RP is one of the leading causes of severe visual impairment in young individuals [15]. Patients with RP often have difficulties with daily activities. Most of them have difficulties in navigation, orientation, and obstacle detection. Among different measures of visual function, VF area has been shown to be the best predictor of poor mobility in patients with RP. Humphrey VF has been shown to be beneficial in assessing the residual central VF of patients. In a large study including data of 928 RP patients [10], the researchers evaluated correlations of MD with visual acuity. They also evaluated potential influences of gender, age, family history and retinal pigmentation on the MD decreasing rate. They found that average VA and MD were 0.79±0.35 and -14.44±8.61 dB respectively. The results showed that when MD was lower than -9.18 dB the visual acuity would be below 1.0 (20/20). The average decreasing value of MD in 10 years’ period was reported as -8.01±3.66 dB and it was correlated to retinal pigmentation but not to gender, age or RP family history.
Most of the recent studies investigated the correlation of VFMD values with other clinical tests. It is known that, OCT examination has provided useful information about the pathology and the prognosis of the disease. The OCT studies showed a shortening of the EZ length and a thinning of the outer retinal layers in eyes with RP. In a study evaluating the progression of OCT findings observed progression in >75% of patients during the 2 year follow up and the the mean annual progression rate of of EZ line was 4.9%. This study was also the first to demonstrate asymmetrical structural progression rate between right and left eye in 19% of patients [16]. A recent study analyzed data of 149 RP patients who reported VF constriction on a central 30-2 Humphrey VF chart. The authors reported that BCVA and VF showed a progressive worsening related to age and disease duration and the progression in VF significantly correlated with the decrease in CMT, EZ length, and macular volume at the central area [5]. Another study including 53 eyes of 27 patients assessed the annual progression rate of photoreceptor atrophy by measuring EZ line in OCT sections through the fovea. During the 4.84 years mean follow up time, the EZ line width decreased with a yearly average rate of 76.4 μm (4.16% / year) which was in accordance with the reported rates between 4.9–10.9% in published literature. [8,9].
To evaluate the efficacy of new treatment options, it is important to measure the severity of the disease. To the best of our knowledge, there is only one study in the literature published by İftikhar et al. including a severity classification system established with the parameters of BCVA, VF and OCT. This classification is applicable for almost all patients regardless of any variations in disease phenotype and may be useful to assess, monitor and compare disease severity in clinical health services and researchs. The authors reported that almost all patients demonstrated a VF extending significantly beyond the edges of their remaining EZ [6]. This led us the opinion that the EZ probably represents organised or densely packed photoreceptors and that there may be scattered or fragmented photoreceptors beyond the edges of the EZ that are alive and functioning [6]. Although this classification is simple and easy to perform, we believe that subjective measures like visual acuity and VF may incompletely demonstrate the patient’s experiences of the daily life and disease severity.
It is known that ERG is a gold standard test for evaluating RP because it is an objective and quantitative measure of global retinal function. Unfortunately, the test is difficult and time-consuming and can be extinguished in the early stage of the disease, when the central visual acuity is still entirely preserved [11]. Because the traditional ERG does not seem to be sensitive enough to indicate the condition of the central retina, other methods have been sought. The mfERG technique, which allows a highresolution mapping of the macular area of the retina seems to be a more promising method for detection of the remaining foveal cone function which can be detectable even in advanced stage of the disease. [12].
In a study by Granse et al. [14] researchers evaluated residual retinal function with three different electrophysiological methods (ffERG, mfERG and mfVEP) in a selected group of RP patients with a remaining small central visual fields. Although the ffERGs were severely reduced in all patients, mfERGs were detectable in most of the patients with reliable responses over 5.0 nV. The mfVEPs also showed measurable amplitudes centrally in most of the patients. These findings corresponded well with the remaining central visual fields. The authors suggested that these two electrophysiological methods, mfERG and mfVEP, might be of clinical importance for evaluating and monitoring the residual central retinal function and small remaining central visual fields in patients with RP.
In another clinical study, researchers assessed central retinal function in patients with advanced RP using the mfERG. They reported that mfERG responses were recordable in at least one area in all successfully tested patients with advanced RP and nonrecordable ffERGs [18].
To the best of our knowledge this is the first clinical study including mfERG in a classification system. We believe that including mfERG as a parameter would increase the value of the classification.
The study includes a large sample size and a wide range of patients in terms of age, sex, mode of inheritance and disease duration. We believe that this classification produces objective measure of disease severity and gives opportunity to compare the results of different treatment modalities.