Optical coherence tomography in photodynamic therapy

Sahni et al presented a new terminology to validate the reliability of optical coherence tomography (OCT) and studied the effect of photodynamic therapy (PDT) in patients with subfoveal, predominantly classic choroidal neovascularisation (CNV) secondary to age related macular degeneration (AMD).¹ The new terminology introduced neuroretinal foveal thickness (NFT), bilaminar foveal thickness (BFT), outer high reflectivity band thickness (OHRBT), intraretinal fluid (IRF), subretinal fluid (SRF), and vitreomacular hyaloid attachment. A heterogeneous group of patients were included. Whereas in three patients OCT scanning was performed before PDT, 53 had already undergone up to seven treatment sessions. The results showed a significant correlation of mean neuroretinal foveal thickness and intraretinal fluid. Additionally, there was a high correlation between bilaminar foveal thickness and visual acuity (VA). The authors also reported a poor agreement between OCT and clinical examination in the detection of cystoid macular oedema (CMO) and subretinal fluid, indicating the usefulness of OCT.

We congratulate the authors and want to add two aspects from our clinical experience. In a previous study we reported the presence of CMO and increased foveal thickness to correlate with a poorer visual outcome in non-treated neovascular AMD.² Sahni et al demonstrated no correlation between VA and IRF and SRF in patients undergoing PDT, giving evidence that a beneficial effect of PDT, especially for the group with exudative AMD associated with CMO, arises. OCT seems to be valuable in predicting functional outcome following PDT.

The current study presented no data on the actual time when the OCT evaluation has been performed, although several authors described remarkable OCT findings following PDT. In a prospective study we evaluated 24 patients (53 PDT sessions) before PDT as well as 2 days and 1 week after PDT by VA and OCT.³ Here, a transient hyperopic shift corresponded with a consecutive increased retinal thickness up to 680 µm on OCT. The cross sectional OCT scan in the macular area revealed a retinal detachment, whereas the anatomy of the neurosensory retina maintained intact, no relevant thickening or schisis-like changes were observed. Previous examinations by Costa et al, determined a few hours after PDT, indicated an increased subretinal leakage on indocyanine green angiography (ICG) and consecutive retinal elevation on OCT, confirming our functional and OCT findings.⁴ Long term OCT findings presented by Rogers et al demonstrated structural alterations in terms of persistent retinal thinning 3 months after PDT.⁵ Therefore, OCT findings before the first PDT are necessary to determine predictable signs for functional outcome. OCT is a useful technique describing morphologal findings of the retina and choroid. Nevertheless, a higher number of patients before the first PDT application have to be evaluated by OCT to define predictable signs elucidating visual outcome.