To achieve and maintain high quality of vision it is important to early diagnose and treat such ocular conditions as amblyopia. Having a reported prevalence of 1—4% amblyopia is one of the leading causes of decreased visual acuity (VA) in children aged 4—5 years [1—4]. And it is worth to remember that fellow eyes of amblyopic patients carry an increased risk of trauma. Studies support a three-fold increase in frequency of eye problems in fellow eyes of amblyopic patients in comparison to that in normal population [5]. Treatment of amblyopia is difficult and requires not only time and attention, but also parents’ motivation to achieve the main goal, which is to gain the highest possible visual acuity. Saint Yves was the first to introduce occlusion as a treatment method for amblyopia in 1722 [6]. Since then it has gained wide use and is still accepted as the most effective treatment choice for this condition. Alternative treatment strategies such as penalisation, pleoptic treatment, and Cambridge vision stimulator (CAM) treatment can be also prescribed to patients, in whom occlusion is not successful enough.

In 1978 Campbell and colleagues developed the so called CAM therapy for amblyopia and reported good results. The method implies presenting a series of slowly rotating line gratings of low to high spatial frequency to the amblyopic eye while the child is playing drawing games over the grating. The underlying principle is that neurons in lateral geniculate nucleus and striated cortex are best stimulated by stimuli with certain spatial frequencies. Hence, the treatment aims to restore the decreased function of neurons [7, 8].

The aim of this study was to evaluate changes in visual acuity and compliance after CAM therapy in patients that initially failed to respond to occlusion or were non-compliant.

A total of 29 patients (29 eyes), who had shown no improvement in visual acuity under occlusion therapy and full spectacle correction over at least a 4-month period, were enrolled. Patients with organic amblyopia and those under 4 and over 10 years of age were not included. Two groups were formed according to patient age (4—7 and 8—10 year olds) and two other groups — according to the type of amblyopia (anisometropic — group 1, strabismic — group 2).

Snellen chart was used for visual acuity testing. Amblyopia was defined as at least two Snellen lines interocular difference. Anisometropia was defined as ≥1.5 D interocular difference in spherical equivalent [9]. Cycloplegic refraction measurements, anterior and posterior segment evaluation, and strabismus testing were performed in all patients. Optical correction was prescribed for refractive errors, whereupon patients whose best corrected Snellen VA was 0.8 or better were excluded from further study. An uncompleted 6-day CAM therapy course was also an exclusion criterion.

CAM vision stimulator (Clement Clarke International (CCI), London, England) was used for treatment. In contrast to the technique described by Campbell et al. [9], in our case this was a 6-day therapist supervised clinic-based course. Six discs with high-contrast gratings of different frequencies that rotate 360째 in one minute were used. Each disc was to be viewed for three minutes. The aim of the treatment was to stimulate visual pathways of the amblyopic eye while carefully patching the fellow eye. Daily occlusions were discontinued for the period of CAM course and visual acuity was remeasured at the end of the therapy. Daily 2-hour occlusions of the better eye were then recommenced in all cases regardless of the age of the patient as well as the type and depth of amblyopia. It was advised to perform near work, such as reading, painting, and playing video games, for at least one hour while occluded. Visual acuity measurements were repeated in 3 and 6 months after CAM therapy.

Statistical analysis of patient data was performed with SPSS 13.0. Mann Whitney U Test was applied to evaluate the differences in the mean age, occlusion time, and follow-up period between the study groups. Wilcoxon Signed Ranks Test was used to evaluate VA changes after the CAM course. A p value <0.05 was considered significant.

This study was approved by the institutional review board (IRB).

Twenty nine patients (29 eyes), of them 17 (58.6%) males and 12 (41.4%) females, were included in the study. Patients’ age ranged from 4 to 10 years with the mean of 7.17±1.62 years.

As for age groups, there were 16 (55.2%) cases of 4—7 year olds (8 males and 8 females) and 13 (44.8%) cases of 8—10 year olds (9 males and 4 females). Distribution of cases according to the type of amblyopia and duration of occlusion therapy before the CAM course is shown in table 1. Duration of occlusion therapy in different age groups is shown in table 2.

