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Myopia is one of the most common eye disorders worldwide, affecting around 25 per cent of the population. In around 2 per cent of the general population, the degree of myopia is above -6DS, and is termed 'high myopia'.
High myopia can be associated with progressive, structural alterations to the globe, including excessive elongation, and degenerative changes in the sclera, choroid, Bruch's membrane, retinal pigment epithelium and sensory retina. In cases where sight and ocular health is threatened, the condition can be termed 'pathological myopia', and is one of the leading causes of registerable blindness worldwide. Loss of vision can occur through several mechanisms, and so, arguably, patients with high myopia should receive at least annual dilated fundus examinations, especially if symptomatic.
Axial elongation results from progressive scleral thinning, which can lead to the formation of a posterior staphyloma. This progressive scleral ectasia can occur in the posterior pole, the macular area, peripapillary area, nasal to or below the disc, or in multiple, compound patterns.1 As the sclera thins and stretches, so too must the overlying choroidal and retinal tissue.2 Attenuation of the RPE gives rise to a pale, tessellated appearance, while choroidal atrophy allows the larger choroidal vessels and eventually the sclera to show through. This change may be seen as pale islands in the posterior pole (Figure 1), a temporal crescent around the disc (which is frequently tilted) and peripheral cobblestone degeneration.
Large breaks in Bruch's membrane, termed lacquer cracks, occur in around 5 per cent of highly myopic eyes, and are seen as fine, irregular yellow lines which often branch and criss-cross at the posterior pole (Figure 2). A complication of lacquer cracks may be the development of choroidal neovascularisation (CNV), which fortunately is amenable to treatment with laser or photodynamic therapy, with a better prognosis than exudative AMD, as the CNV tends to be relatively self-limiting. A Foster Fuch's spot may develop at the site of a CNV, following pigmentary proliferation (Figure 1). Sub-retinal 'coin' haemorrhages may also develop at the site of lacquer cracks, although in the absence of CNV.
Other typical features of degenerative myopia include vitreous degeneration and a high frequency of peripheral retinal lesions such as lattice and snailtrack degenerations, with associated trophic holes or retinal tears. These changes lead to an increased risk of visual impairment from rhegmatogenous retinal detachments. In fact, over 40 per cent of all retinal detachments occur in eyes with high myopia.3 Macular holes also occur more frequently in high myopia, with or without posterior retinal detachment.
Myopic Traction Maculopathy
The above complications of high myopia are well known and are relatively easy to detect with conventional techniques such as direct/indirect ophthalmoscopy, ultrasonography or fluorescein angiography.
However, the posterior retina can also be damaged by the less obvious presence of tractional forces, which may act tangentially to the retina (resulting from overlying epiretinal membranes (ERM), often with multifocal attachments), or in an anteroposterior direction (where there are residual focal vitreoretinal adhesions, following incomplete posterior vitreous detachment).
The combination of traction derived from the presence of ERM and vitreomacular adhesions can be further complicated by the progressive scleral stretching and posterior staphyloma, leading to characteristic macular damage in such forms as retinoschisis, lamellar holes and shallow foveal detachments. Furthermore, such traction may be a contributory factor in the formation of macular holes.4 These pathological findings have been termed 'myopic traction maculopathy' (MTM) by Panozzo et al.5 It is very unusual to see such macular damage caused by similar epiretinal traction in a non-myopic eye.
The early stages of MTM can be difficult to detect with standard investigative techniques, due to the presence of other myopic changes complicating the appearance of the posterior pole, such as tigroid fundus, choroidal atrophy, RPE changes, thinned retina, staphyloma and so on. However, optical coherence tomography (OCT, Zeiss-Humphrey) is one technique that is of great value in the detection of such subtle macular changes.
OCT technology
OCT is a non-invasive, non-contact technique that allows detailed, in-vivo analysis of the human retina. It works in a similar way to B-scan ultrasonography, except that instead of measuring acoustic reflectivity it measures optical reflectivity, thus allowing a depth resolution of as little as 10<03BC>m, as compared with the 150m of B-scan.
Each of the layers in the retina reflects light by a different amount. The OCT displays cross-sectional tomographs of the logarithm of reflectivity in real time, two-dimensional, false colour images (Figure 3). Each 2D OCT scan is comprised of 100 A-scans spanning a length of between 1mm and 10mm (adjustable by the user).
A good correlation has been found between retinal morphology and macular OCT imaging.
Horizontal retinal elements such as the nerve fibre layer at the retinal surface or deeper plexiform layers, as well as the RPE and choroid, show relative high reflectivity, and are shown in warm colours (red to white). Cold colours (blue to black) correspond to the nuclear layers and photoreceptor inner and outer segments, which show relative low reflectivity, due to their vertical macrostructure and regular alignment. Resolution of retinal images is not only dependent on the resolving power of the instrument, but also on the contrast in relative reflectivity of adjacent structures. For example, the OCT cannot discriminate between adjacent tissues that possess matching reflectivity, such as the RPE and choroid, or the photoreceptor outer segments and photoreceptor nuclei.
