Is Cold Weather a Birth Risk Factor for Late Diagnosis and Surgery for Developmental Dysplasia of the Hip or a Predictor of the Actual Cause?

Commentary In their article, Lee et al. provide a compelling case for an association between birth in the 3 coldest months of the year in a subtropical climate and the incidence of late diagnosis and surgery, defined as being after 6 months and prior to 5 years of age, for developmental dysplasia of the hip (DDH). The seasonal variation in DDH has been known for at least three-quarters of a century1 and was emphasized again recently2, but it has never been as well documented as in this large study based on the Taiwan National Health Insurance Research Database. Therefore, the assertion by Lee et al. that the cold weather birth effect has been overlooked in the past is not completely accurate. This database did not contain data for the nonoperative treatment of DDH, such as splinting and bracing, prior to the patient reaching 6 months of age, representing a possible limitation of the conclusions of this study. The authors report 1,296 cases in a population of 2.712 million births over a 12-year period, for an overall prevalence of 0.48 cases per 1,000 population. Of the infants born with DDH, 476 were born in the winter months and 212 were born in the summer months. The annual case incidence fell steadily over the 12 years, which the authors attribute to a national clinical screening program, and they recommend that additional screening be performed at 3 months of age for infants born in the winter months. Also interesting is the probability that educational intervention may have resulted in this steady decline of the incidence of surgically treated DDH over the 12 years of the program. If this seasonal variation can be detected in a subtropical climate, one would predict that the variation would be greater in a temperate or northernmost climate. Indeed, in an indigenous population in northern Saskatchewan3, the prevalence was 7.0 per 1,000 population, partly due to genetic influence and undoubtedly strongly influenced by the universal practice of infant swaddling. There were no breech deliveries in the entire study cohort of the affected population. Of the 125 cases detected in adults in that study, all had been swaddled, and no birth season variation was identified. Thus, the actual responsible factors would appear to be genetic, environmental, and cultural, with seasonal temperature variation being a marker for the need for warm clothing that, if improperly applied, binds the hips in adduction and extension. In the Saskatchewan study, the DDH incidence in first-degree relatives was 15 times the incidence in the general population. This would suggest that the appropriate interventions are obtaining a family history and educating parents at the initial screening visits by emphasizing how vitally important it is to apply warm clothing in such a manner as to promote flexion and abduction of the hips rather than typical swaddling in which the hips are bound in adduction and extension. Lee et al. had a concluding recommendation that an additional hip screening visit at 3 months of age should be implemented for infants born in winter; that could arguably lead to an excessive deployment of resources relative to the magnitude of the problem, the result being arguably a diminishing return on these resources. The additional visit, followed by a likely orthopaedic referral for an ultrasound examination of the hips, to detect an average of 30 to 40 additional cases per year in a country with a population of 23 million would be a larger challenge for lower-resource countries with colder climates despite an even greater seasonal difference in the incidence of surgically treated DDH. Enhanced education of parents about proper application of clothing and blankets to avoid swaddling hips in extension and adduction, with a targeted written reminder when the infant is 3 months of age, would seem to be a more direct and cost-effective approach to mitigate this seasonal effect so convincingly documented by the authors.

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