Fig. 25.3 Evacuation of pus from the macular surface. (a) The flute needle is placed just on top of the pus accumulation, then carefully sunk into it. (b) When the drainage is completed, the damage to the retina by the infection is even more obvious. The image seen here easily explains why pus must not be left on the macular surface

•Place the flute needle first over (Fig. 25.3), then, if this first attempt did not bring drainage, carefully into, the purulent material.

–The material is not simply fluid; therefore very cautious mechanical “nudging” may also be necessary. The soft tip is a good option to do this with, but it will not allow drainage: its internal diameter is too small and the material too sticky.

–Be patient; often the drainage process needs to be suspended and then taken up again later during the operation.

25.2.8 The Surgeon’s Actions

The eye is rotated by the surgeon during much of vitrectomy. The inexperienced surgeon may not coordinate the movement of his two hands properly, which can result in corneal wrinkling, which in turn reduces the sharpness of the image (see Sect. 20.2). The microscope must precisely and simultaneously follow the eyeball (see Sect. 16.7.2).

•Make sure you coordinate the movement of your hands when rotating the eyeball.

–The most common error is to move the eye in one direction with that hand (e.g., to the right with the right hand) and not properly follow it with the other (left) hand.

•Highly myopic eyes have a higher tendency for corneal wrinkling.

–Occasionally the eye is too long for the probe to reach the posterior pole (see Chap. 42); in such cases, it is unavoidable for the eyewall to be pushed in, which may interfere with the image quality. The interference is more conspicuous if a contact lens, rather than the BIOM, is used.

25.2.9 “Chromovitrectomy”

Finally, the issue of the surgeon’s visual aids must be mentioned. Their use is often referred to as chromovitrectomy; these materials help the surgeon delineate tissues that otherwise would be impossible or difficult to visualize. These are discussed in

Chap. 34.

cortical vitreous. The anterior cortex is 2 μ thick; the posterior is 100 μ. There is no cortex over the optic disc.

–The anterior hyaloid face adheres to but is not interconnected with the posterior lens capsule.

–The posterior hyaloid face is also adherent to (typically not interconnected with) the posterior retina, but is glued to it by an extracellular matrix.

–The posterior adherence is stronger than elsewhere at the margin of the macula or the parafoveal area, along the major blood vessels, in areas of certain retinal degenerations,4 and especially at the margin of the optic disc (Weiss ring; see above).

–Both the anterior (to the lens) and posterior (to the retina) adherences weaken with age, but pathologic connections may develop posteriorly at the sites of chorioretinal scars, which can be caused by various diseases, injuries, or even overly strong laser spots.

•The premacular bursa5 is an optically empty, fluid-only space measuring 7 mm in width and 0.6 mm axially, which also connects to the area of Martegiani6 in front of the optic disc. The superior extension of the premacular bursa fuses with Cloquet’s canal and courses through the vitreous, terminating behind the lens.7

26.1.2Vitreous Biochemistry and Its Anatomical and Functional Implications

The vitreous is composed of ~98% water; the rest is made up of collagen fibers (mostly, but not exclusively, type II), hyaluronan, and many other molecules such as chondroitin sulfate, as well as a relatively small number cells (hyalocytes and fibroblasts, see Table 26.1).

The “normal” vitreous is entirely in a gel state: there is no free water content. For the gel to remain so and fill the vitreous cavity completely, both the collagen fibers and the hyaluronan are essential. Without the former the vitreous becomes a viscous fluid; without the latter, it shrinks.

With time the vitreous gel starts to break down; as early as at 4 years of age, the process of syneresis8 begins. The normal collagen-hyaluronan relationship breaks down and free fluid (aqueous) appears in these lacunae. In a person 18 years old, up to a fifth of the vitreous is liquid.9

Aggregated collagen fragments are “swimming” in the lacuna fluid, casting a mobile shadow on the retina, and giving rise to what many people describe as “flying flies.”10

4Such as lattice.

5Also known as posterior precortical vitreous pocket.

6Also called cisterna preoptica.

7Berger’s space; also known as patellar fossa or the space of Erggelet.

8Also called liquefaction; see Fig. 26.1, Chap. 27 and Fig. 54.2d.

9See Fig. 54.1 about the significance of changes in the structure of the vitreous.

10Commonly referred to in textbooks as mouches volantes.

*Listed in alphabetical order. See the text and the appropriate chapters for more details. aIf the eye undergoes scleral buckling, for instance.

b10 mm2.

cLike a pawn for the king in chess.

dThink of a completely bald person vs one whose head is closely shaven.

A more important consequence of the presence of the gel/fluid mixture in the vitreous cavity involves the vitreoretinal interface.

•Posteriorly, the vitreous may separate from the retina completely (PVD), may retain some of its connections (anomalous PVD, which gives rise to VR traction and can present clinically as VMTS, macular edema, macular hole etc.), or vitreoschisis may also develop.

Pearl

Vitreoschisis is an entity that is often, erroneously, diagnosed as a PVD (see Fig. 26.2 and Table 7.1). Vitreoschisis may play a role in EMP or RD development.