The Egyptian Journal of Neurology, Psychiatry and Neurosurgery

: 2016  |  Volume : 53  |  Issue : 1  |  Page : 1--5

Monitoring occlusion therapy in amblyopic children using pattern visual evoked potential

Radwa M Azmy1, Rasha H Zedan2,  
1 Department of Clinical Neurophysiology, Faculty of Medicine, Cairo University, Cairo, Egypt
2 Department of Ophthalmology, Faculty of Medicine, Cairo University, Cairo, Egypt

Correspondence Address:
Radwa M Azmy
MD, Department of Clinical Neurophysiology, Faculty of Medicine, Cairo University, Cairo


Background Amblyopia is a unilateral or bilateral reduction of best-corrected vision that cannot be attributed only to a structural abnormality of the eye. It is a common childhood problem, and can be reversed if treated while the visual system is still maturing. Objective The aim of the present study was to investigate the role of pattern visual evoked potential (PVEP) in the assessment of visual function after occlusion therapy for children with unilateral strabismic amblyopia. Patients and methods Visual function was assessed clinically and using monocular PVEP, elicited by high-contrast checkerboard-patterned stimuli, before and after occlusion of the healthy eye for 1 week for every year of life in 20 children presenting with unilateral strabismic amblyopia. Results PVEP responses were significantly prolonged in latency in amblyopic eyes after the first assessment compared with nonamblyopic eyes. There was no significant difference in the interside amplitude. Assessment after occlusion showed a statistically significant reduction in the latency of the amblyopic eye and statistically significant improvement in the visual acuity. Conclusion PVEP can be used as a method of assessment of visual function after the occlusion therapy for children with unilateral strabismic amblyopia.

How to cite this article:
Azmy RM, Zedan RH. Monitoring occlusion therapy in amblyopic children using pattern visual evoked potential.Egypt J Neurol Psychiatry Neurosurg 2016;53:1-5

How to cite this URL:
Azmy RM, Zedan RH. Monitoring occlusion therapy in amblyopic children using pattern visual evoked potential. Egypt J Neurol Psychiatry Neurosurg [serial online] 2016 [cited 2022 Sep 30 ];53:1-5
Available from:

Full Text


Management of problems in preverbal children is considered challenging for ophthalmologists. Preverbal children may be either unable or unwilling to cooperate [1].

Amblyopia is an acquired unilateral or bilateral decrease of visual acuity without an evident structural or pathologic cause that can be detected by using an ophthalmological examination [2]. It can be defined clinically as a two-line difference in best-corrected acuity between the eyes [3]. Visual impairment is caused by abnormal visual experience during early childhood. The most common types of amblyopia are strabismic and anisometropic amblyopia [3]. Amblyopia has an incidence of 2-2.5%, and is a condition in which visual acuity is subnormal in spite of the absence of obvious pathology [4].

Various attempts were made to assess which factors reflect the final visual outcome after amblyopia treatment [5]. No significant anatomic or physiologic abnormalities in the retina were reported in several studies [6]. Reduced visual acuity in amblyopia probably results from abnormal neuronal network within the primary visual cortex. Neurons of the primary visual cortex lose their ability to respond to stimulation of eyes, in addition to the abnormalities in neurons in the lateral geniculate body [7].

Patching is considered the main line of treatment, though it is not always successful due to noncompliance and recurrence [8]. Patching is based on the closure of the healthy eye to stimulate the amblyopic eye for vision, and helps the part of the brain that manages vision develop more completely [9].

Pattern visual evoked potential (PVEP) is considered an objective way to predict the visual acuity. It also demonstrates the central retinal function and can be used in deciding the type of treatment and follow-up of amblyopic patients [10],[11].

It has been reported that PVEP correlates with the best-corrected Snellen acuity in normal individuals [12]. Refractive errors affect the interpretation of the visual evoked potential (VEP) results, and so it is essential to correct a patient's refractive error, if pattern stimuli are to be used [13].

Treatment of amblyopia should be started before the beginning of the critical period, which is accepted as the proper time for the elimination of congenital problems. The end of the critical period for humans has been suggested between 6 and 12 years of age. The most sensitive age in children regarding amblyopia is the first 2 or 3 years of life, with gradual decrease of sensitivity until 6-7 years of age [14].


The aim of the present study was to investigate the role of PVEP in the assessment of visual function after occlusion therapy for children with unilateral strabismic amblyopia.

 Patients and methods

This study included 20 children presenting with unilateral strabismic amblyopia, who were referred from the Pediatric Ophthalmology Outpatient Clinic at Abul-Reesh Teaching Hospital, after obtaining approval of the local committee. All patients' guardians were informed about the nature of the study, and they signed an informed consent.

