Pathogenetic Mechanisms of Retinal Detachment

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Chapter 95 Pathogenetic Mechanisms of Retinal Detachment

The term retinal detachment is used to describe a separation of the neurosensory retina from the retinal pigment epithelium (RPE). A retinal detachment re-establishes the potential space that exists between the original layers of the embryonic optic cup. This chapter discusses the pathogenetic mechanisms that cause retinal detachments and the most commonly encountered types of detachments. Details of specific types of retinal detachments and their treatment are addressed in other chapters.

Major types

Nearly all retinal detachments fall into one of three general categories based on the underlying cause of the retinal detachment. The most common form is rhegmatogenous retinal detachment, which occurs as the result of a full-thickness retinal break. The second category, traction retinal detachment, occurs when vitreoretinal adhesions mechanically detach the retina from the underlying RPE. In some instances, retinal detachments may involve both a rhegmatogenous and tractional mechanism. A third category, exudative (serous) retinal detachment, is due to a process, such as a tumor or inflammation, resulting in accumulation of subretinal fluid without associated traction or full-thickness retinal breaks.

Accumulation of subretinal fluid is a feature of all retinal detachments. When the normal physiologic forces that maintain contact between the retina and the RPE (e.g., the metabolic pump of the RPE,1 the osmotic pressure of the choroid,2 and the more minor mechanical forces of the interphotoreceptor matrix) are compromised or overwhelmed, a retinal detachment occurs. Various pathologic conditions can upset the balance of the normal transretinal pressure gradient and result in subretinal fluid accumulation.

Rhegmatogenous retinal detachment

Rhegmatogenous retinal detachments are those arising from one or more full-thickness retinal breaks. The term retinal break refers either to a retinal tear or to a retinal hole. Retinal tears are commonly associated with well-defined vitreoretinal traction either on an attached flap or on the retina that had been adjacent to a now free-floating vitreous operculum. In contrast, retinal holes occur more commonly as a result of localized retinal atrophy or deterioration and are not believed to be associated with vitreoretinal traction.35 Clinically, the distinction is not always clear: for instance some operculated retinal breaks have vitreous traction upon nearby retinal vessels and behave as tears with persistent traction upon them.

The characteristics of a rhegmatogenous retinal detachment are: (1) the existence of abnormal mobility of partially liquefied vitreous gel; (2) tractional forces that can precipitate a retinal break; and (3) the presence of a retinal break that will allow the passage of liquefied vitreous into the subretinal space (Fig. 95.1). All three factors need to be present to cause a rhegmatogenous retinal detachment. For example, if a tear or hole is present in the absence of tractional forces and liquid vitreous, it is unlikely that the retina will detach. Examination of postmortem eyes indicates that approximately 5–10% of eyes have full-thickness retinal defects without any apparent detachment.6 Subclinical retinal detachments are defined as having less than 1–2 disc diameters of associated subretinal fluid and usually do not progress, if asymptomatic.7 A spontaneous rhegmatogenous retinal detachment is usually preceded by a posterior vitreous detachment (PVD). With age, fragmentation of collagen fibers and an aggregation of proteoglycans around these fragments is believed to be responsible for destabilizing the vitreous gel and leading to liquefaction (syneresis).8 The subsequent reduced volume of the vitreous gel is associated with the collapse and aggregation of the collagen fibrillar network. The liquefied vitreous gel may coalesce into a large lacuna, which mimics a true PVD.9 When the denser posterior vitreous cortex ruptures, the liquefied vitreous can pass into the subhyaloid space and separate the posterior vitreous surface from the internal limiting membrane of the retina producing a true PVD.9 Both the degree of vitreous liquefaction and the prevalence of PVD are age-related.10 Pools of liquefied vitreous can be seen with the slit lamp in more than 90% of patients after the age of 40 years.11 PVD is found in 27% of patients aged 60–69 years and in 63% of patients after age 70.10 Other factors that hasten increased mobility of partially liquified vitreous include cataract extraction, high myopia, inflammation, and trauma.12

In the presence of a PVD, the condensed vitreous moves about within the vitreous cavity with rotational movements of the eye exerting traction where the vitreous remains attached to the retina. This traction can itself produce a retinal break. A horseshoe-shaped tear, for example, may be the result of such tractional forces.

