Preoperative Pars Plana Measurements Possible With AS-OCT

Preoperative pars plana measurements using anterior segment optical coherence tomography (AS-OCT) enables accurate assessments of highly myopic eyes, according to a study published in Retina. This method may be exploited for the optimization of the entry site in the vitrectomy of these eyes, the authors claim.

This prospective study included 23 eyes (14 women; mean age,: 63.2±11.4 years; mean axial length, 29.24±2.32mm) diagnosed with myopic tractional maculopathy at Yokohama City University Medical Center between January 2022 and June 2022. All eyes underwent pars plana vitrectomy (PPV) and membrane-peeling surgery. 

AS-OCT was used in all eyes for preoperative pars plana measurements of length. A raster scan covering 15X5 degrees (8.3X2.8 mm) over the limbus, consisting of 11 B-scans with an interscan space of 277 µm, was obtained. Automatic real-time (ART) function was set at 36 frames.

The length between the limbus and the end of the ciliary body (which is equivalent to the ora serrata) was measured using the device’s software by 2 independent examiners in 3 different B-scans of the superotemporal and superonasal quadrants. The average between preoperative pars plana measurements was used in the analysis. The mean intraclass correlation coefficient between the individual measurements was 0.91 (95% confidence interval, 0.89-0.93).

Intraoperative measurement of the pars plana was performed as follows: The scleral location over the ora-serrata was identified and marked using scleral indentation (in the superotemporal and superonasal quadrants). The distance between the limbus and the marked location on the sclera was measured. This was done twice to get an average length.The mean temporal and nasal pars plana length using the AS-OCT was 6710±459 µm, and 5610±332 µm, respectively, vs a temporal length of 6671±402 µm and nasal length of 5700±390 µm, according to the intraoperative measurements. There was no significant difference between the 2 groups in either temporal or nasal length (P =.151, P =.221).

The average difference in length between the AS-OCT derived preoperative pars plana measurements and intraoperative measurements was 140±45 µm in the temporal region and 204±31 µm in the nasal region.

The mean length of the entry site from the limbus was 6.2 mm, and 28 mm forceps were used during membrane peeling in 17 eyes (77%). The researchers report no severe intraoperative surgical complications.

Performing delicate surgical procedures, such as membrane peeling, in highly myopic eyes is challenging due to difficulties in reaching the retinal surface with normal-length forceps and the flexibility of forceps with longer shafts. Placing the trocar at a site further than the normal 4 mm from the limbus may facilitate access to the retina. 

The authors explain that AS-OCT can be useful to evaluate preoperative pars plana measurements of length in patients with high myopia. “It provides us with an accurate estimate of pars plana length enabling safe placement of the sclerotomy while also allowing the most posterior entry into the vitreous chamber providing easier access to the retina in these myopic eyes.”

The limitations of the study include a relatively low number of eyes.