Sudden unexpected death in infancy: epidemiologically determined risk factors related to pathological classification MP L’Hoir 1 , AC Engelberts 2 , GThJ van Well 1 , T Bajanowski 3 , K Helweg-Larsen 4 and J Huber 1 Psychosocial and Pathology Departments 1 , Wilhelmina Children’s Hospital, University Hospital for Children and Youth, Utrecht, The Netherlands; Department of Pediatrics 2 , University Hospital Leiden, The Netherlands; Institut fu ¨r Rechtsmedizin der Westfa ¨lischen Wilhelms Universita ¨t 3 , Mu ¨nster, Germany; Danish Institute of Clinical Epidemiology 4 , Copenhagen, Denmark L’Hoir MP, Engelberts AC, Well van GThJ, Bajanowski T, Helweg-Larsen K, Huber J. Sudden unexpected death in infancy: epidemiologically determined risk factors related to pathological classification. Acta Pædiatr 1998; 87: 1279–87. Stockholm. ISSN 0803-5253 Infants that died suddenly and unexpectedly were studied as part of the European Concerted Action on sudden infant death syndrome (SIDS). Three paediatric pathologists, first independently of each other and later in a consensus meeting, classified 63 cases into 3 groups: SIDS (19 cases), borderline SIDS (30 cases) and non-SIDS (14 cases). The interobserver agreement among the pathologists before the consensus meeting was moderate (Kappa = 0.41) and jointly it was higher (Kappa = 0.83). The distribution of epidemiologically determined risk factors was studied over these three groups. Maternal smoking after birth, low socio- economic status and thumb sucking were found more often in SIDS than in the other cases. Inexperienced prone sleeping was a determinant for SIDS, but not for non-SIDS. Previous hospital admission, low birthweight and/or short gestation were associated with borderline SIDS. Non-SIDS cases received more breastfeeding, the parents hardly smoked during pregnancy and after birth, a firm mattress had been used, and more often signs of illness had been reported by the parents, compared with the SIDS and borderline SIDS cases. Bedding factors and both primary and secondary prone sleeping were equally distributed over the three groups which supports the hypothesis that, in SIDS and borderline SIDS, as well as in non-SIDS cases, some similar external and preventable factors might influence the events leading to death. Research should therefore focus on all sudden unexpected deaths, after which subgroups such as SIDS cases can be separately analysed. The postmortem is an essential part of the whole work-up of each case and the results should be interpreted with all other available data to arrive at a sound evaluation of cases and thus form the basis for the prevention of all sudden unexpected infant death.  Cot death, epidemiology, pathology, risk factors, sudden infant death syndrome (SIDS), SUD MP L’Hoir, Wilhelmina Children’s Hospital, PO Box 18009, 3501 CA Utrecht, The Netherlands Sudden infant death syndrome (SIDS) was and still is defined as ‘‘the sudden death of an infant or young child which is unexpected by history, and in which a full post- mortem examination fails to demonstrate an adequate cause of death’’ (1, 2). SIDS is based on exclusion criteria and forms part of the larger group of infants dying suddenly and unexpectedly (SUD). Within SUD there are also infants where major pathological findings form a cause of death. These are designated as non-SIDS. Distinguishing between SIDS and non-SIDS is difficult and arbitrary (3–5). Various studies showed large differences between the number of cases described as ‘‘explained’’ or SIDS cases (6, 7). Efforts have been made towards achieving unifor- mity by using a simple classification of all sudden and unexpected infant deaths into SIDS, borderline SIDS and non-SIDS and recently a higher degree of diagnostic con- cordance has been achieved (8–11). For classic SIDS cases neither clinical nor autopsy information reveals a cause of death. Borderline SIDS means that pathological findings or clinical symptoms are present, but not sufficiently severe to be considered as the cause of death (6). Non-SIDS means that pathology and clinical data which would inevitably have led to the death are present, and thus the death may be called explained. It was expected that many adverse factors found in epidemiological cot death studies (12–14) would be not specific for cot death or SIDS but would be equally unfavourable for babies that had died unexpectedly from whatever cause or disease. These factors might be low socioeconomic status (SES), low birthweight, male pre- ponderance, respiratory infections, winter season and age (15, 16). Categorizing deaths into pure SIDS, borderline SIDS or non-SIDS might be irrelevant when related to epidemiological findings or might show that certain risk factors may be more specific for one or the other group (17, 18). It may be assumed that a factor that would endanger a previously healthy child will also play a role in the death of a child with an infection or other illness. After the initiation of advice to abandon the prone position in favour of supine sleeping for all babies, the sharp decrease in the incidence of cot death went hand in hand with a general decrease in post-perinatal mortality. The Acta Pædiatr 87: 1279–87. 1998  Scandinavian University Press 1998. ISSN 0803-5253