Posterior Precortical Vitreous Pocket Research Articles

Foveal Development and Posterior Precortical Vitreous Pocket Formation.

Oh, Daniel MD1,2,3; Esselfie, Juliet MD1,2; Tsang, Stephen MD PhD1,2,3; Freund, Kenneth Bailey MD1,2; Engelbert, Michael MD PhD1,2,a Author Information

Inter-Eye Comparison in Highly Myopic Patients with Unilateral Myopic Traction Maculopathy

ABSTRACT Purpose To evaluate the risk factors for myopic traction maculopathy (MTM) through inter-eye comparisons of asymmetric ocular features in patients with unilateral MTM. Materials and Method Highly myopic patients with unilateral MTM were enrolled in the study. The results of comprehensive ophthalmologic examinations, color fundus images, and optical coherence tomography (OCT) were reviewed. MTM and myopic atrophic maculopathy was evaluated according to the ATN classification system. The status of the posterior vitreous detachment (PVD), posterior precortical vitreous pocket, vitreoretinal interface abnormalities, posterior staphyloma height (PSH), and the location of the protrusion of the sclera were assessed through OCT. Results Among the 54 eyes of 27 patients 48.89 ± 12.78 years of age, the affected eyes had worse best-corrected visual acuity, a longer axial length (AXL), greater PSH, and higher rates of posterior staphylomas, vitreovascular traction and epiretinal membranes (ERMs) than the fellow eyes (P < .001; P < .001; P < .001; P = .010; P = .002; P < .001). Thirteen cases present obvious anisometropia with an inter-eye AXL difference of more than 1 mm. In the 14 cases without obvious anisometropia, the eyes with MTM still had longer AXL, greater PSH and a higher ERMs rate than the fellow eyes (P = .039; P = .017; P = .001). Besides, in the 7 cases with an inter-eye AXL difference of less than 1 mm and asymmetrical stages of PVD, 5 cases with greater PVD in the affected eyes. Multiple risk factors coexisted in 96% of cases. Conclusions In patients with unilateral MTM, a longer AXL, greater PSH, posterior staphyloma, vitreovascular traction, and ERMs were the main factors contributing to the occurrence of MTM. The process of PVD might involve in MTM development.

Wide-field swept source optical coherence tomography evaluation of posterior segment changes in highly myopic eyes.

To investigate the features in the posterior pole of highly myopic (HM) eyes using a wide-field high-resolution swept source optical coherence tomography (SS OCT). This observational cross-sectional study involved 262 eyes of 139 patients, who were diagnosed as HM and had consecutively been examined by SS OCT in the Ophthalmology Department of the Peking Union Medical College Hospital between March 2019 and December 2019. The characteristics of OCT images were documented and analyzed. In our study, SS OCT could demonstrate the entire layer of the choroid and detect the sclera in all eyes. The mean subfoveal retinal/choroidal/scleral thickness were 204.84 ± 119.86 μm, 92.80 ± 75.78 μm and 394.734 ± 123.09 μm, respectively. 138 eyes (52.67%) had posterior precortical vitreous pocket. Myopic foveoschisis was detected in 110 eyes (41.98%), and significantly associated with the presence of posterior staphyloma. 36 eyes (13.74%) had DSM in our study, of which 8 eyes (22.22%) showed a round dome, 16 (44.44%) were horizontal oval-shaped, 9 (25%) were vertical oval-shaped and 3 (8.34%) were oblique oval-shaped. Both SFCT and SFST were inversely and significantly associated with age and refractive errors. Macular neovascularization was significantly correlated with intrascleral vessels. Different deformation of the sclera and posterior staphyloma were vividly identified on SS OCT images. This study provided a relatively comprehensive picture of posterior pole in HM eyes. Such good visualization of ocular fundus provided by wide-field SS OCT could be useful for the therapy option, disease condition monitoring and pathogenesis investigation.

