Gwiazda, J., Thorn, F., Bauer, J. and Held, R. (1993). Emmetropization and the progression of mani-fest refraction in children followed from infancy to puberty. Clin. Vis. Sci. 8, 337±344.

Hosaka, A. (1988). Population studies Ð myopia experience in Japan. Acta Ophthalmol. (Copenh.)Suppl: 185, 37±40.

Lam, C. S. Y., Goh, W. S. H., Tang, Y. K., Tsui, K. K., Wong, W. C. and Man, T. C. (1994). Changesin refractive trends and optical components of Hong Kong Chinese aged over 40 years. Ophthal.Physiol. Opt. 14, 383±388.

Leung, J. and Brown, B. (1999). Progression of myopia in Hong Kong Chinese schoolchildren is slowedby wearing progressive lenses. Optom. Vis. Sci. 76, 346±354.

Sorsby, A., Sheridan, M. and Leary, G. A. (1962). Refraction and its components in twins. In: MedicalResearch Council Special Reports Series, No. 303, HMSO, London.

Sorsby, A. and Fraser, G. R. (1964). Statistical note on the components of ocular refraction in twins.J. Med. Genet. 1, 47±49.

Teikari, J. M., Kaprio, J., Koskenvuo, M. K. and Vannas, A. (1988). Heritability estimate for refractiveerrors Ð a population-based sample of adult twins. Genet. Epidemiol. 5, 171±181.

Woodru�, M. E. and Samek, M. J. (1977). A study of the prevalence of spherical equivalent refractivestates and anisometropia in Amerind populations in Ontaria. Can. J. Public Health 68, 414±424.

Wu, M. M. and Edwards, M. H. (1999). The e�ect of having myopic parents: an analysis of myopia inthree generations. Optom. Vis. Sci. 76, 377±382.

Yap, M., Wu, M., Liu, Z. M., Lee, F. L. and Wang, S. H. (1993). Role of heredity in the genesis ofmyopia. Ophthal. Physiol. Opt. 13, 316±319.

Young, F. A., Baldwin, W. R., Leary, G. A., West, D. C., Box, R. A., Harris, E. and Johnson, C.(1969). The transmission of refractive errors within Eskimo families. Am. J. Optom. Physiol. Opt. 46,676±685.

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PII: S0275-5408(99)00075-7

Binocular vision problems in children: theirinvestigation and managementBruce Evans

Institute of Optometry, Newington Causeway, London, UK

Primary care optometrists will encounter binocular vision anomalies in about 5% of theirpatients (Stidwill, 1997) and orthoptics is therefore an essential part of optometric practice.The investigation of orthoptic anomalies is a sequential deductive process. There are twomain parallel lines of investigation: (1) comitancy; and (2) the deviation (heterotropia andheterophoria). These are parallel lines of investigation because both comitant and incomitantpatients can be heterophoric or heterotropic.

Comitancy

Comitancy is investigated initially with the motility test (Von Noorden, 1996). There arethree main sources of information in the motility test. First, the patients' eye movements areobserved whilst they follow the pen torch: many incomitancies can be detected through ob-serving the pupillary re¯ex. Second, the patient can be asked about any diplopia, althoughthe reports of some patients can be confusing. The position of gaze of maximum diplopiareveals possible under-acting muscles: the paretic muscle will be in the eye whose image is

E-mail address: bruce.evans@virgin.net (B. Evans).

Conference 2000 Abstracts S11

furthest out. The third source of information in the motility test is to carry out a cover testin di�erent positions of gaze.Incomitancies can be investigated quantitatively by a Hess or Lees screen. A computerised

Hess screen is available making this convenient for practices that have a personal computerin the consulting room (an early version of this was described by Thomson et al., 1990). Newor changing incomitancies require referral for neuro-ophthalmological investigation.

Deviation

The cover test can reveal a great deal of information (Evans, 1997). It diagnoses the typeof deviation (heterophoria or heterotropia, although some microtropias might not bedetected), the direction of the deviation (e.g., eso, hyper, etc.), the magnitude of the deviation(by estimating the amplitude of the eye movement and/or prism cover test). Information canalso be obtained on the person's ability to compensate for a heterophoria (from the smooth-ness and speed of the recovery eye movement) and on the risk of the deviation `building', orincreasing when the patient is tired or ill (from the alternate cover test).It is important to determine whether a heterophoria is compensated or not. In addition to

information obtained during the cover test, patients who can give a reliable subjective resultcan be assessed with the Mallett ®xation disparity unit. This detects whether the patientneeds an `aligning prism' to eliminate any ®xation disparity. This test has a sensitivity of75% and a speci®city of 78% for detecting symptomatic heterophoria (Jenkins et al., 1989).Compensation can also be assessed by measuring the fusional reserve that opposes the hetero-phoria. This can be assessed with rotary prisms or with a prism bar. With young childrenloose prisms can be held in front of an eye whilst the child ®xes a detailed target and thepractitioner looks for a vergence movement. Some information on compensation can also beobtained by testing sensory factors such as foveal suppression and stereopsis.In heterotropia, the cover test can be used to assess the direction and size of the deviation

(Evans, 1997). Additional tests can also be used to assess any sensory adaptation to the stra-bismus. Patients may have normal retinal correspondence with diplopia, harmonious anoma-lous retinal correspondence, or suppression of the binocular ®eld of the strabismic eye.Sensory status should be assessed with tests that do not interfere signi®cantly with normalviewing conditions, such as the Bagolini striated lens test or Mallett modi®ed OXO test(Evans, 1999).

