters, uncles and aunties. Consanguinity among parents and the presence of RP among relatives were inquired. The history of night blindness and eye assessment for RP in the past was also noted.

The best-corrected visual acuity for distance was noted using LEA Symbol chart in the lightbox held at a 3-m distance from the participant. The kinetic field of vision was tested using the Goldmann perimeter (Haag-Streit, Germany). Color vision test was carried out using 16D test of LEA Test kit. Ophthalmologists examined retina using slit lamp bio-microscope (Topcon, USA) and 90D Volk lens. For viewing peripheral retina, the pupils were dilated using two drops of 0.5% tropicamide eye drops. Indirect ophthalmoscope and +20D lens were used. The presence of bony corpuscles (either of black or white), pale disc, and attenuated retinal vessels confirmed a clinical diagnosis of RP. Refraction test was carried out.

For flash ERG (both photopic and scotopic), RETI-compact-model gamma plus (Roland consult Co., Germany) equipment was used. The patient was asked to lay down with eyes closed for 20 min in a dark room. The scotopic ERG was performed. The patient was exposed to normal daylight with eyes kept open for about 10 min. Again, ERG (photopic) was carried out. The ERG report was reviewed by two ophthalmologists, and the identity of patient and clinical findings were masked. They labeled the participant as suffering from RP if ERG had flat or low-amplitude waves complimented with positive clinical findings [Figure 1]. On the next day, the refractive correction was given. If the participant was suffering from presbyopia, near correction was given. To carry out cone adaptation test, LEA Test kit was used [Figure 2]. Tiles of three different colors were placed on a black cloth in front of the participant. A demonstration was given about the test. The participants then performed the test in normal daylight. There are four tiles, each of three different colors, blue, red, and white. The white ones are self-illuminating. The participant arranged four tiles of the same color in three piles. Time of completing the task was noted using a stopwatch. To make the mesopic environment, all windows of the room were closed with dark curtains. One table lamp was kept behind the participant and it was directed downward so that the light did not reflect in the test area. The participant repeated the cone adaptation test. Time of completing the test was noted. If participant took more than 5 min, the test was abandoned and the participant was relieved of the stress of identifying three different colors in the mesopic environment. If the participant successfully completed the test within 10 s, he/she was declared pass. The rest were with defective cone adaptation.

The sensitivity was defined as a number of pass candidates with cone adaptation test among participants who had flat/decreased waves in mesopic ERG and clinical signs of RP. Specificity means a number of cases with test normal among those without flat/decreased waves in mesopic ERG. False positive means those having cone adaptation test failed but ERG normal. False negatives mean participants with failed cone adaptation test and having normal ERG. [7]{Figure 1}{Figure 2}

The number and percentage of subgroups were described to review if the validity of the cone adaptation test differed among subgroups.

The time taken for cone adaptation test, clinical diagnosis of RP, ERG were noted and an average time of each procedure was calculated for comparison.

The cost of cone adaptation testing included the procuring price of the kit, the unit cost of the workforce to explain and supervise the test. The cost of ERG included equipment cost, annual maintenance cost, salary of ophthalmic technician for 15 min of testing one patient in both photopic and mesopic/scotopic scenario, and time of ophthalmologist (5 min) to interpret and explain to the patient about ERG results. All the costs were calculated in US dollars using exchange rates prevailing on February 1, 2014. For international comparison of the cost for ERG testing, we used cost incurred in the USA as a bill to the insurance company. [8] To assess cost-effectiveness, cost of both the cone adaptation test and ERG for one child with RP was used.

 Results



Totally, 32 patients of RP were included in this study. The demographic profile and family history of RP are given in [Table 1]. The mean age was 28 years ± 14 years (range 7-53). Fourteen (42%) children with RP had consanguineous marriage among parents.{Table 1}

All participants had clinical evidence of RP in both eyes and ERG demonstrated flat or decreased waves in the mesopic environment.

Thirty-one RP cases passed cone adaption test in scotopic condition but failed in mesopic condition. Only one patient could complete the cone adaptation test in a mesopic environment in 8 s. The mean time taken by other 31 participants was 82 ± 2 s.

The sensitivity of cone adaptation test was 31/32 × 100 = 96.9% (95% confidence interval 91-100). There was one false negative (3.1%). Specificity was 100% and false positive was 0%.

The mean time of cone adaptation test was 5 min. The mean time for ERG was 20 min and clinical examination including dilation of the pupil was 45 min.

The cost of cone adaptation test was 150 dollars, and it was assumed that it would be useful for 1000 tests. The cost of LED light table lamp was 25 dollars and it was assumed to have a similar life as the test kit. The unit cost of test and lamp was 0.12 and 0.3 US $, respectively. The salary of the ophthalmic technician performing test using 15 min per child was US $ 1.25. The cost of space utilized to perform this test was US $ 1. Thus, the total cost for performing one test was US $ 2.5. The unit cost of ERG that was charged by our "no-profit-no-loss" institution to a private patient was US $ 10 and it was used as an index to compare the cost. The cost of ERG charges in the USA was US $ 150.

 Discussion



Our study found that cone adaptation test is a valid and cost-effective test as a screening tool to detect RP. Consanguinity rate among parents of children with RP was 43%.

The diagnosis of RP primarily is based on the symptoms and clinical findings. Unfortunately, no treatment modalities are currently available. [9],[10],[11] In such situation, eye care providers want to ensure that the patient is diagnosed correctly. Therefore, they often prescribe expensive tests such as ERG and genetic tests. [4] In developing countries, not all patients can afford these expensive tests. Therefore, a search of a reliable and less expensive test is always welcome.

The reliability of cone adaptation test in both photopic and mesopic illumination by normal healthy persons has been documented in literature. [5],[12] Therefore we did not take normal as control to compare.

The parents seek clinician's advice regarding possibilities of retinal dystrophies in their other children. A study to review the usefulness of cone adaptation test in identifying preclinical RP among siblings is recommended. The ethics of screening stresses on the availability of intervention and thus the benefit of detecting condition before symptoms develops. [13] In case of cone adaptation test for RP, early detection could prepare the patient as well as caregivers for low-vision rehabilitation and improve the quality of life.

The consanguinity among parents of children with RP in our study was 43.8%. This information could be useful to understand subtypes of RP such as autosomal dominant RP and autosomal recessive RP. [11] The consanguinity rate in our study was higher than 30% that was reported in a study in rural South India. [14] This seems to support genetic etiology of RP and role of consanguinity as a risk factor for RP.

The risk of other siblings for getting RP, if one child already has RP, is projected to be 19%. If the parents have RP, the risk of RP in other siblings increased to 29% in a study in Japan. [15] In our study, the rate of sibling having RP increased from 22% to 28% among those having a parental history of RP. One should note the sample size of 378 in Japan study compared to 32 in the present study. We wanted to study the additive effect of history of RP among siblings on the validity of cone adaptation test as a screening tool. However, we could not undertake this exercise due to 100% specificity of the cone adaptation test.

 Conclusion



Cone adaptation is a valid and cost-effective screening tool test for RP. The consanguinity rate among parents of an RP patient was high.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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