Macular Oedema

The macula is the part of the retina responsible for sharp vision due to its high density of cone photoreceptors. It is situated at the centre of the back of the retina (the posterior pole), lying about 3mm lateral to the optic disc. It has a central depression known as the fovea centralis. There are nothing but tightly packed cone photoreceptors in this area with no overlying blood vessels. This is where visual acuity is ultimately determined.

Macular oedema refers to the accumulation of fluid within the retina at the macular area (distinct from the condition where the fluid accumulates under the retina).This is frequently - but not always - cystoid in nature (cystoid macular oedema, CMO). There are many reasons why this can happen but most commonly, it is seen:

• Following intraocular surgery:¹ where CMO occurs as a consequence of surgery, it is known as the Irvine-Gass syndrome. It is thought to occur as a result of release of inflammatory mediators within the eye and if it is prolonged or recurring, permanent damage can occur.
• In central (and occasionally) branch retinal vein occlusions:² venous occlusion causes a rise in intra-venous and capillary pressure leading to stagnation of blood, hypoxia of the affected structures and damage to the capillary endothelial cells, so ending in extravasation of plasma constituents. In the case of branch retinal vein occlusion, macular oedema occurs when the temporal vein draining the macula is occluded.
• Other causes of CMO include inflammatory diseases (uveitis, birdshot chorioretinopathy, toxoplasmosis), various types of retinal vascular disease (idiopathic retinal telangiectasia, radiation retinopathy), retinal dystrophies (such as retinitis pigmentosa) and drug-induced (such as can occur with topical adrenaline 2%, particularly in patients without a lens).

When macular oedema occurs in the context of diabetic retinopathy, it is referred to as clinically significant macular oedema (CSMO). It occurs as a result of increased vascular permeability due to histological changes, so resulting in a compromise of the blood-retinal barrier and leakage of plasma constituents into the surrounding area. The oedema can be focal or diffuse - its nature will guide treatment.

The third group of patients prone to macular oedema are those suffering from certain forms of age related macular degeneration (ARMD): exudative ('wet') ARMD. This gives rise to formation of choroidal neovascular membranes which leak fluid and blood under the retina, so resulting in macular oedema.

• CMO - Irvine-Gass syndrome is rare in uncomplicated surgery, occurring in about 1% of cases in a very mild form. More complex surgery results in higher rates (epidemiological data reports very mixed figures) as does the presence of pre-existing inflammation. The incidence of other causes of CMO varies according to the specific pathology but is not generally very common.
• CSMO - if the diabetes is undiagnosed and untreated, there is a 25-30% of developing CSMO: this drops by half in treated patients.³
• Wet ARMD - ARMD tends to occur in patients over 60 and is the leading cause of blind and partial sight certification in the over 65s. Wet ARMD occurs in about 10% of all ARMD cases.

History

The exact nature of presentation will vary with the underlying pathology but essentially, if there is no pre-existing disease, patients will complain of a painless impairment of central vision ± a positive scotoma (a black spot within the field of vision). The visual decline is typically in the region of 6/12 to 6/60.⁴ Patients with ARMD may complain of visual distortion (particularly of straight lines) and a few complain of blue-yellow colour blindness. CSMO may be completely asymptomatic and only be picked up in the diabetic screening clinic/unit.

Examination

The normal macula: what am I looking for?

The macula lies about 2 disc diameters temporally (laterally) to the disc itself. It may be seen as a slightly darker area than the surrounding retina (more so in darker people) with blood vessels arching over and under it but not on top of it. Careful examination will reveal the foveal reflex, a small yellow-white shiny reflection in the centre of the macula - this is the fovea centralis.

Identifying macular oedema

It is virtually impossible to diagnose macular oedema without the help of a slit lamp. A clue might be the loss of the foveal reflex compared to the fellow eye. In wet ARMD, there may also be an associated bleed, seen as a well demarcated deep red patch over the macular area. Even if the view is limited, the history and an abnormal Amsler grid will raise suspicions and should prompt referral.

Once macular oedema is confirmed, the cause is likely to be one of those outlined above. In the young (typically type A personality male) patient, it could also be confused with central serous chorioretinopathy, a generally self-limiting condition which typically arises in times of acute stress whereby there is a localised accumulation of fluid within the retina.

• CMO - if the cause is obvious from the history and slit-lamp examination, it does not generally require further investigation. If there is doubt as to its origin or extent, a fluorescein angiography may be performed in order to clarify the situation.
• CSMO - as for CMO, further investigation is rarely needed unless its extent is not clear in which case fluorescein angiography is appropriate. Patients who are suspected to have ischaemic maculopathy also need angiography.
• Wet ARMD - diagnosis is made on history and slit-lamp examination. A small number of these patients may benefit from photodynamic therapy if they fulfil certain criteria. It is this minority of patients who will undergo angiography to further assess their macula.

Optical coherence tomography (OCT) is a relatively new diagnostic procedure that provides high-resolution, cross-sectional imaging of ocular tissues in vivo.⁵ It can provide a detailed view of the macula, its thickness and any pathology there. However, it is not yet available everywhere and most smaller units rely on clinical appearance ± fluorescein angiography.

The management of macular oedema depends on its aetiology and extent.

• CMO - Irvine-Gass syndrome is typically managed using a combination of topical steroids (e.g. prednisolone 1%) and topical NSAIDs (e.g. ketorolac) over several months. Rarely, these patients need further surgery. Some patients with retinal vascular disease may benefit from laser photocoagulation and all patients with inflammatory disease will need anti-inflammatories to control the underlying disease.
• CSMO - all CSMO patients need laser photocoagulation, irrespective of their visual acuity.⁶ This is a damage limitation exercise in that it reduces the risk of visual loss (by 50%) rather than restoring vision which only happens in a small number of patients. There is photocoagulation of specific vessels (focal treatment) or using a grid pattern of laser burns spaced 1 spot-width apart over the affected area where specific leakage points cannot be identified, sparing the macular area. Intravitreal steroid injection is also looking promising: despite need for vigilance regarding intraocular pressure control and cataract formation (both easily manageable), it does have the benefit of avoiding some of the serious complications that can be associated with laser treatment.⁷
• Wet ARMD - specialist units will carry out photodynamic therapy on selected cases who fulfil certain clinical criteria (the first being the need to have a vision of 6/60 or greater). Unfortunately, only a minority of patients fulfil these standards.

If the oedema is prolonged, scarring or a hole can eventually ensue.

• CMO - this depends on the aetiology but in uncomplicated cases (such as post cataract surgery), recovery is usually complete after several months. In other cases where irreversible processes have occurred (such as in central retinal vein occlusion), the outlook is very bleak with little or no chance of recovery. Where there are inflammatory processes, the duration and severity of the condition will determine overall outcome.
• CSMO - improvement of visual function is rare (where deterioration has occurred). At best, treatment aims to arrest deterioration.
• Wet ARMD - these patients tend to have a poor outcome, even with therapy (which is long, may be painful and is very expensive) - hence the small number of patients treated. Most experience a sudden and rapid deterioration in visual function which irreversible. Vision never fully disappears and peripheral vision will remain normal in the absence of concurrent disease.

Little can be done to anticipate or prevent macular oedema other than in those patients undergoing surgery who have recent or ongoing intraocular inflammation. These patients need intensive steroid cover in order to minimise the risk. Good diabetic control and regular screening to detect CSMO is the best approach to minimise its effects.