Microsurgical observation of the posterior vitreous in patients with vitreous hemorrhage caused by Terson syndrome

PurposeTo describe features characteristic of vitreous hemorrhage in patients with Terson syndrome observed through a microsurgical scope. MethodsBetween May 2015 and February 2019, 12 eyes of 10 patients with vitreous hemorrhage occurring after subarachnoid hemorrhage (SAH) underwent pars plana vitrectomy. ResultsDuring vitreous surgery, we found 10 of 12 eyes did not have posterior vitreous detachment (PVD). Furthermore, we observed in 9 of the 10 eyes without PVD (90.0%) that there was no hemorrhage in the posterior vitreous cavity at the posterior pole while we removed vitreous hemorrhage. We confirmed that this clean space could be the posterior precortical vitreous pocket (PPVP). Conclusion and importanceTerson syndrome may have no hemorrhage in the PPVP regardless of the presence of severe vitreous hemorrhage. The cases presented in our study may suggest one of the mechanisms of Terson syndrome.

CHANGES IN THE POSTERIOR VITREOUS AFTER CATARACT SURGERY ASSESSED BY SWEPT-SOURCE OPTICAL COHERENCE TOMOGRAPHY.

To assess changes in the posterior vitreous caused directly by phacoemulsification with implantation of an intraocular lens, using the enhanced vitreous swept-source optical coherence tomography system (Topcon, Tokyo, Japan). Consecutive patients with cataract without posterior vitreous detachment were enrolled. Swept-source optical coherence tomography examinations were performed 1 day before and several days after surgery, using enhanced vitreous visualization. We compared preoperative and postoperative posterior vitreous status and measured the distance between the internal limiting membrane and the posterior vitreous membrane at 26 locations at the posterior pole. Images of 33 eyes (21 patients) could be analyzed. The perifoveal posterior vitreous detachment was not extended in any eyes, and the internal limiting membrane to posterior vitreous membrane distance before and after surgery did not differ at any location measured (P = 0.071-1.000). The posterior precortical vitreous pocket was dilated in three eyes. Age, gender, axial length, preoperative visual activity, nuclear sclerosis, duration of surgery, and duration between surgery and swept-source optical coherence tomography did not differ between the dilated vitreous pocket group (n = 3) and the unchanged group (n = 30). The height of the posterior vitreous membrane remained unchanged after surgery, although the posterior precortical vitreous pocket was dilated in three patients. Cataract surgery procedures seem to have little influence on the posterior vitreous membrane.

Vitreous anatomy and the vitreomacular correlation.

The presence of a posterior precortical vitreous pocket, referred to as a precortical pocket, implies that the vitreous cortex is formed into a collagen sheet separated from the gel in the macula. Along with strong vitreoretinal attachment at the fovea, the precortical pocket plays a role in perifoveal posterior vitreous detachments, which may lead to macular holes, premacular membranes, and ring-shaped proliferation in diabetic retinopathy. I and my colleagues published pioneer studies of the vitreous in postmortem eyes. Here, the role of the precortical pocket in various vitreoretinal interface diseases is discussed. Swept-source optical coherence tomography showed development of the precortical pocket, the connecting channel, and Cloquet's canal during early childhood. These findings raised the possibility that aqueous humor may drain into the precortical pocket. The physiologic role of the drainage route is also discussed. Crosstalk between the anterior chamber and macula is an attractive hypothesis and remains to be elucidated.

Advances of optical coherence tomography in myopia and pathologic myopia.

The natural course of high-axial myopia is variable and the development of pathologic myopia is not fully understood. Advancements in optical coherence tomography (OCT) technology have revealed peculiar intraocular structures in highly myopic eyes and unprecedented pathologies that cause visual impairment. New OCT findings include posterior precortical vitreous pocket and precursor stages of posterior vitreous detachment; peripapillary intrachoroidal cavitation; morphological patterns of scleral inner curvature and dome-shaped macula. Swept source OCT is capable of imaging deeper layers in the posterior pole for investigation of optic nerve pits, stretched and thinned lamina cribrosa, elongated dural attachment at posterior scleral canal, and enlargement of retrobulbar subarachnoid spaces. This has therefore enabled further evaluation of various visual field defects in high myopia and the pathogenesis of glaucomatous optic neuropathy. OCT has many potential clinical uses in managing visual impairing conditions in pathologic myopia. Understanding how retinal nerve fibers are redistributed in axial elongation will allow the development of auto-segmentation software for diagnosis and monitoring progression of glaucoma. OCT is indispensable in the diagnosis of various conditions associated with myopic traction maculopathy and monitoring of post-surgical outcomes. In addition, OCT is commonly used in the multimodal imaging assessment of myopic choroidal neovascularization. Biometry and topography of the retinal layers and choroid will soon be validated for the classification of myopic maculopathy for utilization in epidemiological studies as well as clinical trials.

