The field of retinal imaging has evolved very rapidly over the course of these past few years. After angiography (with fluorescein and indocyanine green) used for exploration of the vascular network of the retina, the appearance of optical coherence tomography (OCT) at the end of the nineties, was the second revolution in retinal imaging. Firstly Time Domain, and now today Spectral Domain, this non-invasive examination, without any counter-indication, is used to analyse the retinal structure with a resolution of 5 to 7μm and has become an essential examination in the diagnosis and monitoring of mainly macular medical-surgical retinal pathologies.
OCT and macular medical pathologies
1. Exudative diabetic maculopathy
Exudative diabetic maculopathy has benefited a great deal from the contribution made by OCT. This technique firstly permits a much easier diagnosis of maculopathy, even in cases where opalescence of the crystalline is present. It also helps the ophthalmologist to classify exudative maculopathy. Finally, it facilitates patient monitoring. It should also be underlined that OCT has become the benchmark examination in the exploration of exudative diabetic maculopathy, replacing retinal angiography with fluorescein whose role has now been reduced to the screening of ischemic maculopathy.
1.2. OCT diagnosis of diabetic macular oedema
The diagnosis of diabetic macular oedema corresponds to diffuse or localised thickening of the neurosensory retina macula. This thickening can take various forms as we will see in the presentation of the various types of exudative diabetic maculopathies and in most cases leads to a loss of the normal foveolar profile. The threshold for defining retinal thickening varies according to the anatomical markers taken into account by the machine (pigmentary epithelium or Bruch's membrane).
1.3. Assistance with classification of diabetic maculopathy
There are two main types of exudative diabetic maculopathy: diffuse macular oedema, cystoid or not, and focal macular oedema. A diffuse macular oedema appears in OCT as a diffuse thickening of the neurosensory macula retina without any predominant localisation. It is always combined with a loss or even inversion of the foveolar depression and may take on a particular appearance with the presence of cystic intraretinal vacuoles defining the cystoid shape of this oedema (Fig.1).
Fig. 1: Diffuse cystoid macular oedema with vacuoles.
As its name indicates, a focal macular oedema corresponds to a thickening localised in the neurosensory retina, in some cases affecting the foveolar area (Fig. 2).
Fig. 2: Focal macular oedema.
Types of macular oedema combining the 2 types described previously also exist. These are known as mixed macular oedema. In this form, a map of the thickened areas is used for more reliable identification of the areas to be treated, resulting in better results from laser treatment (Fig. 3).
Fig. 3: Map of macular oedema.
Finally, the crucial role played by OCT should be underlined in the identification of diabetic macular traction oedema, the biomicroscopic diagnosis of which is very difficult (Fig. 4).
Fig. 4: Traction macular oedema.
1.4. Assistance with patient monitoring
It is difficult to asses the evolution of exudative diabetic maculopathy by means of a simple biomicroscopic examination. The only two criteria truly demonstrating this evolution are retinal thickness assessed by OCT and the measurement of visual acuity. OCT surveillance of retinal thickness has benefited from the technology known as “eye-tracker”, present on the latest generation OCT, and used to create a macular map including markers defined during a previous examination. This facilitates the comparison of retinal thicknesses between two examinations.
OCT is an essential examination for AMD, particularly exudative AMD. Along with angiographic data it can be used to make the diagnosis and define the type of neovessels involved and, during monitoring, will act as one of the main sources of treatment criteria.
The diagnosis of choroidal neovessels can, in certain circumstances, prove a precious aid, particularly in the diagnosis of occult neovascularization. Due to advertising campaigns, the number of patients consulting for visual problems evocative of choroid neovessels has tended to increase exponentially over these past few years. These are elderly patients who in most cases have a cataract and frequently suffer from pigmentary modifi-cations or macula drusen. Their ophthal-mological examination is therefore often delicate and the formal absence of neovascular processes remains difficult to establish. Within this context, OCT is a fundamental examination since it is so quick to perform and is non-invasive, unlike angiography. It provides us with information regarding the possible presence of any neovascular complication.
Two types of signs indicate the presence of a neovessel: direct signs which are visualisation of neo-vessels and indirect signs, witnessing to neovascular activity within the retina. Choroid neovessels appear in the form of a hyper-reflective area located either under the pigmentary epithelium (occult neovessel) or on the pigmentary neovessel (visible neo-vessel). The indirect signs of neovessels are retinal oedema, with greater or lesser presence of intra-retinal cystic spaces, serous detachment of the neuroretinal and the detachment of the pigmentary epithelium.
