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In practice we are often presented with patients who complain of central scotoma. At our disposal we have visual acuity charts giving an insight into the amount of vision and visual acuity loss. The ophthalmoscope is the primary instrument used by all optometrists and a quick glance should alert us to any obvious cause for the visual loss.

Slit-lamp biomicroscopy allows us to carry out a detailed examination of the fundus and with the right Volk lens a much more detailed 3D view is possible. Using a fundus camera allows us to digitally record our findings for future reference and monitoring. Even with all this equipment at our disposal, at times we cannot ascertain the reason for our patients' complaints.

An optical coherence tomographer (OCT) has given unprecedented, non-invasive access to parts of the retina that were not possible for both optometrists and ophthalmologists. The optometrist now has the ability to view sections of the retina from the vitreous-retinal interface through to the choroidal vasculature. Each and every layer is visualised in real time, allowing us to understand the anatomy and histology of the fundus, but more importantly giving an insight into what are the possible causes of retinal pathology and where they are located within the various retinal layers. OCT also has a 3D macula and optic disc facilities, allowing video footage to be taken and played back. This facility is superb for 'running through' the macular or disc area and noting the changes that are happening or have happened.

In these articles I will describe patients referred to my practice for a second opinion. Most of these patients were complaining of a central scotoma and each patient was investigated with the usual diagnostic equipment as well as the optical coherence tomographer the Optovue RT vue scanner.

Vitreous detachment

Vitreous detachment is an age-related condition which is observed almost daily by each of us in practice. In the author's experience, tractional effects of perifoveal vitreous detachment cause eight distinct macular pathologies. These are namely:

• Psuedo operculum
• Vitreo-macular traction syndrome
• Idiopathic epiretinal membrane
• Idiopathic macular hole
• Tractional cystoid macular oedema
• Tractional diabetic macular oedema
• Inner lamellar macular hole
• Myopic macular retinoschisis.

I will use case histories and fundus photographs together with OCT scans to illustrate some of the macular pathologies associated with symptoms of central scotoma.

Firstly, let us revise the basic pathological process associated with posterior vitreous detachment.

Perifoveal vitreous detachment

Generally, age-related posterior vitreous detachment is an occult, chronic and asymptomatic degeneration, culminating in the separation of the posterior vitreous from the inner limiting membrane. There are four steps to posterior vitreous detachment and this is illustrated in Figure 1. PVD begins with dehiscence in the posterior vitreous cortex overlying the macular area followed by the sudden passage of liquified vitreous into the sub hyaloid space, causing a rapid and smooth separation of the vitreous cortex from the retina, starting posteriorly and progressing peripherally to the vitreous base region. Early perifoveal separation is symptom-free and clinically difficult to detect until the separation of the posterior hyaloid from the optic disc results in the classical Weiss ring.

Case 1: Pseudo-operculum

A 64-year-old female patient was referred for OCT examination because she complained of missing one or two letters when reading and similarly when looking at number plates. She had been referred to a neurologist six months previously when she first complained of the scotoma and the MRI scan revealed a meningioma on the forehead which was successfully removed and unrelated to the scotoma.

Examination revealed 6/9 visual acuity in each eye with a slight central blur in the right eye. Volk examination revealed a slight vitreous condensation on the temporal fundus but no sign of macular or optic disc pathology.

OCT examination was unremarkable, showing a taut, faint line indicative of parafoveal vitreous detachment and a shiny membrane located centrally in the perifoveal region. The fovea appeared normal with a normal foveal pit. The central membrane appears to be the reason for the scotoma and this was not visible with biomicroscope or on fundus imaging. Using a smaller line scale of 3mm the author was able to visualise the central psuedo-operculum in much more detail (Figure 2). The size of the membrane was 85 microns, being one tenth the size of the fovea and creating slightly more than one tenth visual discomfort. The position of the membrane was such that it would cast a shadow and obstruct the central vision, albeit not completely but to an extent that the patient would notice obstructions of letters especially when reading text.

Diagnosis

The patient was diagnosed with posterior hyaloid membrane detachment from the paramacular area and overlying the fovea was a circular psuedo-operculum. This central glistening membrane was the culprit with regards to the central scotoma. The patient was reassured that this was innocuous, age-related degeneration and hopefully non-progressive.