Abnormal traction of the vitreous detected by swept-source optical coherence tomography is related to the maculopathy associated with optic disc pits.

Maculopathy associated with optic disc pits (ODP), which sometimes causes severe visual loss, usually appears in late childhood or early adulthood. However, it has long been unclear how the disease begins to develop at these ages. We evaluated the relationship between vitreous structure and maculopathy associated with ODP. Six patients (seven eyes) with ODP were diagnosed between July 1990 and May 2013. Fundus photographs and swept-source optical coherence tomography (SS-OCT) images were evaluated retrospectively, and the vitreous at the vitreoretinal interface was visualized by reconstructing three-dimensional SS-OCT images. Vitrectomy was performed in the eyes with maculopathy. Among the six patients, five had ODP in one eye each and one patient had bilateral ODP. The pits were mainly located in the temporal quadrant, and maculopathy, including retinoschisis and retinal detachment, was detected in five eyes associated only with the temporal pits. A flat retinal detachment was observed in four eyes and identified within the vascular arcade except in one eye. A posterior precortical vitreous pocket (PPVP) was observed in all eyes except in one eye without maculopathy. Reconstructing images from SS-OCT showed the vitreoretinal interface abnormalities around the optic disc and the macular area in all eyes, which was completely different from the vitreoretinal interface in the normal pediatric eye. Vitrectomy was performed in four eyes with retinal detachment to resect the abnormal vitreous traction. Posterior vitreous detachment was created in two eyes. Retinal reattachment was achieved in three eyes, and subretinal fluid receded in one eye. The visual acuity improved in all four eyes. Abnormal traction of the vitreous due to an abnormality of the vitreoretinal interface, which may be strengthened by the development of a PPVP, generates the maculopathy associated with ODP.

Observation of the Vitreous in Diabetic Macular Edema Using Swept-Source OCT

Purpose: This study describes the relationship between the vitreous and diabetic macular edema (DME) using swept source OCT (SS-OCT). Method: We examined 81 eyes of 58 consecutive patients diagnosed as DME. The condition of the posterior wall of the posterior precortical vitreous pocket (PPVP) was classified into 4 groups based on 12-mm horizontal and vertical scans through the macula using SS-OCT. Classifications were: Group 1, no posterior vitreous detachments (PVDs); Group 2, perifoveal PVD; Group 3, macular PVD; and Group 4, complete PVD. DME types, which included cystoid macular edema (CME) and serous retinal detachment (SRD), were also investigated in each group. The control group consisted of 46 eyes of 39 diabetic patients without DME as shown by OCT. Result: The number of eyes diagnosed as DME for each of the groups was 18, 35, 6 and 22, respectively. OCT images revealed there were 35 (43%) out of 81 eyes with perifoveal PVD, that was accompanied by CME (94%) with or without SRD, which was statistically significant as compared with the other groups (P=0.0005). In Group 2, there were 21 (60%) out of 35 eyes with proliferative diabetic retinopathy (PDR), with this ratio significantly higher when compared to the other groups (P=0.034). Conclusion: A significant correlation exists between CME associated with DME and perifoveal PVD.

Enhanced High-Density Line Spectral-Domain Optical Coherence Tomography Imaging of the Vitreoretinal Interface: Description of Selected Cases

Purpose: We introduce a new method to observe the morphologic features of the vitreous with spectral-domain optical coherence tomography (SD-OCT) by evaluating and describing the posterior vitreous structure of various selected cases using enhanced high-density (HD) line mode. Methods: This is a descriptive case series of SD-OCT images with enhanced HD line at Denver Health Medical Center, University of Colorado School of Medicine, in 2014. The medical records with ocular images for these patients were analyzed. Results: Enhanced HD line in vitreoretinal mode with SD-OCT offered wide images with well-visible vitreoretinal interface. The posterior precortical vitreous pocket (PPVP), often mistaken as a posterior vitreous detachment on standard retina-based cross line mode, was easily identified. Vitreomacular adhesion and traction, absence or presence of connecting channels between PPVP and Cloquet’s canal, liquefied vitreous lacuna, and emulsified silicone oil droplet-layer could be observed. Conclusions: SD-OCT using enhanced HD line in vitreoretinal mode enables clear visualization of the posterior vitreous structure and often reclassifies patients previously thought to have a posterior vitreous detachment.

