Likelihood-based modelling of age-related normal ranges for ordinal measurements: changes in visual acuity through early childhood.

This paper investigates the construction of age-related standards for ordinal outcome data. Asymmetric logistic models are used to describe the age-related changes in the cumulative probabilities for each ordinal level. Maximum likelihood estimation of model parameters allows the use of likelihood ratio tests to ascertain the appropriate model complexity. In contrast to methodologies for constructing age-related standards where the outcome is continuous, we show how the methodology leads directly to centile estimation for individuals. The method is illustrated using visual acuity measurements collected from 2968 children between 2 and 9 years of age made on a 30-point ordinal scale. We show how, in this instance, smoothing of parameters across ordinal categories leads to reduced validity of the centiles, justifying the need for specialized methodology for non-continuous outcomes.

The maturation of linear acuity and compliance with the Sonksen-Silver Acuity System in young children.

The compliance and maturation of visual acuity of 1245 2 1/2- to nine-year-olds was investigated. Testing was undertaken as part of routine surveillance. Ability to letter match, to achieve a binocular measure and to achieve monocular measures for children under three years was 89, 78 and 37 per cent, for children aged three to four years 97, 91 and 66 per cent, and for children over four years 99.9, 99.9 and 99, respectively. A statistical model was developed to give acuity norms by age for use by surveillance and diagnostic teams.

Age-related standards for ordinal data: modelling the changes in visual acuity from 2 to 9 years of age.

The visual acuities of 1140 children between the ages of 2 and 9 years were measured using the Sonksen-Silver Acuity System (SSAS) from a distance of 3 metres. Ordered logistic regression was used to construct smoothly changing age-related reference ranges for the proportions of children at different ages expected to have acuities in each of three ordinal categories. Exponential functions of age fitted the data better than polynomials. The resulting reference ranges can be used to derive the ages at which a given percentage of children achieve each of the acuity cut-offs. However, since it is unlikely that all children will be available for testing at the exact appropriate age, two possible alternatives are introduced and discussed. First, a new ordinal category that would result in a chosen outcome at a pre-selected age is calculated. Secondly, because it may be more feasible to use the current SSAS system whilst varying the distance of measurement, age-related standards showing the distances required to give constant pass-rates with the usual letter-sizes are also presented.

Growth patterns after surgery for virilising adrenocortical adenoma.

Analysis of growth in nine girls after successful surgical removal of a virilising adrenocortical adenoma showed that five girls continued to grow rapidly for up to 18 months after operation and subsequently had normal growth velocities. The other four girls grew at normal rates after surgery. Only one girl showed progressive advance in bone age after operation and she subsequently had precocious puberty.

Protein C deficiency and portal thrombosis in liver transplantation in children.

Changes in plasma protein C and antithrombin concentrations after liver transplantation were monitored in fourteen children and a control group of fourteen adults. In the children, there was a persistent deficiency in the plasma concentration of protein C and a less pronounced deficiency in antithrombin during the early postoperative period, causing a hypercoaguable state. A concomitant rise in plasminogen activator inhibitor further increased the risk of thrombosis by inhibiting fibrinolysis. These changes coincided with the peak incidence of portal vessel thrombosis (4-10 days). Replacement of plasma antithrombin, together with heparin, did not prevent portal thrombosis in two of the children. It is concluded that successful prevention will require protein C replacement together with antithrombin supplements up to, but not exceeding, normal plasma activity.