Treatment

There are four good reasons for treating an orthoptic anomaly: if it is causing symptoms,if it is impairing visual performance, if it will worsen if not treated, or if treatment is indi-cated at that time because the patient may be too old for treatment later. Many cases of con-comitant heterotropia or decompensated heterophoria can be treated in optometric practice(Evans, 1998). Small vertical deviations often cause symptoms and can frequently be cor-rected with prisms.Small or moderate comitant horizontal deviation can sometimes be treated by refractive

modi®cation. Where there is adequate accommodation, exo-deviations are treated by over-minussing or under-plussing the patient. If the exo-deviation is only present during distance®xation then the distance refractive error is modi®ed and bifocals can be prescribed to negateany e�ect of the refractive modi®cation at near. If the exo-deviation is only at near then an`upside-down' executive bifocal can be prescribed (e.g., for an emmetropic patient, aÿ2.00DS with add +2.00 worn upside down gives plano at distance and ÿ2.00 at near).Near eso-deviations can be treated with conventional bifocals or varifocals. The method withrefractive modi®cation is to give the minimum correction that restores the patient to a com-pensated heterophoria, for example, as assessed with the Mallett Unit. Patients are seen everythree to six months and, as long as the deviation remains compensated, the magnitude of themodi®cation to the refractive error is reduced over time.

S12 Ophthal. Physiol. Opt. 2000 20: No 2

Small or moderate exo-deviations are particularly amenable to treatment by orthoptic exer-cises, to increase the convergent (positive) fusional reserves. Various methods are availableincluding haploscopic devices, computerised methods, and free-space stereograms (Evans,1998). Typically, orthoptic exercises are given to the patient to do at home and exercises canwork at any age. Cases that do not respond to exercises and do not have adequate accommo-dation for refractive management may be helped with prisms. People with binocular visionanomalies do not usually adapt to prisms (North and Henson, 1981). Hence they are notlikely to `eat up' prisms, although this can be checked with a prism adaptation test.Practitioners should always be on the lookout for active pathology and binocular

anomalies that do not respond to treatment should be referred. But such cases are rare andmany orthoptic conditions can be treated in optometric practice. The optometrist is oftenconveniently located and may seem less daunting to children than a visit to the hospital.

References

Evans, B. J. W. (1997). Pickwell's Binocular Vision Anomalies: Investigation and Treatment, 3rd ed.Butterworth-Heinemann, Oxford.

Evans, B. J. W. (1998). Optometric management of binocular vision anomalies. Optician 216(5564), 15±22.

Evans, B. J. W. 1999. Anomalous retinal correspondence. Optician 217(5705), 28±35.Jenkins, T. C. A., Pickwell, L. D. and Yekta, A. A. (1989). Criteria for decompensation in binocular

vision. Ophthal. Physiol. Opt. 9, 121±125.North, R. and Henson, D. B. (1981). Adaptation to prism-induced heterophoria in subjects with abnor-

mal binocular vision. Am. J. Optom. Physiol. Opt. 58 (9), 746±752.Stidwill, D. (1997). Clinical survey: epidemiology of strabismus. Ophthal. Physiol. Opt. 17 (6), 536±539.Thomson, W. D., Desai, N. and Russell-Eggitt, I. (1990). A new system for the measurement of ocular

motility using a personal computer. Ophthal. Physiol. Opt. 10, 137±143.Von Noorden, G. K. (1996). Binocular Vision and Ocular Motility, 5th ed. Mosby, St. Louis.

# 2000 The College of Optometrists. Published by Elsevier Science LtdAll rights reserved. Printed in Great Britain

0275-5408/00/$20.00+0.00

PII: S0275-5408(99)00073-3

New methods of investigating ocular functionF. W. Fitzke

Department of Visual Science, Institute of Ophthalmology, University College London, Bath Street,London, UK

Purpose. Recent ®ndings from both post-mortem studies and from basic research of the visualsystem have suggested the need for alternative methods of measuring visual function inpatients. We have investigated psychophysical function which selectively measures di�erentaspects of vision in patients.Methods. Patients with ocular hypertension, glaucoma and other conditions were studied

using both conventional perimetric measures using the Humphrey Field Analyser combinedwith new methods of analysis as well as measurements of motion sensitivity and other mag-nocellular mediated function. Changes in visual function were compared to changes in thestructure of the optic nerve head including those revealed by confocal laser scanning ophthal-moscopy of the lamina cribrosa.

E-mail address: f.®tzke@ucl.ac.uk (F.W. Fitzke).

Conference 2000 Abstracts S13