Ординатура / Офтальмология / Английские материалы / Textbook of Vitreoretinal Diseases and Surgery_Natarajan, Hussain_2008

Textbook of Vitreoretinal Diseases and Surgery

Introduction

Optical Coherence Tomography (OCT) is a new diagnostic tool in the diagnosis and management of Age-related macular degeneration (AMD). It uses light waves (near infrared laser-820 nm) to give cross-sectional images of retinal tissue with a resolution of 10 microns (Stratus OCT). It provides tomographic image and simulates in vivo histopathology. 1

Technique

The scan protocols used in AMD are the fast macular thickness map, macular thickness map, radial lines, line scan and raster lines. Any two of the protocols would provide adequate information. The fast macular thickness map is done first as the protocol takes 1.92 seconds.1 This is very important in patient with ARMD as they have problems in fixation.

OCT complements clinical examination and fundus fluorescein angiography (FFA) in the management of AMD. It aids in the following:

1.Diagnosis and disease categorization:1 It can identify neovascular AMD, primarily understanding the level of lesion characteristic, thus identifying classic and occult choroidal neovascular membrane (CNVM). These are complementary to biomicroscopic examination and FFA.

2.Identify neovascularisation in doubtful cases of dry AMD: In symptomatic patients with extensive soft drusen, OCT helps in detecting occult CNVM.2 It is a step in diagnosis between FFA and Indocyanine green (ICG) angiography.

3.Detects associated changes like Pigment epithelial detachment, Cystoid macular edema and subretinal fluid.3-5

4.Response to treatment: OCT can demonstrate the response to treatment of CNVM viz. PDT or anti VEGF drugs like ranibizumab, bevacizumab, etc.3, 6-9

5.Can monitor treatment response and aid in decision making for re-injection of anti VEGF drugs. Presently this is the most important implication, which has radically changed the treatment approach in AMD. It has evolved the variable dosing of drugs.10,11

Disease Categorization

DRY AMD

Drusen appear as bumps or focal elevations of the RPE. There is no shadowing (Figure 13-1).

Classic CNVM

There is disruption of the RPE with an area of increased reflectivity in the foveal center. There is increase in the retinal thickness with a cystoid space (Figure 13-2).

Occult CNVM

There is an elevation of the RPE layer with an area of enhanced reflectivity under it. There is increase in the retinal thickness suggestive of Occult CNVM. The ICG angiography demonstrates

150 the large plaque lesion (Figures 13-3 and 13-4).

Textbook of Vitreoretinal Diseases and Surgery

FIGURE 13-7: FFA and OCT images 6 months post-treatment showing intraretinal cystic change. The patient underwent retreatment

FIGURE 13-8: Following retreatment, note the disappearance of the cystic space

OCT-guided Treatment of AMD

Approach towards treatment of AMD has undergone paradigm shift over time. OCT-guided management has become the standard of care in the era of VEGF inhibitors. Presently the treatment of AMD is with the use of two drugs—ranibizumab, a fully humanized antibody fragment has short half life and greater affinity for VEGF.14 Bevacizumab is the other drug which is off-label; full length

154 monoclonal antibody with long half-life.15

Optical Coherence Tomography in Age-related Macular Degeneration

FIGURE 13-9A: Pretreatment ICGA, FFA and OCT images of an occult CNVM

In MARINA and ANCHOR16, 17 studies, with monthly injections of ranibizumab, the mean change of visual acuity over time showed a 17 letter difference between the treated group and the sham group. Further 25-34% patients showed visual improvement of 15 or more letters.

In clinical practice, replicating clinical trial methodology is not possible. Moreover, the drug is expensive and may show variable response. In addition, the possible risks of systemic side effects also exist.

The treatment could be based on the following:

1.Empiric dosing

2.Visual acuity

3.Anatomic characteristics.

EMPIRIC DOSING

The PIER trial18 was started to evaluate whether quarterly dosing of ranibizumab would have the same result as the MARINA trial. The mean change of visual acuity over time showed a drop in the curve to just below the base line compared to +6-7 letters in the MARINA study. The 3-line gainers dropped to 13%. Hence, empiric dosing is not the right approach. It was further observed that the

initial gain (3 month data) was similar to the MARINA trial as both the studies had the initial three 155 monthly injections.

Textbook of Vitreoretinal Diseases and Surgery

FIGURE 13-9B: Post-treatment FFA and OCT showing good resolution of CNVM following treatment

VISUALACUITY

The functional change could be used to decide on treatment and retreatment. However this has been shown to be unreliable as the ETDRS chart recording, necessary for using visual acuity as a guide for treatment, is not available in many clinics. Further, there is a variation in vision in patients of AMD and the functional change is not reflected in the anatomy of the macula. Hence this would be quite unreliable, though could be an additional parameter in the decision to retreat.

ANATOMIC CHARACTERISTICS

This is a true reflection of the activity and OCT is the best tool to evaluate.

The OCT could be assessed in the following ways:

Quantitative

Here the analysis is done by averaging the retinal thickness using the various scans. The two commonly used measurements are the central macular thickness and the central subfield. The problem with this method is that patients with AMD have improper fixation leading to various errors. Therefore,

156 boundaries are incorrectly placed by the instrument leading to errors in retinal thickness.19

Optical Coherence Tomography in Age-related Macular Degeneration

FIGURE 13-10: Intra-retinal cysts (arrow)

FIGURE 13-11: OCT image showing diffuse retinal edema (long arrow), sub-retinal fluid (arrowhead) and sub-RPE fluid (short arrow)

Presence of subretinal fluid, retinal pigment epithelial detachment (RPED) and eccentric CNVM leads to significant errors when using the standard analysis protocols. Errors in central macular thickness can occur in 62.2% patients with macular pathology.20 Incorrect boundaries and off center artifacts contribute to these errors. Measurements of retinal thickness with the analyzed images are significantly lower than when measured on the original image in patients with AMD.21

Qualitative

The OCT image is evaluated for the following features:

1.Intraretinal cysts (Figure 13-10)

2.Diffuse retinal edema (Figure 13-11)

3.Subretinal fluid (Figure 13-11) and

4.Sub-RPE fluid. (Figure 13-11),

All the OCT scans should be evaluated for the above four findings, which would indicate activity.

Goff et al3 used a combination of quantitative and qualitative assessment when using bevacizumab as monotherapy or in combination with PDT. They reported resolution of the subretinal fluid and retinal edema following treatment but the RPE detachment a took longer time. Emerson et al9 used the presence of intraretinal or subretinal fluid as a guide for retreatment with bevacizumab in AMD. The PrONTO study11 on 40 patients using OCT as guide to treatment with Ranibizumab showed the

mean change in visual acuity was 9.1 letters, similar to the outcome of MARINA and ANCHOR 157