Investigating the impact of measurement uncertainty in the radiographic surveillance of hip displacement in cerebral palsy: A Monte Carlo simulation.

Hip surveillance programmes have greatly improved the management of hip dysplasia in children with cerebral palsy. Reimer's migration percentage is the most common index for quantifying hip dysplasia from planar radiographs. However, measurement uncertainty could undermine the diagnostic accuracy. A Monte Carlo simulation was created to investigate the impact of measurement error on decision making in hip surveillance programmes. The simulation was designed to mimic the annual surveillance of children with cerebral palsy (Gross Motor Functional Classification System levels III-V) between 2 and 8 years of age. Simulation parameters for the natural history of hip dysplasia and measurement error were derived from published data. At each measurement interval, the influence of uncertainty in the measurement of Reimer's migration percentage on decision-making was investigated. The probability of a child being indicated for intervention in error during the course of the simulation was relatively high, particularly in the highest functioning cohort where the positive predictive value of Reimer's migration percentage was at best 70% and at worse less than 20%. Including a rate of progression term within the decision-making algorithm had a negative effect on positive predictive power. This simulation suggests that hip surveillance programmes are sensitive to detecting genuine hip dysplasia but can have poor positive predictive power, potentially resulting in unnecessary indication for intervention.

The effect of anaesthesia on the assessment of hamstring length in spastic cerebral palsy.

The decision to surgically lengthen the hamstring muscle group in a child with spastic cerebral palsy is based, in part, on the range of motion measurements made in the clinic, particularly, the popliteal angle. This measurement is often repeated when the child is under anaesthesia, before surgery. We were interested to know whether a systematic difference exists between the popliteal angles measured under these conditions, and to understand under which condition the measurement is more reliable. The effect of anaesthesia on the measurement of range of motion has not been reported previously. We repeated the measurement of popliteal angle in 15 children with spastic cerebral palsy (30 limbs, mean age: 8 years 3 months), on the evening before surgery when they were conscious, and again when they were under anaesthesia. The means of measurements made under the two conditions were not significantly different (p=0.17, paired t-test). Repeated measurements of popliteal angle when the patient was conscious were, however, less reliable than those made under anaesthesia (standard error of 15 and 5 degrees, respectively). This resulted in poor agreement between measurements made on the same patient under the two conditions (95% limits of agreement were -19 and 25 degrees). Our study supports the common practice of surgeons of reassessing hamstring length under anaesthesia before single level and multilevel surgeries and incorporation of these measurements into their decision-making.