Shaken Impact Syndrome

Karen Barlow and Robert Minns1 report a one in 4065 chance of a child in Scotland having NAHI by age 1 year. They give the annual incidence of shaken impact syndrome as being 24·6 per 100 000. Even though the study sample size and data structure were not described in detail, these risks are clearly the same.

Their analysis treats NAHI and shaken impact syndrome as identical, even though part of their report defines shaken impact syndrome as a subgroup of NAHIs. Intuitively, the syndrome that includes an acute encephalopathy with subdural haemorrhages, cerebral oedema, retinal haemorrhages, and fractures, occurring in the context of an inappropriate or inconsistent history, commonly with evidence of other impact or malicious injuries, is an extreme form of non-accidental head injury. We think that the risk estimates should be different.

The effects of variation in the data are not described adequately by the cited binomial CI. In their earlier, retrospective study of children younger than 5 years, Barlow and Minns showed appreciable variation beyond that which would be explained by a binomial model.2 For example, in 1990 they identified four cases of NAHI and, in 1994, 26 cases. With such variation, and even with fuller reporting, it is difficult to justify basing an annual incidence on a study of 18 months' duration.

We are not convinced that the new term shaken impact syndrome is required. Caffey3 used the term whiplash shaken baby syndrome, and Duhaime and colleagues4 the simpler term shaken baby syndrome. We think that the best term would be one that makes few or no assumptions about mechanisms of injury, which are contentious.

Barlow and Minns did not assess how many children had injuries with these clinical features that were classified as accidental. This information is essential to study regional variations in incidence and to decide whether these arise because of differences in diagnostic classification. Changes in classification might be affected by time trends and substantially alter assessments of new policies.

The investigators say that their measurement of incidence for NAHI in infants younger than 1 year is precise and will help in assessing the impact of new policies. We encourage the government to consider policies that, although unpopular, might lead to a lower incidence of disabling non-accidental injuries in children. We suggest that estimates from several sources are used when assessing the effect of new laws.