OCT can aid in identifying, monitoring and quantitatively assessing various posterior segment conditions including macular oedema, AMD, macular hole, central serous retinopathy and epiretinal membrane. Furthermore, it is also capable of scanning and evaluating the optic disc, and thus can be used in the assessment of glaucoma. It has been used in several studies looking at myopic traction maculopathy.
Prevalence
Several studies have attempted to determine the prevalence of macular changes in high myopia, as found by OCT. Baba et al6 looked at 134 eyes of 78 consecutive patients with high myopia (>-8.00DS), with and without visual symptoms, attending a high myopia clinic in Tokyo.
The participants were divided into two groups, those with posterior staphyloma (78 eyes of 45 patients) and those without (56 eyes of 33 patients). This study looked for the particular feature of foveal retinal detachment without macular hole, which is thought to be a precursor to macular hole formation. They found a prevalence for foveal retinal detachment of 9 per cent (seven eyes) in the group with posterior staphyloma. No eyes without posterior staphyloma showed this feature. All seven eyes with foveal retinal detachment also showed severe myopic fundus changes (focal chorioretinal atrophy or bare sclera), with vision ranging from better than 20/50 to below 20/200. Surprisingly, two eyes showed better VA than 20/50, suggesting that in eyes with shallow retinal detachments, oxygen and nutrient diffusion from the choriocapillaris to the photoreceptors might be sufficient, allowing the photoreceptors to survive to some extent.
Takano and Kishi found a much higher prevalence of foveal retinal detachment, with 34 per cent of highly myopic eyes with posterior staphyloma showing foveal retinal detachment or retinoschisis. However, the study group was much smaller, only 32 eyes, and also included pseudophakic patients, who are at a greater risk of retinal detachment following cataract surgery.7
Panozzo et al 5 studied 125 eyes, looking for the more general features of epiretinal traction related macular damage. Epiretinal traction was found in 46.4 per cent (58 eyes) and retinal damage in 34.4 per cent (43 eyes). Macular retinoschisis was found to be the most frequent form of macular damage, affecting 25 out of the 125 eyes, followed by retinal thickening, lamellar hole and shallow retinal detachment.
Surprisingly, of the 43 eyes with macular abnormalities found by OCT, only 14 patients were judged to be symptomatic (defined as a worsening of at least two Snellen lines in visual function in the last six months in one eye). Although this finding may be the result of some participants having more longstanding damage, it could also suggest that OCT can detect subtle, early changes before they impact on vision.
Although there is some discrepancy between the prevalence values quoted in these studies, it is clear that MTM and its associated sequelae are a significant pathological finding in high myopia, and should be considered as a separate cause of visual loss to the more well-known complications of high myopia, such as rhegmatogenous retinal detachments, choroidal atrophy, CNV and so on. Arguably, routine OCT scanning of high myopes would therefore be beneficial.
Treatment
Treatment for MTM is still in its early stages, and further study is needed to establish full indications for treatment.
However, in some centres, most notably in Japan and Italy, surgery in the form of vitrectomy, indocyanine green enhanced internal limiting membrane peeling and gas tamponade has been trialled, in order to treat myopic foveoschisis. Ikuno et al (2004)8 performed this operation on six eyes of five patients with high myopia, with no serious complications occurring, including macular hole formation or retinal detachment. Best corrected visual acuity increased by more than two lines in all patients six months postoperatively, and microperimetry showed a smaller central scotoma following surgery. Similarly, Panozzo et al 9 obtained a mean visual recovery of two lines of VA in 18 consecutive cases undergoing this form of surgery for MTM. Although MTM has been studied in detail in various centres around the world, with OCT scanning becoming fairly routine, in the UK this condition is still in the early stages of recognition and treatment.
In a much smaller-scale study carried out at Manchester Royal Eye Hospital (unpublished data), 14 highly myopic eyes (>-6.00DS) underwent full ophthalmological investigation, including OCT. Six of these eyes showed abnormal retinal morphology by OCT. Surgery in the form of vitrectomy has not yet been carried out on these patients, but remains a possibility for the future.
There follow two case studies of patients seen at MREH, with signs of MTM.
Case Study 1
A 54-year-old Caucasian female, was referred for ophthalmological opinion by her local optometrist as her vision was found to be reduced, R>L, without an obvious cause found by ophthalmoscopy. The patient complained of some central vision loss and distortion.