A thorough ophthalmological examination was carried out for all patients, including Snellen visual acuity in verbal children, cycloplegic refraction, muscle balance, and ocular motility and fundoscopy.

Monocular pattern-reversal visual evoked potential (PR-VEP) examination was performed before and after the occlusion therapy of the nonamblyopic eye using the same machine and under the same settings. The nonamblyopic eye was occluded using an eye patch for 4 h daily, either continuously or interrupted, for a period of 1 week for every year of age. Spectacles were used for the correction of error of refraction.

PVEP was carried out for all patients using Neuropack M1 (Nihon Kohden, Japan) apparatus. The active electrode was placed at Oz with the reference electrode at Fz according to the 10-20 international system. A separate electrode was attached at vertex Cz and connected to the ground, in a manner adhering to the ISCEV guidelines [15]. The patient sat on a comfortable chair or on their parent's lap at a distance of about 60-70 cm from a black-and-white, checkerboard-patterned, 20 inch, monochrome screen after the correction of vision. Gold disc electrodes were used for recording. The electrode impedances were kept below 5 kÙ. The mean luminance of the checkerboard was 50 cd/m 2 , the checks' size was 60 min arc, and the contrast between black and white squares was 100%, at a frequency of 1.0 Hz. The PR-VEP waveform consists of N 75 , P 100 , and N 135 peaks [16]. By 6 months of age the peak latency of the main positive peak of the pattern reversal VEP for larger checks (>30′) is usually within 10% of the adult values [14].

VEPs are recorded when the child is in an attentive behavioral state to maintain attention and fixation; recording trial is paused if the child is distracted, and is then resumed as the child regains adequate attention.

Statistical analysis

Statistical analysis was performed using a commercially available statistical software package (SPSS; IBM Inc., Chicago, Illinois, USA). T-tests and paired samples tests were used to compare VA, amplitudes, and latencies of amblyopic eyes with sound eyes. Pearson's and Spearman's correlation analysis and Fischer's exact test were also used to evaluate correlation of visual acuity and VEP measurement. A P value of less than 0.05 was considered statistically significant.


This study included 20 children: seven (35%) boys and 13 (65%) girls. Their ages ranged from 3.5 to 6 years with a mean age of 4.3 ± 0.68 years. Overall, 40% of the children presented with unilateral strabismic amblyopia had their right eye affected and the remaining 60% their left eye. Only 65% of the children were verbal and old enough to undergo the VA assessment before and after the occlusion therapy.

PVEP test was carried out before and after occlusion of the nonamblyopic fellow eye (sound eye); the data collected is shown in [Table 1] and [Table 2] in the form of mean and SD of P 100 latency and amplitude of the amblyopic and sound eyes, as well as minimum and maximum latencies and amplitudes of the examined eyes.{Table 1}{Table 2}

In the preocclusion examination, the P 100 latency was relatively delayed in the amblyopic eye compared with the sound eye with insignificant interside differences (range: 0.5-9 ms), and a statistically significant difference between the amblyopic and sound eyes' mean latency in the tests before and after occlusion.

On comparing the tests before and after occlusion, the value of the P 100 latency on PVEP showed improvement in 60% of the cases in the form of a reduction in the P 100 latency (range: 0.9-7 ms, mean: 4.6 ms) [Figure 1] with a statistically significant difference (P < 0.05) between P 100 latency in the amblyopic eyes before and after occlusion. In contrast, there was no statistically significant difference (P > 0.05) as regards the mean P 100 latency in the sound eyes before and after occlusion.{Figure 1}

The amplitudes of the P 100 responses showed no significant interside difference in tests before and after occlusion. There was no significant absolute or relative change in the amplitudes of the P 100 responses on comparing the tests before and after occlusion.

Overall, 30% of the children who had their VA assessed showed improvement in the VA with an average of ±1 line improvement in the postocclusion assessment. The vision in the remaining 35% verbal children did not change. Postocclusion improvement in VA and in P 100 response latency were statistically significant (>0.05) [Table 3]. There was no significant correlation between the postocclusion reduction in the P 100 latency of the amblyopic eye and improvement of the VA.{Table 3}

Postocclusion alternation of strabismus was noticed in 20% of the cases. The sound eye showed a nonsignificant minimal increase in P 100 latency in 35% of the patients. There was no significant correlation between the delay of the P 100 latency in the sound eye latency and alternation of strabismus [Table 4].{Table 4}


Managing preverbal children with amblyopia can be challenging. VEP offers an objective tool for diagnosing and managing amblyopia in preverbal children [17]. VEP can be applied to diagnose amblyopia, and monitor the effect of treatment, thus allowing prognosis of future treatment [6].

