EXPERIMENTAL AND CLINICAL TRANSPLANTATION

Key words : Kidney transplantation, Liver transplantation, Optical coherence tomography angiography, Vascular density

Introduction
Routine ophthalmologic examination is crucial in pre-operative and postoperative follow-up of renal transplant (RTX) and liver transplant (LTX) patients, as it is known that ocular structures can be affected by these surgeries. After RTX, retinopathy associated with worse transplant function and microcirculation damage has been shown in previous research.¹ However, RTX itself can also stabilize preexisting retinopathy of patients by improving the systemic circulation of the patients.^1,2 Systemic inflammation, regional microcirculatory changes, and substantial systemic hemodynamic adaptations are typical in individuals with end-stage liver disease.^3,4 The hemodynamic changes and macrocirculation have been studied previously; however, the microcirculation changes in these patients are still a naive topic. Only a reduced chorioretinal thickness correlated with endothelial dysfunction, which resolved after LTX, has been reported previously, which may act as a dynamic reflection of microcirculation.⁵

Macular optical coherence tomography angiography (OCTA) is a noninvasive, reliable, and quick retinal imaging method widely used in ophthalmology. By calculating the motion contrast caused by the movement of red blood cells in the chorioretinal microvasculature, OCTA allows direct visualization of perfused vessels in superficial and deep retinal layers and quantitative evaluation of chorioretinal microvascular structures.⁶

Organ failure, poor responsiveness to vasoactive medications, and higher mortality are all related to microcirculatory dysfunction. The retinal vasculature is a well-established noninvasive indicator of the systemic microvascular health.⁷ We are aware of no published reports that outline the macular OCTA findings in transplant patients. Our purpose in this study was to determine if there are any retinal microvascular changes in RTX patients and LTX patients that may reflect subclinical vascular pathologies noninvasively.

Materials and Methods
This retrospective, cross-sectional study protocol was approved by the Ba?kent University’s local ethics committee and adhered to the guidelines of the 1975 Declaration of Helsinki. We included pediatric patients who had undergone RTX or LTX surgery from January 2012 to December 2021 at Başkent University Department of General Surgery, as well as age-matched and sex-matched healthy participants (control group). Three groups were formed for the analysis. Group 1 consisted of the LTX patients, group 2 consisted of the RTX patients, and group 3 was the control group, which consisted of healthy participants matched for age, sex, and spherical equivalent.

Patients between ages of 4 and 18 years were included. The data of best-corrected visual acuity, slit-lamp biomicroscopy, dilated fundus examination findings, and OCTA parameters were recorded for all patients. The demographical characteristics, ophthalmologic examination findings, and OCTA images of the patients were evaluated. Only patients with a signal strength ?6/10 on OCTA were included.

Exclusion criteria for both groups were the presence of severe media opacities, such as corneal opacities, cataracts, or dense vitreous hemorrhage, that would interfere with OCTA image quality; refractive errors of 6 diopters or more; any additional retinal and systemic vascular diseases; chronic and uncontrolled hypertension and diabetes; any optic nerve diseases; and any history of ocular trauma or intraocular surgery. Patients with retinopathy related to their primary disease were also excluded.

The macular images were obtained with an OCTA imaging system (Avanti RTVue XR, version 2017.1.0.151; Optovue, Inc.). The AngioAnalytics software of the Optovue system, which contains an automated segmentation algorithm, was used to evaluate the vessel density (VD) measurements of the superficial capillary plexus (SCP) and deep capillary plexus (DCP) in the macular 6 × 6-mm region and in the perifoveal region, as well as the foveal avascular zone (FAZ) area in square millimeters (mm2). From 3 ?m below the internal limiting membrane to 15 ?m below the inner plexiform layer, the SCP was identified. From 15 to 70 ?m below the inner plexiform layer, the DCP was described. The Avanti device is equipped with internal software to automatically calculate the VD values. The program creates a 1.0-mm diameter circle centered on the macula, which is defined as the foveal area. A 2.0-mm-wide annulus surrounding the fovea is defined as the parafovea, whereas a 3.0-mm-wide annulus around the parafovea is classified as the perifovea.⁸ The whole image macular VD was calculated in the 6 × 6-mm area around the fovea. Furthermore, the zones were separated into 4 quadrants (temporal, nasal, inferior, and superior) as well as 2 equal hemispheres (superior and inferior). The signal strength values were provided automatically to define the image quality based on the intensity or brightness of the reflected light during scanning.

