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Myopes, squints, and scans
  1. CAROLINE J MAcEWEN
  1. Department of Ophthalmology
  2. Ninewells Hospital and Medical School, Dundee DD1 9SY

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    Why should some people with severe axial myopia develop a marked esotropia combined with hypotropia? Although rare, this unusual form of restrictive extraocular muscle imbalance is well recognised. Various suggestions regarding its aetiology have been proposed. Several point to an underlying anatomical cause; the large, myopic globe leading to an imbalance of the structures within the orbit which results in a limitation of normal ocular rotations.12 In this issue of the journal Krzizok and co-workers (p 625) present magnetic resonance imaging (MRI) and surgical findings in a series of such patients with high myopia and strabismus. Their high definition magnetic resonance scans demonstrate that the lateral rectus muscle takes a pathological course, developing an inferotemporal deviation at the equatorial plane, in a significant percentage of these patients. This finding is confirmed at operation and surgery aimed specifically at correcting the abnormal path of the muscle proves curative.

    This paper illustrates well how newer and more accurate imaging techniques can be helpful in improving understanding of the aetiology of extraocular muscle disorders and how this understanding can lead to more defined methods of management. It also reinforces the concept that when considering incomitant strabismus it may not always be possible to explain the clinical picture in terms of the usual simplistic concepts of extraocular muscle weakness or restriction. In some cases it is necessary to consider abnormal muscle anatomy or function and occasionally a clearer view of the intraorbital dynamics is necessary to understand the exact mechanism. Indeed, the role of the connective tissue system within the orbit and its connection with the extraocular muscles is often overlooked as an important factor in the genesis of a number of eye movement disorders.3

    High quality MRI has undoubtedly improved our understanding of the intraorbital contents. Compared with conventional computed tomography it provides excellent soft tissue differentiation which delineates the muscle anatomy in detail. Thus, in addition to the relative relation with other intraorbital structures, the extraocular muscles themselves can be evaluated with regard to abnormalities in muscle size or shape as well as any variations in the origin, insertion, or pathway. Three dimensional reconstruction is an exciting extension of the standard two dimensional scan and, as this provides a fuller anatomical picture, may assist in improving the understanding of certain conditions such as blowout fractures, lost or dropped muscles, postoperative overcorrections and undercorrections, and craniofacial disorders.

    Anatomical detail, however, may not be sufficient to unravel the entire story and, as described in Krzizok et al’s paper, functional assessment of the muscles can be carried out by examining scans taken in different positions of gaze. Information is thus obtained about the contraction/relaxation characteristics of each muscle, as demonstrated by the length and breadth features in different locations and in different planes. ‘Dynamic’ images are usually made up by building a cine loop of multiple static orbital scans, taken as the patient fixates on a series of targets.45 These ‘fixed eye’ dynamic scans do not, therefore, represent a real time picture and so give no details about the velocity of movements and therefore only provide limited information. ‘Moving eye’ methods of examining the extraocular movements have been described4and seem to give a more helpful and relatively accurate portrayal of the actual movements of the eyes; these, however, are very demanding on the patient and cannot currently be considered as feasible. At first glance it may be considered that such imaging techniques will only be useful in the rarer and more unusual types of ocular motility disturbance which have an obvious intraorbital or functional anomaly. However, utilising information derived from such cases may lead to an improvement in the knowledge of many commoner conditions. There are a surprising number of disorders which can readily be identified as being candidates for high quality dynamic scanning—for instance, the alphabetical patterns (A, V, and X) and DVD, as well as some rarer disorders such as Duane’s and Brown’s syndromes, strabismus fixus, and the adherence syndrome.

    Imaging can also contribute to the identification and monitoring of pathological processes within the muscles. The understanding and management of dysthyroid eye disease has been improved by the use of MRI, using the STIR (short tau inversion recovery) sequence. This distinguishes the amount of active muscle inflammation and therefore acts as an index of disease activity and, taken with the clinical findings, allows this disease involving the extraocular muscles to be staged and treated appropriately.67

    There are therefore a variety of ways in which imaging may assist in motility disorders. This field is constantly advancing and improvements in resolution, special sequences, three dimensional reconstruction, and real time techniques are inevitable and should ensure that MRI becomes an increasingly valuable tool which contributes further to the understanding, and therefore management, of ocular motor disturbances.

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