In retinal detachment, the normal forces maintaining neurosensory retinal attachment to the RPE are overwhelmed by opposing forces causing retinal detachment. The forces maintaining retinal adhesion are discussed in detail elsewhere in the text and include the Na+/K+-ATPase metabolic pump of the RPE, the osmotic pressure of the choroid, and the weaker forces of the interphotoreceptor matrix. The opposing forces include hydraulic dissection caused by the intraocular fluid swirling through the retinal break held open by vitreoretinal traction.13 The location of the residual vitreoretinal adhesion on the retina often determines the location of the retinal break. Specific clinical situations will lead to similar vitreoretinal adhesion and characteristic tears. Aphakia and pseudophakia are commonly associated with peripheral retinal breaks at the posterior edge of the vitreous base near the ora serrata. The retina in this region is relatively thin and less developed. Ocular trauma can result in retinal breaks, three-quarters of which are retinal dialyses. A retinal dialysis following trauma usually occurs in either the superonasal or inferotemporal quadrants. Dialyses, in general and following trauma, are more common in the inferotemporal quadrant although most of the dialyses in the superonasal quadrant are associated with a definite history of preceding trauma.14 Giant retinal tears (circumferential retinal breaks of ≥90°) commonly arise from circumferential vitreous traction in the region of the posterior vitreous base,15 although tears in the anterior vitreous base can occur with Stickler syndrome.16 This inherited condition is associated primarily with the COL2A gene and also with the COL11A1 gene.17 It carries a very high lifetime risk for rhegmatogenous retinal detachments.16,18 Retinal detachments originating from the posterior pole (macula) or the intermediate zone between the posterior pole and the equator are characteristic to high myopia. Posterior pole retinal detachments are very rare, accounting for less than 1% of retinal detachments in the USA, but the prevalence may be much higher in Japan.19 Most of the nontraumatic retinal breaks located in the posterior pole are secondary to macular holes and associated with a posterior staphyloma.20 In some myopic macular holes and detachments, tangential vitreoretinal traction based on epiretinal membranes after incomplete posterior vitreous separation “vitreoschisis,”21,22 are speculated to be pathophysiologic factors.23

However, even when tangential traction is completely released by removing the internal limiting membrane during a vitrectomy, the retinal detachment may still persist. Some authors believe that operative failure in these instances is related to axial elongation and posterior staphyloma formation.24 The incidence of retinal detachment associated with cataract extraction has decreased with the changes in surgical technique over the years, from intracapsular cataract extraction and aphakic correction to phacoemulsification, combined with the placement of a posterior chamber intraocular lens in the presence of an intact posterior lens capsule.25 The reasons for this change are postulated to include: reduced forward shift of the vitreous, reduced shrinkage of the vitreous secondary to hyaluronic acid loss from the eye, and support of the vitreous body by the posterior chamber intraocular lens and posterior lens capsule. Furthermore, in eyes with an intact posterior lens capsule after extracapsular cataract extraction, surgical or Nd:YAG laser posterior capsulotomy is associated with an increased incidence of subsequent retinal detachment.26 Nevertheless, the risk of retinal detachment after cataract extraction continues despite these changes. Rowe et al.27 predicted that 10 years after either phacoemulsification or extracapsular cataract extraction, the cumulative probability of retinal detachment was 5.5 times higher than expected. The question of whether patients have inherent risk factors that are manifested by manipulations during cataract extraction, arises when considering phakic fellow eyes. Sharma et al.28 evaluated fellow eyes of patients experiencing a pseudophakic rhegmatogenous retinal detachment and noted that 7.8% developed a phakic retinal detachment over a mean follow-up period of 57.4 months. Neuhann et al.29 reviewed the risk of pseudophakic retinal detachment in a large series of very high myopic eyes. To the surprise of many they found that pseudophakia means no additional risk. Their explanation is based on the facts that shifting the risk of retinal detachment to early age with increasing axial length of the eye30 is a consequence of a concomitant increase in vitreous mobility from premature posterior vitreous separation.31 At the time of cataract extraction, vitreous mobility and risk of retinal detachment are at a summit for some time. Therefore, the clinically relevant risk of pseudophakia in emmetropia and low myopia, becomes irrelevant in very high myopia. Various intraocular inflammatory and infectious conditions can cause vitreous gel liquefaction, PVD, and retinal breaks. Ocular toxoplasmosis,32 ocular toxocariasis,32,33 and pars planitis34 are associated with morphologic vitreoretinal changes that can lead to vitreoretinal traction and retinal breaks. The acute retinal necrosis syndrome and cytomegalovirus retinitis are forms of infectious retinitis that can result in multiple small breaks along the border between atrophic and normal retina and within necrotic retina.4,11,35

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