Use of OCT Imaging in the Diagnosis and Monitoring of

Age Related Macular Degeneration By

Dr. doaa ali mahmoud (md)

1. Introduction

• Optical Coherence Tomography (OCT) is a non-invasive, high-resolution imaging technique that has been introduced in the clinical practice at the beginning of the last decade. The first application of this method has been recorded in the field of ophthalmology.

• Retinal diseases such as Age-related Macular Degeneration (AMD), central serous chorioretinopathy, macular hole, vitreo-macular interface syndrome and diabetic maculopathy have taken advantage of this relatively new imaging method. Among these, AMD is by far the ocular condition that has benefited the most from the enormous advantages offered by OCT, in terms of diagnosis, response to treatment and monitoring.

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Pathology of AMD

Despite recent progress, age-related macular

degeneration (AMD) remains the leading cause of

vision loss in high-income countries, and its

incidence appears to be increase probably due to

longer life span and improved methods of detection.

Loss of visual acuity typically results from

progressive degeneration of the

choriocapillaris, retinal pigment epithelium

(RPE), and photoreceptors although the earliest

manifestation of the disease appears to be

abnormalities within Bruch’s membrane.

There are two variants: atrophic (dry) and

neovascular (wet/exudative), with the latter

being less common and accountable for

more severe visual loss. Involvement is

usually bilateral although there may be

asymmetry in progression.

Exudative ARMD precursors:

(a) Soft drusen can be recognised by

irregular tapering edges and a fluffy eosinophilic content.

(b) Basement membrane deposits (other names: basal

linear, basal laminar are different to soft drusen in their location, staining characteristics, and ultrastructural appearance. The deposits form broad strips beneath the RPE and have a striated pattern

Retina - Central degenerationARMDDeposits - Basement membrane deposit

1-Dry type:AMD there is also concomitant thickening of the collagenous

layers within Bruch’s membrane, degeneration of elastin and

collagen within Bruch’s membrane with calcification of Bruch’s

membrane, increased levels of advanced glycation end products,

and accumulation of lipids as well as exogenous proteins. [These

changes may serve as a hydrophobic barrier to impede the

passage of fluid and nutrients between the choroid and outer

retina resulting in relative ischemia.

Apearance of the macula:

1 Atrophic ARMD: geographic atrophy characterised by

pale areas of patchy loss of the RPE. These areas may grow in size and coalesce.

a well demarcated area of depigmentation is seen at the macula. The depigmented oval areas have a petaloid pattern and are surrounded by a rim of hyperpigmentation . At the histological level, there is total atrophy of the outer retina in the region.

Wet type:Neovascular AMD Neovascular AMD represent 50% to 65% of advanced AMD, while the other cases correspond to atrophic forms without neovascularization.

Risk factors influencing the course of wet AMD are:

similar to those of age-related maculopathy (ARM) and essentially comprise risk factors for arteriosclerosis (hypertension And hyperlipidemia), environmental, and lifestyle factors. However, recently identified genetic factors also probably play a very important role in the pathogenesis of AMD

Most CNV can be classified into three main categories:

- occult,

- classic,

-and/or mixed;

They May also be recognized as:

-Type 1: Pre-epithelial,

▬ Type 2: Sub-epithelial (sub-RPE), and/or

▬Mixed.

Exudative ARMD: at the early stage, small capillaries are

present between the RPE and Bruch’s membrane

This abnormality is referred to as a type 1 SRNVM

and progression takes the form of an established

Fibrovascular membrane in which arterioles And

venules can be identified. A vicious circle arises when

the small capillaries leak or burst with further

inflammation

and fibrovascular repair. Subsequently, interference

with photoreceptor metabolism is followed by atrophy

and degeneration of the outer retina.

2-WET ARMD: characterised by the presence of :

subretinal neovascular membrane. There may be associated subretinal exudation, haemorrhage, or a fibrovascular (disciform) scar. The haemorrhage may

track through the retina into the vitreous. Pigmented pithelial detachment (PED) may develop in subretinal neovascularisation with serous exudate or haemorrhage.

A type 2: subretinal neovascular membrane in exudative

ARMD refers to an extension of fibrovascular proliferation

between the RPE and the neural retina (subretinal space).

This potential space provides an easy extension of haemorrhage from the fragile capillaries.

