20.1 OCT-Based Classification of Vitreomacular Adhesion

In the normal state, all eyes have a complete adhesion of the vitreous to the inner retinal surface at birth. As the eye ages, a complex series of events causes the vitreous cortex to detach from the inner retina. This phenomenon is called posterior vitreous detachment (PVD) and is a normal aging process. ^{1 ,​ 2 ,​ 5 ,​ 6} However, it can be accelerated by blunt trauma, inflammation, and intraocular surgery. PVD usually begins in the perifoveal region and progresses over many years to involve multiple sites in the peripheral fundus before finally coalescing into a complete separation of the vitreous cortex from the macula and the optic nerve, with the appearance of the Weiss ring. For the most part, this is a nonpathological process. In some eyes, the vitreous cortex detaches inadequately or incompletely from the inner retinal surface. This phenomenon is termed an anomalous PVD. ^{5 ,​ 6 ,​ 7} According to the IVTS Group Classification System, ⁵ an anomalous PVD is defined as a partial vitreous detachment with an anomalous persistent attachment in the macular region resulting in tractional distortion of the retina. An anomalous PVD may then lead to pathological features of VMI disease. ^{5 ,​ 6 ,​ 7}

In the anatomical OCT-based IVTS Group Classification System, vitreomacular adhesion (VMA) is equivalent to a stage 1 PVD in the Uchino, Gaudric, and Johnson classification systems. ^{1 ,​ 2 ,​ 6 ,​ 8} The anatomical OCT-based IVTS Group Classification System ⁵ defines VMA as a specific stage of vitreous separation (Table 20.1 , Fig. 20.1). It is defined as a perifoveal separation of the vitreous cortex with continuing VMA within a 3-mm radius of the foveal center, associated with a completely normal retinal morphology. The detached posterior hyaloid may appear on or above the retinal surface. If it is above the retinal surface, the angle between the detached vitreous cortex and the inner retinal surface is acute. VMA is usually asymptomatic and appears as a normal finding on OCT during the natural course of the progression of PVD. Over time, the vitreous usually separates completely from the inner retinal surface as part of the normal physiologic aging process.

Using the anatomical OCT-based classification system, ⁵ VMA is subclassified by the size of the VMA as measured using the caliper function tool on OCT into (1) focal VMA, where the size of the VMA is ≤1500 μm; or (2) broad VMA, where the size of adhesion measures more than 1500 μm (Table 20.1, Fig. 20.1). The area of VMA is measured roughly parallel to the retinal pigment epithelium, and small regions of dehiscence (<1 mm) between the vitreous and the inner retina that may be present within the zones of a broad VMA are disregarded. When VMA is not associated with any other ocular disease, it is termed isolated VMA. In cases where VMA is associated with other macular abnormalities, such as diabetic macular edema, age-related macular degeneration, or retinal vein occlusion, it is termed concurrent VMA (Table 20.1 , Fig. 20.1). It should be noted that this OCT-based definition of VMA ⁵ does not take into account the symptoms, as specific visual symptoms may be subjective and may be caused by a separate disease process.

20.2 OCT-Based Classification of Vitreomacular Traction

During an anomalous progression of PVD, ^{5 ,​ 6 ,​ 7} excessive traction on the macula can result and cause changes in the foveal contour, compromising vision. ⁹ The anatomical OCT-based IVTS Group Classification System ⁵ defines VMT as a perifoveal vitreous separation with continuing vitreomacular attachment within a 3-mm radius of the foveal center, associated with an abnormal retinal morphology (Table 20.1 , Fig. 20.1 , Fig. 20.2). An abnormal retinal morphology may involve distortion of the fovea, intraretinal structural changes such as pseudocyst formation (Fig. 20.2), macular schisis, cystoid macular edema (CME), subretinal fluid, elevation of the fovea above the retinal pigment epithelium, or any combination of these. Unlike VMA, where the vitreous separation is virtually always related to a normal physiologic process, VMT is the result of an anomalous progression of PVD.

