Diagnosis of Posterior Vitreous Detachment
Conventionally, PVD is defined by the presence of the Weiss ring, corresponding to the site of previous attachment to the optic disk, within the vitreous cavity. This, however, may cause underdiagnosis of some PVDs, as the Weiss ring may be destroyed or fragmented during the process of vitreous separation. The morphological variations of Weiss rings were characterized by Akiba et al, and they found that the majority of Weiss rings were in fact incomplete. Furthermore, the presence of a Weiss ring may not necessarily be associated with complete separation of the PHM.
The presence of PVD can be more accurately diagnosed by the detection of the detached PHM within the vitreal cavity. In most patients, this can be seen by slit-lamp biomicroscopy as a characteristic, mobile membrane that is glossy and crinkled within the vitreous cavity (Figure 1). Where the gel is collapsed, the PHM can be observed directly in the anterior vitreous space with the slit-lamp alone using oblique illumination (offset by 10°) to provide specular and retro-illumination of the PHM. A 90-diopter (D) condensing lens is required if the membrane is lying more posteriorly.
In order to determine the extent of PHM separation, it is important to examine the vitreous dynamically; this is done by observing the vitreous cavity following swift eye movements vertically and horizontally. This allows the PHM to be examined in its entirety for the presence of defects. Not infrequently, a dehiscence in the PHM can occur during separation from the macula, resulting in a tear in the membrane.[10,11] This is seen clinically as a rhexis-like defect in the PHM with remnants left on the retina, usually manifesting as an epiretinal membrane over the macula (Figure 3). In a study by Hikichi et al, 12% of patients with PVD demonstrated a definite PHM defect over the macula. It is likely, however, that the prevalence of this defect may be higher, as PHM defects can be difficult to visualize.
Posterior hyaloid membrane with a visible defect (black arrow) and Weiss ring (white arrow).
Dynamic vitreous examination also allows for appreciation of the quality of the internal vitreous gel architecture and for detection of the presence of pigment granules within the anterior vitreous or studded on the PHM; these aspects are important in the assessment of possible PVD-related complications.
The anterior limit of the PVD can be variable, ranging from the equator to the ora serrata in most healthy eyes, and it has been inferred as the vitreous base by some authors. The extent of PHM separation can be difficult to assess clinically, although in cases of retinal detachment, the PHM insertion is frequently visible as a white line on the detached retina during indirect ophthalmoscopy with indentation (Figure 4). Identification of the line can be useful in determining the anteroposterior location of horseshoe retinal tears, as they will always lie along this line.
White line (arrow) representing site of insertion of detached posterior hyaloid membrane into retina. (Courtesy of JD Scott)
© 2005 Comprehensive Ophthalmology Update, LLC
Cite this: Posterior Vitreous Detachment: Current Concepts and Management - Medscape - Jul 01, 2005.