CHAPTER 63 Surgical strategies for AMD
Introduction
The retinal pigment epithelium (RPE) holds the strategic position of the metabolic gatekeeper between photoreceptors and the choriocapillaries and hence plays a key role in maintaining retinal function. The RPE/photoreceptor complex suffers cumulative damage over a lifetime, which is thought to induce age-related macular degeneration (AMD) in susceptible individuals with a genetic predisposition (including dysregulation of the complement system). Two main routes of the disease occur: a slower progressing atrophic form where most patients currently have limited therapeutic options, and a potentially rapidly blinding exudative/neovascular form1,2. For the latter, intravitreal injections with anti-angiogenic drugs have now replaced laser photocoagulation or photodynamic therapies, offering a highly effective, yet palliative treatment for neovascular AMD. Still, long-term effects remain to be elucidated, and a proportion of patients lose vision despite pharmacologic therapy. A thorough review of intravitreal anti-angiogenic treatments is beyond the scope of this article; the interested reader is therefore referred to comprehensive articles on this matter (see for example, refs 3,4).
This chapter describes the current state for surgical approaches to AMD, and touches on future perspectives. Within this context, the use of the RPE cell as a ‘therapeutic agent’ is a potential strategy, as healthy RPE cells would, in theory, restore the functions of their degenerated counterparts (Fig. 63.1). The ultimate success of these cell-based therapies, however, clearly depends on delivery and maintenance of cells in a properly polarized state, capable to perform most, if not all, complex functions of the RPE. While in dire need for patients with advanced AMD, it is unlikely, in the authors’ opinion, that clinically feasible photoreceptor replacement will be achieved in the mid-term, and this aspect of retinal tissue engineering is therefore not discussed.
Epidemiologic considerations and terminology
Age-related macular degeneration is the leading cause of visual impairment in industrialized countries in patients over 50 years of age5,6. The prevalence of AMD increases with age so that up to one-third of individuals aged 75 and older suffer from some form of AMD7,8. Currently, it is estimated that 1.75 million individuals suffer from this disease in the United States and about 7 million are reported to be ‘at risk’9. Higher expectations on quality of life demanded by the expanding aging population hence result in increasingly significant morbidity from AMD10.
Clinical features
AMD is believed to be caused by progressive deterioration of RPE, Bruch’s membrane, and the choriocapillaris–choroid complex, which consequently leads to subsequent damage of the photoreceptor cells1,2,11. The hallmark of the disease and first visible changes are drusen, which present clinically on funduscopy as subretinal yellowish-white dots. According to their size and margin they are divided into hard and soft drusen. The risk for progression to advanced disease stages correlates to number, size, and density of drusen12. Soft drusen can coalesce and form pigment epithelial detachments (PED). Of these PEDs, 30–50% will become vascularized and may cause severe visual loss within the following years13. In patients with bilateral drusen and good vision in both eyes, the annual incidence of new atrophic or exudative lesions is approximately 8% over 3 years14.
In AMD, the RPE cells may become dysfunctional, which is seen in pigment irregularities with either hyper- or hypopigmentation often accompanied by drusen, which results either in the non-neovascular form with progression to geographic atrophy (‘atrophic AMD’) or in neovascular AMD (‘exudative, wet AMD’) with choroidal neovascularization (CNV) accompanied by exudation and hemorrhage, and subsequent cell death and scar formation10,15. Neovascular membranes secondary to AMD, particularly in patients on anti-coagulants, are prone to extensive bleeding into the subretinal space or vitreous. The hemorrhage in turn prevents metabolic exchange between the RPE and outer neural retina, and is retinotoxic16.
Fundamental principles
Dysfunction of the RPE may alter the extracellular environment for photoreceptors and Bruch’s membrane and thereby contribute to a variety of sight-threatening diseases including AMD. A thorough discussion of its underlying fundamental principles is beyond the scope of this article, as it encompasses factors such as, genetic polymorphisms of the complement system17,18, accumulation of intralysosomal degradation products affecting RPE function19, autoimmune mechanisms20, and Bruch’s membrane aging21.
Medical treatments
CCR3, an eosinophil/mast cell chemokine receptor, has been specifically shown to be expressed in choroidal neovascular endothelial cells in patients with CNV due to AMD. Its suppression in mouse models with induced CNV is more effective, yet less toxic than anti-VEGF treatments and it therefore represents a new, very promising therapeutic strategy in exudative AMD patients22.
Indications for surgery
Most AMD patients suffer from the slowly progressive early dry form, for which there currently are no efficient therapies. Some patients will convert into neovascular AMD, which can now be successfully treated by intravitreal injection of anti-VEGF drugs. However, with long-standing disease and/or associated structural damage to the RPE and surrounding structures, treatment responses may be disappointing and warrant the search for alternative strategies. Surgical approaches in the past have included excision of CNV, and macular translocation as well removal of massive subretrinal hemorrhages. In these instances, a reconstruction of the submacular architecture or maculoplasty would represent a curative treatment21,23. Currently discussed AMD subforms for which a surgical intervention may be considered include:
Patient selection and assessment
The most crucial factor for successful postoperative visual rehabilitation is preop macular function. Da Cruz and associates recommend, besides obtaining a history of duration of visual loss and BCVA, performing microperimetry, fixation analysis, and multifocal ERG24. High resolution, spectral domain OCT with 3D rendering allows for better assessment of subretinal pathology. The prognostic value of pre- and postop RPE autofluorescence imaging has recently been addressed, whereby a normal autofluorescence signal would indicate a functioning photoreceptor–RPE complex 24,25. Particularly for patients with primary GA of the RPE, it may serve as an important adjunct to determine remaining subfoveal RPE coverage.
Surgical techniques
Macular surgery for massive hemorrhages
Circumscribed submacular hemorrhages can sometimes be liquified by intravitreal injection of 25–75 µg recombinant tissue plasminogen activator (rt-PA) and gas, followed by face-down positioning for several days to displace the lesion peripherally. The optimal therapeutic window for this intervention is within 2 weeks after the initial event; early treatments are clearly preferable. After this time, the blood clot may begin fibrotic remodeling which can induce strong adhesions within the adjacent neural retina. All subsequent therapies become difficult and fibrinolysis no longer works26.
With extensive bleeding more invasive measures may become necessary. These may encompass, depending on the extent of the hemorrhage, a vitrectomy with a paramacular retinotomy, or 180° retinotomy, removal of the hemorrhage and with subsequent gas, or silicone-oil fill of the eye; see Figure 63.227.
Macular surgery for choroidal neovascularization
Surgical interventions in exudative AMD had their beginnings in excisions of subfoveal CNV28,29, which, however, usually resulted in simultaneous removal of the RPE ‘trapped’ within the neovascular complex30