This is the first in a series of three articles. It aims to develop an understanding of the processes involved in the different kinds of retinal detachment. The next two articles put forward a practical and structured approach to patients who present with symptoms of flashes and floaters including;
1 The questions that will elucidate risk factors and rule out innocuous conditions (what to ask).
2 Observations (what to look for and how to look for it).
3 Assessment of this information to develop a diagnosis and management plan.
There is also a discussion of national guidelines for referral and the changing role of optometrists in locally commissioned schemes with their own protocols.
Maintenance of attachment
Retinal detachment is the separation of the neurosensory retina (NSR) from the underlying retinal pigment epithelium (RPE).
Figure 1: Large retinal detachment with neurosensory retina separating from the pigmentary retina
In the embryo these two layers come into apposition when the optic vesicle invaginates to form the optic cup, but anatomical connection between them is limited to interdigitation between photoreceptor outer segments and microvillae on the RPE cell surface, with the help of the extracellular matrix which may act as glue. The most important factor is hydrostatic force, which maintains attachment by moving water outwards from the vitreous into the choroid. This movement can either be active (RPE pump) or passive (osmotic gradient). These forces of attachment can be overwhelmed allowing the accumulation of fluid in the potential space between the NSR and the RPE. The mechanisms of retinal detachment have traditionally been classified as rhegmatogenous, tractional and exudative. These are set out in Table 1.
Table 1: Types of retinal detachment (PVD = posterior vitreous detachment)
Exudative retinal detachments (ERD)
These are the rarest form and some optometrists may rarely encounter them. The list of conditions that can cause them is long and diverse (see table 2).
Table 2: Aetiology of exudative retinal detachment
Under normal conditions water flows from the vitreous to the choroid. Flow in this direction is achieved by the relative hyperosmolarity of the choroid with respect to the vitreous and by the RPE which actively pumps ions and water. Medical conditions which interfere with either of these mechanisms can lead to accumulation of fluid in the subretinal space. For example, pregnancy induced severe hypertension (eclampsia or pre-eclampsia) causes choroidal arteriole occlusion with breakdown of the outer blood retinal barrier. This changes the composition and osmolarity of the choroidal interstitial fluid. In addition, the RPE pump is disabled through hypoxia. Pre-eclampsia is in fact one of the rarest causes of exudative detachment, but it has given rise to a very widespread myth that the strains of child birth can lead to retinal detachment. Optometrists can definitely reassure their patients that this is not the case.
Figure 2: Exudative detachment related to choroidal melanoma
In ERD there is no break in the NSR so there is no communication between the subretinal fluid and the vitreous. The subretinal fluid in an ERD is rich in protein and is therefore heavier than the vitreous so it will always settle through gravity to the lowest part of the eye, which is inferior when the patient sits upright and at the posterior pole when the patient is supine. Shifting fluid is unique to ERD.
Table 3: Aetiology traction retinal detachment
Traction retinal detachment (TRD)
Traction retinal detachment also occurs in the absence of retinal breaks. Here the neurosensory retina is separated from the RPE by bands within the vitreous or membranes on the retinal surface, which undergo contraction. These can occur as a consequence of penetrating trauma but the most common cause is proliferative diseases that are characterised by progressive retinal ischaemia, such as diabetes (figure 3), sickle cell retinopathy, retinal venous occlusion or retinopathy of prematurity (table 3).
Figure 3: Traction retinal detachment caused by proliferative diabetic retinopathy
In the case of proliferative diabetic retinopathy, fibrovascular stalks grow from the retina into the vitreous cavity where they anchor to the posterior hyaloid membrane which becomes taut and acts as a firm scaffold. Fibrovascular tissue has a tendency to contract, which is beneficial in wound healing, but in this situation it pulls the loosely attached neurosensory retina away from the RPE. In rare cases a retinal break can develop, usually near a site of retinal traction, which leads to a rapidly progressing combined tractional/rhegmatogenous detachment.
Rhegmatogenous retinal detachment (RRD)
The term rhegmatogenous is derived from the Greek word “rhegma”, meaning a break, rent or fissure. It encompasses all detachments where a break in the retina is present, which are much more common than those caused by traction or by exudation of subretinal fluid. RRD has an annual incidence of around 12 per 100,000 people1 and probably accounts for 85% to 90% of all detachments.
Figure 4: Retinal tear with associated detachment
Not all retinal breaks have the same propensity to cause a retinal detachment. Table 4 shows that the vast majority of RRDs are initiated by a posterior vitreous detachment (PVD), which causes a flap tear (figure 4) - or rarely a giant retinal tear. These are more likely to progress to a sight threatening detachment because they have a combination of all three of these features:
1 Retinal break.
2 Liquefied vitreous to flow through the break.
3 Continuing vitreo-retinal contact which holds the break open.
Table 4: Type of break responsible for RRD in a prospective two-year survey2
Traction is often said to exist when the vitreous cortex remains attached to the flap of a tear. This is classified as ‘dynamic’ traction because it is induced by eye movements. It expands the area of detachment by causing the retinal break to gape open, encouraging flow into the subretinal space. This is in contrast to the ‘static’ traction seen in proliferative diabetic retinopathy, which actually pulls the sensory retina away from the RPE.