Pathopathology of psuedo-operculum

In the macular region vitreous retinal adherance is strong and distributed diffusely beyond the temporal vascular arcade throughout the posterior pole. It is thought with early posterior vitreous detachment the vitreous cortex/posterior hyaloid membrane separates from the retina and encounters the increased vitreomacular adhesion. This vitreomacular adhesion acts likes a draw string, reducing the central vitreous cortex into a small circular defect overlying the fovea. This theory is confirmed by our simulated 3D macular imaging showing the central dehisced vitreous cortex above the macula and in the parafoveal region we can also see the strong attachment of the vitreous. When this contracts, imagine a draw string bag when you pull the strings. The whole central aperture reduces and in this instance the dehisced vitreous cortex gets pushed centrally.

Vitreomacular traction syndrome (VMTS)

Case 2

A 49-year-old female patient had been experiencing central blur especially with reading and her specialist could not find any obvious cause for the scotoma. This patient was at her wit's end as the central scotoma was affecting her day to day performance as a school secretary. On examination she showed a corrected acuity of 6/9+3 in the right eye and 6/6 in the left eye. There were no obvious signs of cataract or macular pathology. OCT examination with repeated checks showed subtle vitreomacular traction syndrome with a tiny pinch at the fovea, revealing a perifoveal pseudocyst. The pinch caused a bulge at the fovea, obscuring the normal depressed foveal pit (Figure 3).

Diagnosis

Our patient was told that she had mild vitreomacular tractional syndrome which caused a pinch at the fovea and resulted in the central scotoma. On the vertical cross-scan we could see the tiny line of posterior vitreous detachment and looking at the shape of the central fovea there must be a faint attachment which has caused the slight distortion of the fovea. She was referred back to the specialist to see if it was worth trying anti-inflammatory eyedrops such as Acular for easing the traction or alternatively waiting to see if the traction would ease on it own accord. The wait and see plan is the best alternative.

Case 3

An 80-year-old retired chemist attended the clinic for a routine annual examination complaining of a slight visual scotoma centrally. His vision a year ago was 6/9 and at this visit it reduced to 6/12 with no obvious ophthalmoscopic signs of macular or optic disc pathology. OCT examination revealed vitreomacular traction with a pagoda-shaped foveal thickening and a large perifoveal cyst (Figure 4).

Diagnosis

The patient was told that he had definite moderate vitreomacular tractional syndrome. This was characterised by focal adhesion of the hyaloid membrane at the fovea and associated with this was cystoid macular oedema.

He was referred to an eye specialist as a non-urgent case, again stressing to the patient that the vision may not improve and should the traction continue then there is a small likelihood that the vision may deteriorate and the specialist may contemplate vitrectomy to ease the traction.

Pathopathlogy and occurrence of VTMS

Vitreomacular traction syndrome is an idiopathic condition in which an incomplete posterior vitreous detachment with residual attachment and traction at the macula causes visual deficits.

In our elderly patient the traction caused a cystoid macular oedema. The cyst is thought to result from intraretinal splitting at the fovea where the oblique course of the Henle's fibre layer and the vertical Muller's cells holding the inner and outer retina provide a weak infrastructure against the vitreous traction. Our elderly patient's simulated 3D macular scan shows the raised foveal area due to adherance of the vitreous at the centre and again the OCT gives us a valuable insight into the mechanism of vitreous traction.

It is known that 10 per cent of vitreomacular traction syndrome patients will experience spontaneous resolution of cystoid macular changes following posterior vitreous detachment making surgery unnecessary. It is therefore prudent to wait several months before attempting vitrectomy.

VTMS is clinically invisible and very rarely picked up using slit-lamp biomicroscopy (8 per cent) while OCT confirms the diagnosis each and every time.

Mechanisms of VTMS

One theory suggests that vitreous traction can lead to breaks in the internal limiting membrane, allowing glial cells access to the retinal surface and providing much firmer posterior vitreous adhesion. A second theory suggests that in the absence of retinal breaks, migration and proliferation of glial cells may be a secondary response to VTMS and this theory is confirmed by histological tissue samples showing presence of fibrous astrocytes and bits of internal limiting membrane in VTMS.

In the next article, some cases of epiretinal membrane and macular hole formation will be described.

Kirit Patel practises in Radlett, Hertfordshire

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