The FAZ area and VD values in the SCP and DCP were the parameters measured with the OCTA method. In addition, we evaluated the VD values in each layer in whole image macula; perifoveal, parafoveal, and foveal; inferior hemisphere and superior hemisphere perifoveal/parafoveal; and nasal, inferior, temporal, and superior perifoveal/parafoveal regions. The FAZ area in square millimeters in full retinal vasculature was calculated automatically by the FAZ assessment tool.

The data were analyzed with SPSS statistical software (version 25.0). Descriptive statistics were used to define the data. Analytical evaluations were made to compare the groups. For continuous variables we used the Mann-Whitney U test, and for categorical data we used the chi-square test. P < .05 was considered statistically significant.

Results
Data were reviewed for 175 patients, of whom 87 had undergone RTX and 88 had undergone LTX surgery. Of these 175 recipients, 10 RTX and 16 LTX patients fulfilled the inclusion criteria and were enrolled in this study. We included 32 eyes of 16 LTX patients (group 1: 11 male, 5 female), 20 eyes of 10 RTX patients (group 2: 4 male, 6 female), and 64 eyes of 32 healthy controls (group 3: 16 male, 16 female). There were no statistically significant differences in sex between group 1 and control group 3 or between group 2 and control group 3 (P = .08 and P = .43, respectively).

The median age in group 1 was 13.5 years (interquartile range [IQR], 7.75 years), group 2 was 12 years (IQR, 6 years), and group 3 was 12 years (IQR, 3 years). There were no statistically significant differences between group 1 and group 3 or between group 2 and group 3 (P = .61 and P = .93, respectively). The median spherical equivalent of group 1 was +0.50 diopters, group 2 was +0.25 diopters, and group 3 was +0.50 diopters. There was no significant difference between the groups (P > .05 for all comparisons).

All patients had normal ophthalmologic examination findings. The distribution of the primary diagnosis of the patients was evaluated. Among the patients who had undergone LTX, 62% of the patients had biliary atresia, 12% had Alagille syndrome, 12% were idiopathic, 6% had citrullinemia, and 6% had hepatoblastoma. Among the patients who had undergone RTX, 40% had vesicoureteral reflux, 20% had cystinosis, 20% had nephronophthisis, and 20% were idiopathic. Three patients had controlled systemic hypertension in our study population. One patient from group 1 used enalapril, and 2 patients from group 2 used amlodipine.

For OCTA images, the median image quality scores were 7 (range, 6-9) in group 1 and group 2 and 7.5 (range, 6-10) in group 3 (P = .30). The OCTA features including acircularity index (AI), flow area, FAZ, and VD values of the SCP and DCP were noted (Table 1 and Table 2). The median FAZ area was 0.31 mm² (IQR, 0.15 mm²) in group 1, 0.29 mm² (IQR, 0.23 mm²) in group 2, and 0.22 mm2 (IQR, 0.09 mm2) in group 3. In binary comparisons of the FAZ area values, there was a significant difference between group 1 and control group 3, whereas group 2 was found to be similar to control group 3 (P = .01 and P = .19, respectively). The median AI was 1.09 mm² (IQR, 0.03 mm²) in group 1, 1.10 mm² (IQR, 0.03 mm²) in group 2, and 1.09 mm² (IQR, 0.03 mm²) in group 3. Binary comparisons of the AI values showed no significant difference between group 1 and group 3 or between group 2 and group 3 (P = .45 and P = .17, respectively). Significant reductions in VD values for both SCP and DCP were observed in both the LTX group and the RTX group compared with the control group (Figure 1 and Figure 2).

Discussion
We used OCTA to evaluate retinal microvascular alterations in pediatric patients who had undergone LTX or RTX surgery, in an attempt to discover any evidence of microcirculatory dysfunction.

The FAZ is a specialized retinal region with the highest cone photoreceptor density and oxygen consumption, and it usually has a regular, circular, or elliptical configuration in healthy people.^9,10 Previous studies have shown that the size of the FAZ tends to increase in eyes of patients with systemic diseases that affect vascular structures, such as diabetes or hypertension.^11,12 The mechanisms for FAZ area enlargement are thought to be progressive capillary dropout and capillary closure.^9,13 Our results show that FAZ was enlarged in both the RTX group and the LTX group compared with the healthy control group, although the difference was only found significant in the LTX group.