It is important to remember that some cases of RRD do not present with symptoms of PVD, because they are caused by other mechanisms. These will have different predisposing factors, for example atrophic round hole detachments tend to occur in younger and more myopic individuals.
Posterior vitreous detachment (PVD)
The vitreous is a gel which consists of 98% water and 2% solid elements, mainly collagen fibrils. The organisation of collagen fibrils is not uniform throughout the vitreous. The outer boundary, or cortex, is characterised by much more densely packed lamellae which are bound together to form a tough outer membrane (also known as the posterior hyaloid membrane). It is not elastic, which is in contrast to the majority of vitreous gel which will stretch in response to tension. It has molecular structures which integrate with the underlying inner limiting membrane (ILM) of the retina.
In posterior vitreous detachment, these relatively weak bonds are overcome and the two layers separate. In contrast to this, the attachment is extremely strong at a zone known as the vitreous base, which is 3-4mm wide and straddles the ora serrata. Here the collagen fibrils are so well integrated into the cytoskeleton of the retina that the posterior hyaloid membrane and the ILM cannot be separated. This means that a PVD can progress towards the posterior border of the vitreous base but no further. Other areas of relatively strong attachment are; around the optic disc, at the macula, over blood vessels and at the edges of lattice degeneration.
The process of PVD is not completely understood, but it includes the perforation of the posterior hyaloid membrane, allowing liquefied vitreous, with the aid of eye movement, to dissect the vitreous cortex from the ILM. The extent of a PVD can be localised, partial or total (up to the posterior border of the vitreous base). The rate of progression can sometimes be slow, taking months or years. PVD appears to be a normal age-related event. In emmetropes it is rare under the age of 40, but post mortem studies3 show that two thirds of people over 60 years old will have undergone this change, rising to a figure of 87% in those over 80. The vast majority of these will not be aware of this event and will not encounter any problem. However, there is the potential for a tear to occur at a point of increased adhesion as the remainder of the cortex continues to separate from the ILM (figure 5). This is much more likely in acute PVD, when a domino effect in destabilised vitreous gel causes many of the collagen fibrils to aggregate together, causing shrinking of the solid component of the gel, known as vitreous collapse (figure 6). It is known that when PVD occurs as a sudden abrupt event, which causes symptoms, there is a 10% to 15% chance of a retinal break4 (most likely a flap tear).
Figure 5: Retinal tear
PVD symptoms
Photopsia
Photopsia are experienced as brief recurrent flashes of light, which are more evident in dark conditions or following abrupt saccadic eye movements. They are due to traction on the NSR by the posterior hyaloid membrane at some point where they still remain in contact. It is tempting to think that this would be at the point where a flap tear occurs, but photopsia are a symptom of PVD and not of retinal detachment. Photopsia are nearly always seen in the temporal field and do not correspond to the location of a tear, if one is present. The posterior hyaloid membrane always remains attached at the vitreous base where the anatomical links are unbreakable. It is this area of the retina that initiates photopsia, when force is applied to it through movement of partially liquefied gel.
Figure 6: Vitreous collapse
Floaters
When a patient experiences sudden onset floaters in one eye they could indicate a PVD. A single ring or crescent shape could be a Weiss ring (figure 7). A cloudy haze or a shower of black dots, streaks or cobwebs, could be due to haemorrhage into the vitreous, when the detaching vitreous cortex encounters stronger adhesion over a retinal blood vessel. Alternatively dots could be pigment liberated from the RPE through a tear (tobacco dust).
Figure 7: Large annular floater subsequent to a posterior vitreous detachment
Subjective reduction in vision
It is important to remember that when the vitreous cortex leaves its habitual position adjacent to the retina and comes further forward in the vitreous cavity, it too can cause visual symptoms. These often amount to a vague feeling that something is obscuring the vision. It may or may not appear to drift in and out of view. These apparently non-specific symptoms should not be ignored if they are monocular and definitely of sudden onset. One study5 of PVD symptoms showed that subjective reduction in vision (which did not always correspond to poor Snellen acuity), was highly predictive of the presence of both detachments and breaks without detachment.
In the next part of this series we will build on this understanding of the pathophysiology of retinal detachment, to develop a structured approach to the assessment of patients who present with symptoms of flashes and floaters.
Dr Graham Macalister is a specialist optometrist at Moorfields Eye Hospital
References
1 Mitry D, Charteris,D, Yorston D, et al (2010). The epidemiology and socioeconomic associations of retinal detachment in Scotland: a two-year prospective population-based study. Investigative Ophthalmology & Visual Science 51(10), 4,963-4,968.
2 Mitry D, Singh J, Yorston D, et al (2011). The predisposing pathology and clinical characteristics in the Scottish retinal detachment study. Ophthalmology, Jul 2011, vol 118, no 7, p 1,429-1,434.
3 Hikichi T, et al (1995). Comparison of the prevalence of posterior vitreous detachment in whites and Japanese. Ophthalmic Surg 26(1): 39-43.
4 D’Amico DJ (2008). Clinical practice. Primary retinal detachment. New England Journal of Medicine 359(22), 2,346-2,354)
5 Dayan MR, Jayamanne DG, Andrews RM, Griffiths PG (1996). Flashes and floaters as predictors of vitreoretinal pathology: is follow-up necessary for posterior vitreous detachment? Eye, vol 10, p 456-458.