A large number of studies using conventional PVEP showed both increased latency and reduced amplitude of the responses from the amblyopic eye [18],[19], while in this study, the P 100 latency was relatively delayed compared with the sound eye with insignificant interside difference, considering the young age of the patients and that latencies in the high normal range for one or both eyes does not necessarily exclude an abnormality.

In the present study, the PVEP responses of the amblyopic eye in the preocclusion sitting showed no statistically significant difference concerning the amplitude of P 100 response, compared with the sound one, which is in agreement with a study by Halfeld Furtado de Mendonça et al. [17], although according to another study [20], VEP responses of the amblyopic eye had lower amplitude than that of the sound eye.

In amblyopic eyes, reportedly, the amplitudes of P 100 responses were significantly smaller and the latency significantly longer and these changes correlated to the reduction of VA [21]. Some authors suggested that an increase in the P 100 amplitude reflects vision improvement during amblyopia treatment [22].

Studies carried out on strabismics and anisometropes showed an increase in the P 100 latency in PVEP in both groups of patients, but the P 100 amplitude was reduced only in the anisometropic group [2]. Some authors stated that there was no relationship between mean VA and mean P 100 amplitude [17].

Our results not only showed insignificant interside difference before or after occlusion concerning the amplitude, but the amblyopic eye showed, at times, even higher amplitudes than did the sound one; these findings are in agreement with those of another study [23]. Some authors noted that most of the strabismic amblyopes showed little or no interocular difference in amplitude when using large checks [24], which were inconsistent with the checks used in this study. Nonsignificant interside amplitude difference could possibly be attributed to using large checks in the present study in addition to the possible lack of continuous focusing, which could affect the amplitude especially when taking into consideration the young age of our patients. In addition, this reinforces the suggestion that the sound eye cannot be considered completely normal. Some authors even attracted attention to the possibility of a negative effect on the sound eye caused by prolonged patching [25].

The present study did not detect amplitude improvement, yet a reduction in latency was observed with a mean of 4.6 ms, within a short period of follow-up. Authors reported visual improvement proportionate to the increase in the N 1 P 1 amplitude in anisometropic amblyopic patients treated with the occlusion therapy for 6 months [22].

In many studies, it was shown that P 100 wave amplitude increased and latency of P 100 decreased with the increase in visual acuity [22],[26],[27]. Similarly, a study by Cabi et al. [28] reported that the latency values decreased and the amplitude values increased in the PVEP records with an improvement in visual acuity in a study carried out on patients with strabismic amblyopia, their ages ranging from 7 to 16 years. Assessments before and after occlusion were performed 6 months apart. These results are in agreement with those of the present study as regards the decrease in latencies but not the increase in amplitude, which can be attributed to the young age and shorter duration of the follow-up. Although longer durations are better in assessing the treatment, this cannot be guaranteed because of patient noncompliance. The improvement in latency was not statistically correlated to the improvement in visual acuity because only 65% were verbal and old enough to have their visual acuity assessed.

Examination of the patients before and after occlusion therapy revealed conversion of unilateral amblyopia to alternating amblyopia in 20% of the patients. PVEP after occlusion revealed a minimal nonsignificant delay in the sound eye compared with the preocclusion test. The delay of the P 100 latency of the sound eye could not be significantly correlated to the conversion to alternating amblyopia.

We conclude that PVEP can be used as a reliable tool for monitoring the effect of occlusion therapy on the visual function in unilateral strabismic amblyopia, especially in young age. In addition, we recommend longer periods of follow-up and multiple assessment sessions.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