Investigation results were as follows:
Refraction: -18.25/+0.50x120 -17.00/+0.75x35
VA: 6/24+ 6/15
Pelli Robson contrast sensitivity: 1.45logCS (~2.8 per cent) 1.45logCS (ie slightly reduced)
Axial length: 32.30mm 31.82mm
Visual fields: Central loss of sensitivity L>R
FFA: Normal R+L
Ophthalmoscopy of the right eye showed an area of choroidal atrophy surrounding the disc, a rather pale, thin retina, but no obvious macular pathology to account for the patient's reduced vision. The left eye showed an area of chorioretinal atrophy at both the disc and the macular area (Figure 4). The patient was also noted to have a posterior pole staphyloma in both eyes.
Figures 5 and 6 show a horizontal and vertical 10mm scan through the fovea of the right eye. The OCT scans clearly show a disruption of the normal retinal morphology, with what looks like epiretinal tissue causing traction and puckering of the retina. This abnormality was not detected on routine ophthalmoscopy.
Case Study 2
A 72-year-old Caucasian male chronic glaucoma sufferer has had previous unsuccessful surgery in the left eye for posterior pole retinal detachment secondary to a macular hole.
He has a longstanding history of poor vision in the right eye, with indirect ophthalmoscopy showing a possible shallow retinal detachment, although this was difficult to confirm, for reasons discussed above. Fundus photographs (Figure 7) show extensive myopic degeneration in the right eye, and the area of detachment in the left eye can be seen where the picture is slightly out of focus.
Refraction: -15.00CS +3.25DS (pseudophakic)
VA: 6/60 6/75
Axial length: 30.41mm Unable to record accurate measurement in view of retinal detachment and poor fixation.
OCT was used to investigate further, and Figures 8 and 9 show horizontal and vertical scans through the fovea of the right eye.
Figure 8 shows the presence of an epiretinal membrane leading to tangential traction, causing the retinal tissue to display a corrugated appearance, with many schitic spaces.
Figure 9 shows antero-posterior traction caused by what looks like a vitreoretinal adhesion, namely a long strand of vitreous pulling on the retinal tissue, again causing schitic spaces and a puckered appearance.
Figure 10 shows an enhanced OCT image of the left eye, in which the retina can be seen to have detached completely from the back wall of the globe, with the macular hole still present.
Conclusion
It has been demonstrated in numerous studies that traction maculopathy and its associated macular damage are a frequent finding in high myopia, often related to ERM or incomplete vitreous separation.
These features can be difficult to diagnose using standard examination techniques, requiring instead the use of OCT to study the configuration of the retina in detail. MTM should be suspected in cases of high myopia where there is visual impairment and metamorphopsia but with negative findings for foveal haemorrhage, neovascularisation or progressive atrophy.
However, MTM has also been found to be present in asymptomatic patients. If left untreated it is thought that MTM may progress to the formation of a macular hole, suggesting a need for routine OCT scanning to detect the condition early on. Selected cases may be managed with vitrectomy, which may be used as a prophylactic treatment for highly myopic eyes at high risk of macular hole development.10
References
1 Curtin BJ. The posterior staphyloma of pathologic myopia. Trans Am Ophthalmol Soc, 1977;75:67-86.
2 Kanski J. Clinical Ophthalmology Fourth Edition. P425-428.
3 Kanski J. Clinical Ophthalmology Fourth Edition. P367.
4 Benhamou N, Massin P, Haouchine B et al. Macular retinoschisis in highly myopic eyes. Am J Ophthalmol, 2002;133:794-800.
5 Panozzo G, Mercanti A. Optical coherence tomography findings in myopic traction maculopathy. Arch Ophthalmol, 2004;122:1455-1460.
6 Baba T, Ohno-Matsui K, Futagami S et al. Prevalence and characteristics of foveal retinal detachment without macular hole in high myopia. Am J Ophthalmol, 2003;135:338-342.
7 Takano M, Kishi S. Foveal retinoschisis and retinal detachment in severely myopic eyes with posterior staphyloma. Am J Ophthalmol, 1999;128:472-476.
8 Ikuno Y, Sayanagi K, Ohji M et al. Vitrectomy and internal limiting membrane peeling for myopic foveoschisis. Am J Ophthalmol, 2004;137:719-724.
9 Panozzo G, Mercanti A. OCT helps visualize traction maculopathy in high myopia. Ophthalmology Times, Oct 15, 2002.
10 Kobayashi H, Kishi S. Vitreous surgery for highly myopic eyes with foveal detachment and retinoschisis. Ophthalmology, 2003;110:1702-1707.
Emma Sharples is an optometrist working at Manchester Royal Eye Hospital. The author would like to thank Paulo Stanga, consultant vitreoretinal surgeon and Robert Harper, principal optometrist at Manchester Royal Eye Hospital, for their help in compiling this article