The AI value indicates the degree of FAZ border deformation by comparison of the perimeter of the FAZ and perimeter of a circle with the same area as the FAZ.¹⁴ The AI was found to be similar between the groups in our study, whereas FAZ area was altered. Our results for FAZ and AI are comparable with the conclusion of a recently published meta-analysis. In that meta-analysis study, the AI was found to be less affected in diabetic retinopathy compared with FAZ, and the authors commented that AI may not be a sensitive indicator for detection of microvascular changes.¹⁴

Another finding in our study was statistically significant reduction in VD values for the macular SCP and DCP. We observed significantly reduced VD values in both the LTX group and the RTX group compared with the healthy control group. Although this is the first study to investigate the VD values measured by OCTA of transplant patients, previous OCT studies have shown significant changes in retinal and choroidal measurements.^5,15,16 Retinal and choroidal changes have been observed in RTX patients, as reported by van Dijk and colleagues.¹⁶ They evaluated ophthalmologic findings of RTX patients on a regimen of low-dose steroids who had no visual complaints, and retinal abnormalities were found in 59% of the patients. Subclinical central serous retinopathy, epiretinal membrane, and increased subfoveal choroidal thickness were noted in these patients. The authors concluded that retinal changes are common in RTX patients and that these changes could be related to both the renal disease and the effect of steroid use.¹⁶ Maldonado and colleagues¹⁵ have reported a case of neonatal liver failure with macular edema and subretinal fluid that resolved spontaneously after LTX. Also, Gifford and colleagues⁵ have observed that retinal and choroidal thicknesses were affected in patients with end-stage liver disease and LTX recipients. The authors showed that retinal thickness, macular volume, and choroidal thickness were reduced in end-stage liver disease. These changes all resolved after transplant surgery and correlated with the circulating markers of endothelial dysfunction.⁵ Therefore, Gifford and colleagues⁵ have suggested that, by monitoring these changes during patient follow-up, OCT measurements could demonstrate a noninvasive window to the microcirculatory status of these patients. In our study, we have not compared the choroidal and retinal thickness measurements, but we have found that retinal vasculature was affected in transplant patients.

Previously published reports have proved that retinal and choroidal microvascular alterations are related to systemic circulatory dysfunction.¹¹ The microvascular retinal damage can occur as a result of underlying systemic macrovascular disorders and therefore could act as an indicator of end-organ damage and also could predict cardiovascular risk and mortality in many diseases such as cardiovascular diseases, stroke, cardiac failure, nephronopathies.^17-19 Systemic hypertension, which is known to affect the microvasculature, can cause retinal damage and was found to be associated with reduced VD values in macular SCP and DCP correlated with disease severity.^17-23 In a study by Lim and colleagues,¹⁹ patients with hypertension were divided into groups according to disease duration, and the authors found that chronic hypertension for more than 5 years resulted in decreased retinal blood flow, whereas hypertension for less than 5 years did not. As systemic hypertension is known to affect retinal VD values, to overcome this effect in our study, we excluded patients with chronic uncontrolled systemic hypertension. Also, a reduction in VD values in both SCP and DCP was identified previously in diabetic patients without retinopathy.^24-27 Microvasculature alterations in retinal OCTA images were also shown to be a potential predictor of cardiovascular risk profile in coronary artery disease patients.^28,29 Retinal microvascular changes and reduced parafoveal VD values associated by renal function deterioration were also shown in chronic renal disease patients by Yeung and colleagues.³⁰ Based on these previous published studies, our results showing the altered VD values in SCP and DCP in LTX and RTX patients can be evaluated as a reflection of systemic vascular damages.

The most important limitation of our study was the lack of preoperative data of the patients. This was caused by the retrospective cross-sectional design of the study. In addition, the postoperative medications may have influenced our results. Therefore, in future studies, preoperative measurements should be recorded, and the effects of postoperative medication should be investigated. Also, this study had limited statistical power because of the small number of cases. We had a limited number of patients, and we only excluded patients with uncontrolled hypertension. Three patients in our study group had controlled systemic hypertension, and despite the fact that their hypertension was under control with medication, this condition may have influenced our results; therefore, future studies should include this in the exclusion criteria.

Despite the limitations, to our knowledge, this is the first study to investigate the OCTA changes in RTX and LTX patients. We observed several vascular alterations in transplant patients compared with the healthy group. To improve the knowledge regarding microvascular changes in transplant patients and the systemic correlations of these changes, further studies are needed that compare preoperative and postoperative findings.

Conclusions
The OCTA method could be a useful tool for noninvasive evaluation of a patient’s vascular profile, and it holds the potential to be used in the algorithms for diagnosis and prognosis of the postoperative follow-up of transplant patients.

Volume: 20 Issue: 5 May 2022 - Supplement - 3

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