We retrospectively reviewed the medical records of a total 18 consecutive patients with a single active toxoplasma retinochoroiditis lesion examined between January 2018 and February 2020 at our department. This study was driven in accordance with the Declaration of Helsinki, current Turkish legislation and with the approval of local ethics committee (Approval ID: 2021/04 − 01).
Complete past medical and family history was obtained. Full ocular examination including visual acuity evaluation, intraocular pressure measurement, slit-lamp biomicroscopy and dilated fundus examination was performed in all cases. All clinical descriptions were made according to Standardization in Uveitis Nomenclature (SUN) criteria. At baseline, SS-OCTA examination (DRI OCT Triton Plus® SS, Topcon Corporation, Tokyo, Japan) was performed in addition to ancillary techniques of fluorescein angiogarphy and optical coherence tomography. The commercial DRI Triton instrument operates with a wavelength of 1050 nm and a scan speed of 100.000 A-scans per second. It has a transversal resolution of 20 µm and an axial resolution of 7 µm. All study eyes were scanned using a 6x6 mm image centered on the active lesion site. OCTA images (superficial capillary plexus (SCP), deep capillary plexus (DCP), outer retinal and choriocapillaris (CC) layer slabs) were obtained with the automated layer segmentation performed by the instrument. Measurements were acquired by M.K., F.A., or B.A.. OCT-A examination was performed at least three times and the best one was selected for the image analysis.
Nineteen eyes of 18 patients with a single acute toxoplasma retinochorioditis lesion were examined but a total of 14 eyes of 13 patients were included as good quality OCTA images could not be obtained in the remaining patients .Though the diagnosis of active toxoplasma retinochoroiditis was established on the clinical criteria suggested by Holland et al  all patients were also tested for Toxoplasma serology and received a combination of sulfamethoxazole and trimethoprim (160 mg/800 mg twice a day for four to six weeks) and azithromycin (500 mg once a day for nine days). Oral corticosteroid therapy (0,5 mg / kg / day prednisolone equivalent) was added only in patients with sight-threatening macular involvement 48 hours after the administration of antitoxoplasma drugs. Systemic corticosteroid therapy was ceased with a rapid taper before the discontinuation of antiparasitic therapy.
OCTA examination was repeated at each follow up visit.
The SS-OCTA of 6x6 mm the CC slab images were analyzed for each lesion. First, the original gray level images were converted to binary using thresholding. The pixel values below and above the threshold value were assigned as 0 and 1, respectively. While black pixels represent non-detectable flow signals, white pixels represent healthy retinal tissue . Here, the selection of the threshold value has critical importance on the result. Since the use of a fixed threshold value may cause difficulties due to varying image characteristics across the patients, it was determined automatically with Otsu's method . This method chooses the threshold value that can minimize the in-class variance of black and white pixels. In this respect, the “imbinarize” function of Matlab was used.
After the binarization process, the center of the disease was marked by the clinician. Then, by taking the expert indicated point as the center, a grid of concentric circles of increasing diameter from the center of the lesion site (i.e. 500, 1000, 1500, 2000, 2500 µm) was created to identify the most representative lesion area to analyze. The optimal diameter was chosen as 1500 µm to cover all of the lesions (Fig. 1).
Finally, for each circled area, the numbers of 0 (i.e. black) and 1 (i.e. white) valued pixels were calculated. The ratio of the number of black or white pixels to the total number of pixels is defined as the pixel index. The pixel index was counted first at the time of diagnosis and at the first follow up visit when the chorioretinal scar formation was noticed. We considered the lesion as a healed lesion when a pigmented atrophic scar with a relatively sharp margin was observed together with a decreased vitritis.
All patient data were recorded in the IBM SPSS Statistics (version 24.0, IBM Corp, 66 Armonk, NY, USA) software package and statistical analyses were performed using paired t-test. Normality of the variables was examined using histograms. The correlation was analyzed using Spearman’s correlation coefficient. The results are presented as mean ± standard deviation, and a p value < 0.05 was considered statistically significant.