Refractive errors were distributed as follows: astigmatism and hypermetropia — 19 (65.5%) cases, astigmatism and myopia — 4 (13.8%), hypermetropia — 4 (13.8%), and mixed astigmatism — 2 (6.9%).

The follow-up period generally ranged from 6 to 24 months with the mean of 14.8±5.2 months, but was a bit shorter in the anisometropic group (13.8±5.3 months) than in the strabismic group (15.9±5 months) (p>0.05).

The mean Snellen VA after the CAM course generally improved from 0.41±0.19 to 0.56±0.23. The detailed dynamics of VA by groups are given in table 3.

While before the CAM course low visual acuity (of 0.4 or lower) was found in 10 (34.48%) cases, after the therapy this number reduced to 7 (24.13%) cases. An immediate increase in VA by two or more lines after CAM therapy was registered in 11 (37.93%) cases, of them 7 from the strabismic and 4 from the anisometropic group. At 6 months this number rose to 15 (51.7%) (8 strabismic and 7 anisometropic cases). There were also 6 patients who did not improve after CAM therapy (two of these, however, showed a 1-line increase in VA at the 3-month follow-up visit).

Treatment complications such as diplopia, amblyopia in the occluded eye, intermittent strabismus, and latent nystagmus did not occur in any of the patients.

Many different methods for the treatment of amblyopia have been tested up to date, however, occlusion therapy is the most preferred due to its usability and inexpensiveness [10]. Patient compliance is the major predictive factor of therapeutic success under occlusion therapy, which ranges from 45% to 82% [11, 12]. There is currently no consensus on the optimal duration of occlusion. A Pediatric Eye Disease Investigator Group (PEDIG) study revealed no significant difference between full-time and part-time (6 hr/day) occlusion in deep amblyopia as well as between 2 hr/day and 6 hr/day occlusion in moderate amblyopia [13, 14]. In accordance with PEDIG recommendations, we prescribed to our patients 2 hr/day occlusions after the end of the CAM therapy course.

Since the introduction by Campbell and colleagues, CAM therapy has proved to be an alternative to ocular occlusion in patients with amblyopia. The method consists in activation of visual neurons in the visual cortex by gratings of different spatial frequency. In a study conducted by Banks et al. in 22 amblyopic eyes, the mean visual acuity increased from 6/40 to 6/17 after six sessions of CAM stimulation [15]. The results we achieved are very similar. The mean Snellen VA before the CAM course was 0.41±0.19 and thеn increased to 0.56±0.23 (p<0.01). There was a study, however, by Fricker et al. that compared the effect of television games played either with stripes superimposed on the screen image or without them. A 20 min/week course in 50 patients with amblyopia revealed no major difference between the results obtained during the sessions or follow-up period in subjects exposed to stripes and those from the control group [16]. These findings do not support the effectiveness of gratings used in CAM therapy.

The upper age limit for successful amblyopia treatment is 9—11 years, according to different studies [8, 17]. The age of our subjects did not exceed 10 years. We evaluated the effect of age on treatment results. We found that the increase in visual acuity was higher in 4—7 year olds (68.95%) than in 8—10 year olds (46.19%), though the difference was not statistically significant. Dorey et al. studied 39 subjects that had failed to respond to previous home-based occlusion therapy over 9 months on average. They were prescribed therapist supervised intensive occlusion therapy for 5 days in a pediatric ophthalmic ward. Twenty six (67%) children gained at least one line of acuity in their amblyopic eye and five (13%) gained three or more lines. In our study 23 (79.31%) patients gained at least one line of acuity and 11 (37.93%) gained two or more lines after the CAM course. These results suggest that CAM therapy may be as effective as intensive occlusion therapy in patients that had failed to respond to home-based occlusions.

The mean follow-up period in our study was 14.8±5.2 months (6—24 months). An increase in VA by two Snellen lines or more was observed in 11 (37.93%) cases at the end the CAM course and in 15 (51.72%) cases on the last visit. We think that a rapid acuity gain after CAM stimulation contributes to better compliance to further occlusion therapy and higher final visual acuity.

We do not have any commercial or proprietary interest in the product used in this study. We have no financial support to disclose.