2.2. Patient monitoring
The protocol for keeping a check on patients suffering from AMD requires monthly monitoring. In addition to visual acuity, the parameter evaluated to decide on possible re-injection of anti-VEGF is the aspect under OCT. Indeed, in case of persistence or relapse of exudative signs (retinal oedema, serous retinal detachment, presence of intraretinal cystic spaces) a further injection is recommended. OCT is therefore the preferred examination for the monitoring of AMD patients, well ahead of angiography (Fig. 5).
Fig. 5: AMD with neovascularization and cystoid retinal oedema before and after intra-vitreous anti- VEGF injection.
3. High myopia
High myopia is often complicated by macular haemorrhaging, the aetiology of which is often difficult to identify. Information obtained using angiography is highly perturbed by the mask effect of the blood present in the retina. Within this context one will understand the possibilities offered by OCT which permits fine analysis of the retinal structure, providing information in the majority of cases on the possible diagnoses.
4. Central serous chorioretinitis
OCT makes an enormous contribution to central serous chorioretinitis. It facilitates diagnosis by showing a bubble of serous detachment in the neuroretina that may be associated with small detachments of the pigmentary epithelium without any underlying sub-epithelial abnormality. Angiography is therefore optional in most cases at the time of diagnosis if the OCT appearance and the context are typical; it will be done only to assess evolution of the pathology.
OCT and surgical macular pathologies
In surgical macular pathologies, OCT is used to make the surgical diagnosis in difficult pathologies and also to direct surgery.
1. Making the surgical diagnosis
OCT is particularly useful in certain cases where diagnosis with a slit lamp is difficult. One such difficult diagnosis is macular retinoschisis of high myopia. This separation of layers of the neuroretina can be difficult to see in myopic staphyloma. It is mainly OCT that is used to make a positive diagnosis, by showing thickening of the neuroretinal, schisis and possible foveal detachment. Discovery of a full thickness macular hole is a surgical indication (Fig. 6a and 6b).
Fig. 6a: High myopia macular retinoschisis without macular hole.
Fig. 6b: High myopia macular retinoschisis with macular hole.
The syndrome of idiopathological vitreo-macular traction or diabetic retinopathy is a pathology of the vitreomacular interface for which OCT can provide a definite diagnosis. Whether adherence is spread over the macular surface (Fig. 7) or only foveolar, surgery is required if the OCT image is accompanied by a macular syndrome. Within the context of a diabetic traction macular oedema, aspect under OCT can counter-indicate the performance of staggered macular photocoagulation.
Fig. 7: Diabetic traction macular oedema in Time Domain with spread traction.
OCT also enables us to diagnose macular hole threats easily and to monitor their evolution which can be spontaneously resolutive. These stages 1A and 1B were very difficult to show with biomicroscopy and are not surgical.
OCT is used to make the difference between a macular hole and a lamellar hole, which does not require surgery (Fig.8).
Fig. 8: Stage 3 macular hole with detached edges.
2. Orienting and defining surgery
OCT is used to measure the diameter of macular holes by placing the two cursors on the inner edges of the hole. A multicentric study has shown a higher rate of closure when the limitator is removed during vitrectomy in macular holes larger than 400 μm. Moreover, a face down post-operative position is also beneficial in holes larger than 400 μm.
Finally, small macular holes of less than 250 μm can close spontaneously and there is a rule that no surgery is offered before the patient is seen again a month later (Fig. 9).
Fig. 9: Stage 2 small diameter macular hole.
3. To assess the functional and anatomical benefit of surgery.
OCT can show raised and cystic edges on macular holes, which would be a better prognosis than atrophic edges (Fig. 10).
Fig. 10: Stage 3 macular hole with cystic edges.
Finally OCT can be used very easily to visualise post-surgical closing of the macular hole (Fig. 11a and 11b).
Fig. 11a: Stage 2 macular hole before surgery.
Fig. 11b: Same macular hole 1 month after posterior vitrectomy.
OCT is a non-invasive examination, essential in the care of medical and surgical macular pathologies, which enables, amongst other things, either diagnosis in difficult cases such as high myopia retinoschisis or the monitoring of developments in the pathology and the orientation of the therapeutic decision, particularly in AMD.