Posterior Precortical Vitreous Pocket in Children

Purpose: The purpose of the present study was to investigate the profile of posterior precortical vitreous pocket (PPVP) in children using spectral-domain optical coherence tomography (OCT) and to compare the profile with that of adults.Materials and methods: In this retrospective comparative study, the height at the fovea and the horizontal diameter of PPVP in healthy children and adults in the upright position were measured based on OCT (Spectralis, Heidelberg Engineering GmbH, Heidelberg, Germany) images. PPVP diameter was divided into nasal length and temporal length based on the center of the fovea. The proportion of temporal length to the entire horizontal diameter was calculated as temporal length/horizontal diameter of PPVP. The association of age with the height and the proportion of temporal length to the entire horizontal diameter were evaluated in each group. The two values were also compared between the two groups.Results: Sixty-three eyes of 63 children (mean age: 6.9 ± 1.8 years) and 44 eyes of 44 young adults (mean age: 32.1 ± 5.3 years) were included. The height of PPVP at the fovea was 355.3 ± 80.3 µm in children and 367.8 ± 75.6 µm in adults. The proportion of temporal length to the entire horizontal diameter was 0.45 ± 0.05 and 0.50 ± 0.03. The proportion was significantly greater in adults than in children (p < 0.001), whereas the height (p = 0.349) was not different between the two groups. In children, there was a positive association between the age and the proportion of temporal length to the entire horizontal diameter (p = 0.001).Conclusion: The results suggest a progressive temporal extension of PPVP during childhood.

The Premacular Bursa's Shape Revealed In Vivo by Swept-Source Optical Coherence Tomography

To resolve the controversy surrounding the shape and relationship of posterior vitreous spaces by characterizing the connections between the premacular bursa, the area of Martegiani, and Cloquet's canal. Comprehensive posterior vitreous maps were created using swept-source optical coherence tomography (SS OCT) in a cross-sectional study. The posterior vitreous of 102 eyes of 51 volunteers 21 to 54 years of age without ocular pathologic features was imaged using SS OCT. The DRI OCT-1 Atlantis 3D SS OCT (Topcon Medical Systems, Oakland, NJ) was used to acquire scans of the posterior vitreous over an 18×18-mm area. Posterior vitreous spaces and their relationships were identified. The premacular bursa was identified in all 102 eyes and was found to extend superiorly beyond our scanning ability at a variable angle. No discernible superior borders could be identified. Instead, a connection of the bursa with the preoptic area of Martegiani or its extension, Cloquet's canal, was found in 101 of 102 eyes. This connection occurred at a variable distance from the optic nerve, where it formed a flat and broad superior channel. The skyward direction of this channel was found to be gravity dependent in all 14 eyes of the 7 subjects examined in various head positions. Although SS OCT was able to identify vitreous degeneration, the above changes were present in 28 eyes even without any discernible vitreous degeneration. The premacular bursa, also called the posterior precortical vitreous pocket, was found to continue superiorly beyond the posterior pole without a detectable border. The bursa fused broadly with the extension of the preoptic area of Martegiani, namely Cloquet's canal, or the hyaloidal tract of Eisner. These findings suggest that there is a direct anteroposterior connection between the retrolental and premacular and preoptic spaces already existent in the eyes of young adults before the occurrence of vitreous degeneration. This observation may have important implications with respect to the movement of intrinsic and extrinsic mediators between the anterior and posterior segments.

