VII: Diagnostic trials: Simple measures of validity and reliability

The evaluation of a new diagnostic or imaging device has to focus both on its diagnostic validity and reliability. Cohen's kappa coefficient can be used to assess the agreement of the new device's findings with those of an established diagnostic reference, it therefore presents a surrogate measure of validity. Additional measures of validity are sensitivity and specificity, which assess the order of agreement with the reference for positive and negative reference findings, respectively. The latter, however, can only be applied for binary reference findings. If discordant findings are observed, the McNemar test can be used to detect a diagnostic shift between diagnostic novum and reference. The estimation of reliability can be based on the independent parallel rating of a patient subset by two clinical readers; again the kappa coefficient measures the agreement of the parallel ratings and therefore represents a surrogate measure of reliability. The McNemar test can be used to detect an observer bias.     

Epikeratophakia: the surgical correction of aphakia. Update: 1982

Sixty-five epikeratophakia procedures have been performed in 63 patients; visual acuity data have been tabulated on 31 patients with 4 to 30 months follow-up. Patients with more than a year of follow-up showed stable keratometry readings. Early patients achieved 70% of the predicted dioptric correction; more recent patients have achieved 87% with improvements in the lathing procedure, tissue handling, and surgical technique. Visual acuities improve with time. At any given time after surgery, acuities measured with a hard contact lens are better than those measured with spectacles; the decrease in spectacle acuity is probably a result of irregular refraction at the graft surface. The gap between contact lens and spectacle acuity decreases with time. A number of patients achieve postoperative visual acuities better than their preoperative acuities, and most achieve final spectacle acuities within a line or two of their preoperative acuities.     

Accommodation and presbyopia : part 1: physiology of accommodation and development of presbyopia

Accommodation is a dynamic change in the dioptric power of the eye. According to the widely accepted and experimentally confirmed theory of Helmholtz, it is achieved by release of zonular tension with contraction of the ciliary muscle and consecutive modelling of the shape of the crystalline lens by the elastic lens capsule. The ability to accommodate is gradually lost with age (presbyopia). Because of difficulties in examining the accommodative apparatus in vivo, many theories, in part contradictory, about the mechanism of accommodation and the origin of presbyopia have been developed. In recent years experimental studies have greatly increased the knowledge about the acommodative apparatus and suggest a multifactorial aetiology of presbyopia. A better understanding of the physiology of accommodation and presbyopia can contribute to the development of effective treatments.