Abstract
Purpose.:
To observe posterior precortical vitreous pockets (PPVPs) using swept-source optical coherence tomography (SS-OCT).
Methods.:
We performed SS-OCT in both eyes of 58 volunteers (36 men, 22 women) using 12-mm horizontal vertical scans through the macula and optic disc. To minimize age-related changes (liquefaction or posterior vitreous detachment), all subjects were a mean of 26.2 years (range, 22–40 years). The refractive errors ranged from −9.5 diopters (D) to +3.0 D. To estimate the PPVP size, we measured the height between the fovea and the anterior border of the PPVP and the maximal width in the 12-mm horizontal scan through the fovea and disc.
Results.:
SS-OCT visualized the PPVPs as boat-shaped lacunae in the macular area bilaterally in all subjects (maximal width, 3114–9887 μm; mean width, 6420.6; central height, 208–1877 μm; mean height, 708.1 in the right eyes, with no significant difference in the left eyes). There was a significant correlation between the PPVP height and myopic refractive error. The posterior wall of the PPVP was a thin vitreous cortex, thinnest at the fovea. The septum was between the nasal border of the pocket and Cloquet's canal, which extended forward and tilted superiorly in all cases. A channel connected Cloquet's canal and the PPVPs bilaterally in 54 (93.1%) of 58 cases.
Conclusions.:
SS-OCT clarified the boat-shaped PPVP structure in vivo. Although the central height increased with the myopic refractive error, the width was unchanged. A channel connecting Cloquet's canal and PPVP suggested the route of aqueous humor into the PPVP.
We performed SS-OCT (DRI OCT-1 Atlantis; Topcon, Tokyo, Japan) consecutively in both eyes of 58 healthy volunteers (36 men, 22 women) while they were sitting. This OCT system has an A-scan repetition rate of 100,000 Hz, and its light source operates in the 1-μm wavelength region. The light source is a wavelength-tunable laser centered at 1050 nm with a 100-nm tuning range; the axial resolution is 8 μm, the lateral resolution 20 μm, and the imaging depth 2.3 mm in tissue. The ocular fundus was scanned in six horizontal 12-mm scans that included the entire extent of the PPVPs, the centers of which were aligned at the plane through the optic disc and fovea, and two vertical 12-mm scans through the fovea and optic disc.
The subject ages ranged from 22 to 40 years (average, 26.2 ± 0.6 years). No eyes had an ocular disease. Subjects over 40 years of age were excluded to minimize age-related changes in the vitreous such as liquefaction or posterior vitreous detachment.
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The refractive powers were measured using a commercial topographer (Autoref-Topographer RT-6000; Tomey Corporation, Nagoya, Japan) and ranged from −9.5 diopters (D) to +3.0 D (average, −3.4 ± 0.3 D) in the right eyes and from −9.5 D to +3.0 D (average, −3.2 ± 0.3 D) in the left eyes.
During the OCT examination, we placed the scanner head backwards for anterior focusing, which allowed us to capture the anterior extent of the PPVP in the B-scan images. After obtaining the SS-OCT images, we adjusted the contrast to visualize the gel, liquefied pocket, and the cortex.
To estimate the size of the PPVPs, we measured the height between the fovea and the anterior border of the PPVP, according to our previous report,
11 and the maximal width in the 12-mm horizontal scan through the fovea and the disc (
Fig. 1).
All values are expressed as the mean ± standard error of the mean with the range. The differences between groups were analyzed using the Mann-Whitney U test or Wilcoxon signed-rank test. The correlations between groups were analyzed using Pearson's correlation coefficient test. P values less than 0.05 were considered significant.
The study was conducted according to the tenets of the Declaration of Helsinki. The institutional review board ethics committee approved the study. All individuals provided informed consent after having received a detailed explanation of the purpose of the study.
PPVPs first were observed in autopsy eyes in which the gel was stained with fluorescein.