Evolution of Vitreomacular Detachment in Healthy Subjects

Development of posterior vitreous detachment (PVD) plays an important role in vitreomacular diseases. Spectral-domain optical coherence tomography (SD-OCT) with noise reduction can visualize a posterior precortical vitreous pocket (PPVP) and classify PVD stages according to the state of the posterior wall of the PPVP. To describe the role of the PPVP in early-stage PVDs in healthy individuals. We performed biomicroscopy and SD-OCT in the right eyes of 368 healthy volunteers (188 males and 180 females; mean [SD] age, 57.1 [19.4] years; range, 12-89 years). The condition of the posterior wall of the PPVP was classified into stages according to the biomicroscopic findings and SD-OCT images: stage 0, no PVD with PPVP (134 eyes; mean [SD] subject age, 38.7 [16.5] years; range, 12-76 years); stage 1, paramacular PVD with PPVP (47 eyes; mean age, 55.2 [10.3] years; range, 36-77 years); stage 2, perifoveal PVD with PPVP (27 eyes; mean age, 62.0 [8.7] years; range, 46-81 years); stage 3, vitreofoveal separation with persistent attachment to the optic disc (19 eyes; mean age, 65.8 [6.2] years; range, 55-80 years; stage 3a, vitreofoveal separation with an intact posterior wall of the PPVP in 12 eyes; stage 3b, vitreofoveal separation with a defect in the posterior wall of the PPVP in 7 eyes; and stage 4, complete PVD (141 eyes; mean age, 73.2 [8.3] years; range, 48-89 years). Ages in each PVD stage. The posterior wall of the PPVP initially detaches at the paramacular area and extends to the perifoveal area, which results in a perifoveal PVD. A vitreofoveal detachment may develop with or without a defect in the PPVP. When the vitreous detaches from the optic disc, a complete PVD develops. An anatomical feature of the PPVP may play a role in the development of a perifoveal PVD.

Modified Technique for Inducing Posterior Vitreous Detachment through the Posterior Precortical Vitreous Pocket during Microincision Vitreous Surgery with a Wide-Angle Viewing System

Purpose: To introduce a simple technique to create a posterior vitreous detachment (PVD) through the posterior precortical vitreous pocket (PPVP) during microincision vitreous surgery using a wide-angle viewing system (WAVS). Methods: This study included 20 eyes of 20 consecutive patients (mean age, 65.6 years; range, 56-76) with a stage 2 or 3 idiopathic macular hole who underwent 25-gauge microincision vitreous surgery using a WAVS and phacoemulsification surgery. A triamcinolone suspension was injected into the vitreous cavity. The triamcinolone facilitated visualization of the posterior wall of the PPVP. Using a vitreous cutter, we suctioned the outer margin of the PPVP. After a resultant small break of the posterior hyaloid membrane developed and enlarged rapidly, a PVD easily expanded at the periphery. Results: Using this technique, we induced a PVD without close proximity to the optic disk and macula. Conclusion: This technique is an easy and safe method for creating a PVD during microincision vitreous surgery using a WAVS.

Non-aspiration technique to induce posterior vitreous detachment in minimum incision vitrectomy system

Complete removal of the posterior hyaloid is an important procedure in vitrectomy for various vitreoretinal disorders. Although posterior vitreous detachment (PVD) can be usually induced by active aspiration of vitreous...

AGING CHANGES OF VITREOMACULAR INTERFACE

The posterior precortical vitreous pocket (the pocket) is a premacular liquefied lacuna, the physiologic presence of which was reported in autopsy eyes. We describe the morphologic features of the pocket in normal individuals. We performed spectral-domain optical coherence tomography in 102 eyes of 51 normal subjects of various ages in a sitting position. The pocket was identified as an optically lucent space anterior to the posterior fundus in 85 of 102 eyes (83.3%) in the age-group. The vitreous cortex was extremely thin at the central fovea, and it gradually thickened along with age in the perifoveal area. A shallow perifoveal posterior vitreous detachment developed in 16 of 32 eyes (50%) of subjects aged >51 years. The lamellar structure of the vitreous cortex was seen in 7 of the 32 eyes (22%) of subjects aged >51 years. Spectral-domain optical coherence tomography confirmed the presence of the pocket in all ages. Thickened vitreous cortex and perifoveal posterior vitreous detachment were physiologic findings in older individuals. The lamellar structure of the vitreous cortex may be related to the vitreoschisis.

Observations by Spectral-Domain Optical Coherence Tomography Combined with Simultaneous Scanning Laser Ophthalmoscopy: Imaging of the Vitreous