1Pine Brook NJ. Overcoming challenges in amblyopia. Detection using visual evoked potential (VEP). Diopsys VEP technology offers an answer. PRNewswire, 2012. Available at challenges in amblyopia detection using visual evoked potential vep. [Last accessed on 2015 Jan].
2 Heravian J, Daneshvar R, Dashti F, Azimi A, Ostadi Moghaddam H, Yekta AA, Esmaily H. Simultaneous pattern visual evoked potential and pattern electroretinogram in strabismic and anisometropic amblyopia. Iran Red Crescent Med J 2011; 13 :21-26.
3 American Academy of Pediatric Ophthalmology. Strabismus panel. Preferred practice pattern guidelines. Amblyopia, 2007. Available at: [Last accessed on 2015 Jan].
4 Heravian J, Ostadi-Moghaddam H, Yekta AA, Hasanabadi H, Mahjoob M. Pattern visual evoked potential in response to monocular and binocular stimulation in normal and amblyope subjects. Iran Red Crescent Med J 2008; 10 :69-74.
5 Iliakis E, Moschos M, Hontos N, Tsalouki JK, Chimonidou E. The prognostic value of visual evoked response latency in the treatment of amblyopia caused by strabismus. Doc Ophthalmol 1996; 92 :223-228.
6 Dickmann A, Petroni S, Salerni A. Unilateral amblyopia: an optical coherence tomography study. J AAPOS 2009; 13 :148-150.
7 Bach M, Hawlina M, Holder GE, Marmor MF, Meigen T, Vaegan, Miyake Y. Standard for pattern electroretinography. International Society for Clinical Electrophysiology of Vision. Doc Ophthalmol 2000; 101 :11-18.
8 Wong AM. New concepts concerning the neural mechanisms of amblyopia and their clinical implications. Can J Ophthalmol 2012; 47 :399-409.
9 Marmor MF, Zrenner E. Standard for clinical electro-oculography. International Society for Clinical Electrophysiology of Vision. Doc Ophthalmol 1993; 85 :115-124.
10Chung W, Hong S, Lee JB, Han SH. Pattern visual evoked potential as a predictor of occlusion therapy for amblyopia. Korean J Ophthalmol 2008; 22 :251-254.
11No authors listed]Guideline thirteen: guidelines for standard electrode position nomenclature. American Electroencephalographic Society. J Clin Neurophysiol 1994; 11 :111-113.
12Ohn YH, Katsumi O, Matsui Y, Tetsuka H, Hirose T. Snellen visual acuity versus pattern reversal visual-evoked response acuity in clinical applications. Ophthalmic Res 1994; 26 :240-252.
13Donnell J, Creel DJ. Visually evoked potentials. In: KolbH, FernandezE, NelsonR, editors. Webvision: The Organization of the Retina and Visual System. Salt Lake City, UT:University of Utah Health Sciences Center; 2011.
14Odom JV, Bach M, Barber C, Brigell M, Marmor MF, Tormene AP, et al. Visual evoked potentials standard (2004). Doc Ophthalmol 2004; 108 :115-123.
15Odom J, Bach M, Brigell M, Holder GE, McCulloch DL, Tormene A, Vaegan. ISCEV standard for clinical visual evoked potentials (2009 update). Doc Ophthalmol 2010; 120 :111-119.
16Chiappa KH. Evoked potentials in clinical medicine. 2nd ed. New York: Raven Press; 1990.
17Halfeld Furtado de Mendonça R, Abbruzzese S, Bagolini B, Nofroni I, Ferreira EL, Odom JV. Visual evoked potential importance in the complex mechanism of amblyopia. Int Ophthalmol 2013; 33 :515-519.
18Heravian J, Ostadimoghadam H, Yekta AA, Hasanabadi H, Mahjoob M. Pattern VEP in response to monocular and binocular stimulation in normal and amblyope subjects. Iran Red Crescent Med J 2008; 10 :69-74.
19Wright KW, Erikson KJ, Shors TJ. Detection of amblyopia with P-VEP during chloral hydrate sedation. J Pediatr Ophthalmol Strabismus. 2000; 24 :170-178.
20Arden GB, Barnard WM, Mushin AS. Visually evoked responses in amblyopia. Br J Ophthalmol 1974; 58 :183-192.
21Kubová Z, Kuba M, Juran J, Blakemore C. Is the motion system relatively spared in amblyopia? Evidence from cortical evoked responses. Vision Res 1996; 36 :181-190.
22Oner A, Coskun M, Evereklioglu C, Dogan H. Pattern VEP is a useful technique in monitoring the effectiveness of occlusion therapy in amblyopic eyes under occlusion therapy. Doc Ophthalmol 2004; 109 :223-227.
23Wanger P, Nilsson BY. Visual evoked responses to pattern-reversal stimulation in patients with amblyopia and/or defective binocular functions. Acta Ophthalmol (Copenh) 1978; 56 :617-627.
24Beneish R, Lachapelle P, Polomeno RC, Lake N. Pattern VEP differences in strabismic and anisometric amblyopia. Clin Vision Sci 1990; 5 :271-283.
25Odom JV, Hoyt CS, Marg E. Effect of natural deprivation and unilateral eye patching on visual acuity of infants and children. Evoked potential measurements. Arch Ophthalmol 1981; 99 :1412-1416.
26Shao C, Sun Y, Han W, Pu L, Zhang Y, Gao L, Yang H. The application of the pattern visual-evoked potential in the diagnosis and treatment of amblyopia. Yan Ke Xue Bao 1995; 11 :108-110.
27Parisi V, Scarale ME, Balducci N, Fresina M, Campos EC. Electrophysiological detection of delayed post retinal neural conduction in human amblyopia. Invest Ophthalmol Vis Sci 2010; 51 :5041-5048.
28Cabi C, Sayman Muslubas IB, Aydin Oral AY, Dastan M. Comparison of the efficacies of patching and penalization therapies for the treatment of amblyopia patients. Int J Ophthalmol 2014;:480-485.