Using the anatomical OCT-based classification system, ⁵ VMT is subclassified by the size of vitreomacular attachment as measured using the caliper function tool on OCT into (1) focal VMT, where the size of vitreomacular attachment measures ≤ 1500 μm; or (2) broad VMT, where the size of vitreomacular attachment measures > 1500 μm (Table 20.1 , Fig. 20.1). As with VMA, when VMT is not associated with any other ocular disease, it is termed isolated VMT. In cases where VMT is associated with other macular abnormalities, it is termed as concurrent VMT (Table 20.1 , Fig. 20.2).

As PVD progresses, residual cortical vitreous may be left on the retinal surface, even in the setting of a complete, or stage 4, PVD with Weiss ring. This occurrence is termed vitreoschisis. ^{10 ,​ 11} The residual vitreous remnant may serve as a nidus for proliferation, thus forming an epiretinal membrane (ERM). ERM typically contains glial cells and histiocytes that collect and attach to the vitreous remnant on the retinal surface. ¹² Contracture of an ERM may cause further traction on the macula, resulting in distortion of the retinal morphology. Sometimes, VMT and ERM are seen on OCT simultaneously (Fig. 20.2).

20.3 OCT-Based Classification of Full-Thickness Macular Hole

The Gass classification of macular holes is widely used clinically and was based on careful clinical examination; this classification predates both the invention of OCT and the use of vitrectomy to repair macular holes. It divides macular holes into four stages (Table 20.2). ^{13 ,​ 14} Over the past few years, OCT has increased the understanding of the anatomy and progression of macular holes. The anatomical OCT-based IVTS Group Classification System ⁵ defines a full-thickness macular hole (FTMH) as an anatomical defect in the fovea that involves an interruption of all the neurosensory retinal layers from the inner limiting membrane (ILM) to the retinal pigment epithelium. This definition requires such a detection on at least one OCT B-scan. Sometimes, small FTMHs are missed by examining only one OCT B-scan, and so, a series of closely spaced OCT B-scans is a more appropriate way to examine for an FTMH on OCT.

Using the anatomical OCT-based classification system, ⁵ an FTMH is subclassified based on the following parameters: (1) size of the hole, (2) status of the vitreous, and (3) cause of FTMH.

20.3.1 Size

Using the caliper function tool on OCT, the aperture size of the FTMH is measured roughly parallel to the retinal pigment epithelium at the narrowest opening of the hole in the mid retina or at the photoreceptor tips (Table 20.1 , Fig. 20.3). Using this function, FTMH is classified as small, medium, or large based on its aperture size on OCT. OCT-based measurements of the aperture size of FTMH predict medical and surgical treatment outcomes. ^{15 ,​ 16 ,​ 17} A small FTMH is defined as a FTMH with an aperture size measuring ≤ 250 μm (Table 20.1, Fig. 20.3). This cutoff is obtained from studies showing that small FTMHs are associated with a small rate of spontaneous closure and a rate of closure approaching 100% with vitrectomy. ¹⁶ Small FTMHs are also associated with the best response (approximately 60%) to pharmacologic vitreolysis. ¹⁶ A medium FTMH is defined as an FTMH with an aperture size measuring > 250 μm and ≤ 400 μm (Table 20.1 , Fig. 20.3). Postsurgical FTMH closure rates for medium FTMHs approach approximately 90% with complete removal of the residual hyaloid, with or without peeling of the inner limiting membrane. ^{15 ,​ 17} Compared with small FTMHs, medium FTMHs have a lower response rate (approximately 50%) to pharmacologic vitreolysis. ¹⁸ A large FTMH is defined as an FTMH with an aperture size measuring > 400 μm(Table 20.1 , Fig. 20.3). Roughly half the FTMHs are large FTMHs at diagnosis. They have a 90 to 95% closure rate afater vitrectomy with inner limiting membrane peeling and a 75% closure rate after vitrectomy without inner limiting membrane peeling. ¹⁹ Pharmacologic vitreolysis has been tried in a few eyes with large FTMHs, but no anatomical success has been reported so far in this group. ¹⁶

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