To determine the ability to detect normal vitreous structure, evolving posterior vitreous detachment (PVD), and related vitreoretinal changes with combined spectral-domain optical coherence tomography (SD-OCT) and scanning laser ophthalmoscopy (SLO). Observational cross-sectional study. Simultaneous SD-OCT and SLO imaging instruments (SD-OCT/SLO) were used to image both eyes of patients with symptoms of PVD. The vitreous cortex, preretinal lacunae, hyaloid, and its relations to the retinal surface were analyzed. In addition, ultrasound was performed in a subset of patients to determine the stage of PVD. Two-hundred two eyes of 113 subjects were scanned. There was a high correlation between diagnosis of complete PVD by clinical examination and OCT (95 vs 93 eyes, respectively; kappa, 0.82). A partial PVD was detected more frequently by SD-OCT/SLO than by biomicroscopy examination (45 vs 7 eyes; P < .0001). Ultrasound was performed in a subset of 30 eyes. A high agreement was found between ultrasound and SD-OCT/SLO results for both complete PVD (kappa, 0.933) and incomplete PVD (kappa, 0.91). Vitreous cortex was detected in 181 eyes, and posterior precortical vitreous pocket was detected in 85 eyes. The effects of PVD, including vitreoretinal traction, paravascular lamellar holes, and fine changes at the fovea, could be visualized reliably in detail only with SD-OCT/SLO. In all these eyes, SD-OCT/SLO allowed improved visualization of the vitreoretinal relationship. SD-OCT/SLO provides unprecedented in vivo information about the physiologic and pathologic vitreous structure; it allows an extremely detailed analysis of the vitreoretinal interface, and it is particularly useful for defining focal changes and PVD.

Vitreous findings in a patient with Terson's syndrome

In Terson's syndrome a sudden increase in intracranial pressure due to spontaneous subarachnoid hemorrhage or head trauma may result in intraocular hemorrhage. A patient with bilateral vitreous hemorrhages underwent vitrectomy in both eyes with excellent result. In the right eye a glistening membrane was found with blood underneath, covering the macula. This was probably the internal limiting membrane (ILM). At the vitreous base and in the equatorial zone, attachment plaques are present between the ILM and the Müller cells. Because attachment plaques are missing in the posterior zone, where the ILM is much thicker, a retinal hemorrhage is capable of detaching the ILM from the retina in that area. The presence of attachment plaques is considered in relation to centripetal vitreous traction, which is absent in the area of the posterior precortical vitreous pocket (PPVP). The posterior wall of the PPVP coincides therefore probably exactly with the thick part of the ILM in the posterior zone.

Clinical Manifestations of Posterior Precortical Vitreous Pocket in Proliferative Diabetic Retinopathy

In diabetic retinopathy, fibrovascular tissue is prone to form around the macular area in a circular pattern. Posterior precortical vitreous pocket is a physiologically presented premacular liquefied lacuna of the vitreous. The authors studied how a posterior precortical vitreous pocket affects the clinical feature of proliferative diabetic retinopathy (PDR). The authors examined the vitreous of 148 diabetic eyes from 114 patients with a ring- or C-shaped fibrovascular tissue surrounding the posterior fundus. A cone-shaped partial posterior vitreous detachment was a common feature of the vitreous. A posterior vitreous detachment was prevalent outer to the ring-shaped proliferation. In the posterior fundus inner to the ring, only the gel component of the vitreous was detached from the fundus with the cortical vitreous still attached onto the retina. The premacular liquefied lacuna was identified as the posterior precortical vitreous pocket. The profile of this pocket became obvious when vitreous hemorrhage settled within the posterior precortical vitreous pocket. The ring-shaped proliferation was formed along its the outer margin. These results suggest that a posterior precortical vitreous pocket plays a key role in determining the pattern of proliferation in some types of diabetic retinopathy.

Posterior Precortical Vitreous Pocket

We examined the vitreous of 84 human autopsy eyes by biomicroscopy after staining the gel component with fluorescein and immersing the specimen in water. A noteworthy finding was the presence of a posterior precortical vitreous pocket that was basically located immediately anterior to the posterior fundus surrounded by the temporal vascular arcades. The posterior wall of the posterior precortical vitreous pocket was composed of a thin layer of vitreous cortex. The anterior border was contoured by the formed vitreous. The posterior precortical vitreous pocket seemed to expand or become confluent with adjacent lacunae in the vitreous. The posterior precortical vitreous pocket was present in all of 48 eyes with no or incomplete posterior vitreous detachment and in 19 of 36 eyes with posterior vitreous detachment. An oval-shaped defect of the premacular cortical vitreous was seen in 10 of 36 eyes with posterior vitreous detachment. The posterior precortical vitreous pocket was consistently present in front of the macula regardless of the type of vitreous liquefaction.