1 Triamcinolone-assisted vitrectomy allowed intraoperative visualization of the PPVP.
2 Time-domain OCT depicted the vitreous cortex when it was slightly detached from the retina; however, the imaging failed to show the inner structure of the vitreous. Noise-reduced SD-OCT visualized the PPVP in most eyes. However, because of the low sensitivity of the vitreous and limited scan length, SD-OCT occasionally failed to visualize the entire PPVP.
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In the current study, SS-OCT with a 12-mm scan length visualized for the first time the entire structure of the PPVP. Because of high penetration and high speed, SS-OCT was superior to SD-OCT for depicting the choroid and the vitreous compared. If the vitreous was enhanced by increasing the brightness and contrast in the OCT images, the vitreous structure always was observed if it was present, although the quality of the retinal images decreased due to excessive brightness and contrast.
The PPVPs appeared as boat-shaped vitreous lacunae in the macular area, and the posterior wall of the PPVPs was a thin vitreous cortex attached to the retina. Worst described the bursa premacularis as a pear-shaped sack with its own outer membrane.
9 The bursa premacularis is situated on the convexly elevated posterior vitreous membrane (
Pars patelliformis membranae vitrealis), which forms subbursal premacular space. The PPVP is probably the same space as the bursa premacularis described by Worst, except there are no membranes, just a layer of premacular cortical vitreous that are adherent to the macula in young adulthood.
While the posterior wall of PPVP is a thin vitreous cortex itself, the anterior border is vitreous gel. The mean height of the PPVPs was 708.1 μm (range, 208–1877) and the mean maximal width was 6420.6 μm (range, 3114–9887) in the right eyes. The PPVPs were symmetrical in both eyes of each individual; thus, the size of the PPVPs did not differ in the left eyes. The height of the PPVPs increased along with the myopic refractive error (
Fig. 5). Using SD-OCT, we examined the size of PPVPs without highly myopic eyes and reported that the mean height was 0.5 ± 0.2 mm at the fovea and the horizontal diameter was a mean of 6.0 ± 0.8 mm.
5 The dimensions of PPVPs in the current study were slightly larger than our previous report because this study contained high myopia probably. In the vertical scans, the superior portion of the PPVP always enlarged when the participants were sitting (
Fig. 2B), as we reported previously.
11 Gravity may affect the shape of the PPVPs.
A noteworthy finding in the current study was a connecting channel between the nasal side of the PPVPs and the temporal side of Cloquet's canal in 93.1% of cases, the presence of which suggested that the PPVPs are not isolated lacunae. If Cloquet's canal extended to the posterior chamber behind the lens (Berger's space), aqueous humor may drain into the PPVPs via the connecting channels. We described the role of PPVPs in vitreoretinal interface diseases, such as macular holes and idiopathic premacular fibrosis.
6,7 The connecting channel may have a pathophysiologic role in PPVPs. No connecting channel was seen in either eye of four subjects, in whom the refractive errors were emmetropia (two cases); hyperopia (one case); and low myopia (one case;
Fig. 5). These cases had small PPVPs with thick septums between the PPVPs and Cloquet's canal, and the anterior edge of the septum had a lamellar structure (
Fig. 4). It is unclear whether the connecting channel collapsed in these cases as a result of the overlying vitreous gel.
Worst described two channels between the bursa and Cloquet's canal.
9 One channel was a superior branching channel that arose from the roof of the bursa and merged into Cloquet's canal. However, using SS-OCT, we could not observe the orifice of the superior branching channel in the roof of the PPVP. Because the imaging depth of SS-OCT is 2.3 mm in tissue, we could not explore the anterior extent of the superior branching channels. Another the lower branching channel was connected to the bottom of the bursa and the Martegiani's space in Cloquet's canal. The connecting channel in the current study appeared to coincident with the lower branching channel described by Worst.
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In conclusion, SS-OCT showed the detailed structure of PPVPs in vivo. The configuration of the PPVP is boat-shaped. Although its central height increased with increasing myopia, its width was unchanged. A channel connected Cloquet's canal and the PPVPs, which suggested the route of aqueous humor into the PPVPs.