Continuing leakage and haemorrhage from the capillaries

lead to large submacular fibrous scars. This is termed a disciform scar. Continuous subretinal traction may lead to retinal distortion and rupture of the retina and its blood vessels may be followed by bleeding into the vitreous. Haemorrhage may be complicated by a massive retinal Detachment.

Retina - Central degenerationARMD - Wet/NeovascularSub-RPE (Type I),(2)

1- ocult CNV:OCT showed a minimal, regular, thin PED. However,

separation of the RPE from Bruch’s membrane (BM) that became clearly visible, confirmed the presence of a PED with a homogeneous, moderately reflective cavity And minimal shadowing.

• The exudative reaction was very minor,causing only a slight increase in retinal thickness, but no SRF or cysts . The outer nuclear layer was also thinned over the juxtafoveal PED, which extended as far as the center of the fovea.

OCCULT CNV Initial, dormant lesion, discovered on clinical examination of the fellow eye .

• Lesion at the initial clinical stage, after onset of the first symptoms .

LARGE, ADVANCED, OCCULT CNVLongstanding vascularized PED, present for more than one year

Classic CNV:

Clinical applications of OCT in AMDThe antero-posterior sections on OCT reveal the succession of the retinal layers and of the Retinal Pigmented Epithelium (RPE), as well as the presence of any spaces between theselayers. The information offered by OCT is detailed, simple and easily interpretable.

Basis of OCT interpretation:

The OCT signs in AMD are extremely valuable for the ophthalmologist.

- Retinal thickening (the thickness of the retina is measured between the internal limiting membrane and the RPE) is determined by the exudation from the choroidal new vessels. -The occult Choroidal Neovascularization (CNV) is revealed by the constant presence of the elevation or detachment of the RPE band. Frequently, the occult CNV is suggested byvarious alteration of the RPE: irregularities, fragmentation, thickening, thinning.

The subretinal fluid appears as diffuse infiltration or as the constitution of cystic spaces in the macular area.

The classic CNV is translated on OCT as hyper-reflective zones adjacent to or away from the RPE. They must be differentiated from other hyper-reflective structures: fibrous tissue, exudate, pigment, pseudo-vitelliform material.

Other signs of prognostic value can be visualized at the level of the outer retinal layers

(outer nuclear layer and external limiting membrane): hyper-reflective spots and areas of densification. They prove the progression of the disease.

Limits of OCTOCT cannot precisely describe a CNV network, nor can it

define its nature: active or pre-fibrotic. Therefore, the OCT scan must be interpreted in correlation with the fundus photography, direction of scan and ideally, the angiography .

Another limitation of OCT is revealed in evaluating the extension of the geographic atrophy (GA). In a recent study, the GA areas identified in SLO scans were significantly larger than the ones detected on the OCT maps. Spectralis OCT showed significantly more mild and

severe segmentation errors than 3D and Cirrus OCT. Taking into account the fact that GA is a frequent form of AMD, this limitation should be resolved in order to identify and

document RPE loss in a realistic manner.

• Associated findings

1- Drusen

subretinal pale areas are either small and dis-crete (hard) or are larger with indistinct edges (soft).

2 -Irregular pigment hyperpigmentation (clumping).

3- Much interest has also centred on changes in Bruch’s membrane

(for example calcification and lipid accumulation)

Which could interfere with metabolism of the RPE cells.

• The following deposits between Bruch’s membrane and

• the retinal pigment epithelium are commonly found in

• A1 Hard drusen are easily identified clinically as tiny,

• discrete, non-pigmented elevations beneath the retina

• between the basement membrane and Bruch’membrane,

• but are not considered to be precursors of neovascularisation.

• 2 Soft drusen possess indistinct borders and may be more

• easily identified with fluorescein angiography. These structures are located between the cell basement membrane and Bruch’s membrane and are implicated in the attraction of blood vessels beneath the RPE (choroidal/subretinal neovascularisation).

Retina - Central degenerationARMD Deposits - Soft drusen

3 A third deposit has been variously termed basal linear/basal

laminar/basement membrane deposit :

This material is located between the cell membrane and the basement membrane and is so thin that it can be difficult to identify clinically. This deposit is often found in relation to neovascular tissue proliferating beneath the macula. Some authors use the term “basal linear deposit” as synonymous with confluent soft drusen .

Shadowing by a large vessel (arrow) in the peri-papillary region is clearly visible.

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