Urolithiasis, idiopathic hypercalciuria and insularity

Abstract

Sirs, Edvardsson et al. recently published an interesting article in this Journal on the high incidence of kidney stones in Icelandic children [1]. These investigators reported the incidence of kidney stones to be 5.6–6.3 per 100,000 Icelandic children, which is very much higher that the incidence rates reported for other communities. Hypercalciuria was the most common metabolic risk factor identified (78%). In a much earlier study carried out in 1971, Holmes reported a high incidence of kidney stones in the population of the Fiji Islands [2]. Metabolic studies of these patients did not reveal any evident biochemical abnormality which could cause nephrolithiasis, and the authors came up with the theory that an excessive intake of curry could be nephrotoxic [3] and even published an Editorial stating this in the “British Medical Journal” [4]. To the best of our knowledge, nothing has been published since on this novel theory. A high prevalence of nepholithiasis (14.4%) has also been described in Caucasians living on the island of Puerto Rico [5]. In 2000, we studied the urinary elimination of calcium in 549 school children from the island of La Gomera and in 100 children of a control group from the island of Tenerife in the Canary Islands [6]. We calculated the prevalence of hypercalciuria in the control group to be 3.8% and that of the study group to be 16%. The results showed that the prevalence of hypercalciuria was higher in communities from isolated areas with a history of inbreeding (28.4%) than in those with good transportation facilities and thus interacting with communities other than their own (10.6%). We found that the risk of presenting hypercalciuria was 2.85-fold higher in children with four grandparents who were inhabitants of La Gomera than in those who do not have any grandparents from this island. The frequency of hypercalciuria in siblings of subjects with hypercalciuria was 50%. Historically speaking, the rate of consanguinity on the island of La Gomera has been very high. When we looked at the ecclesiastical files from the 18th and 19th centuries, we noted that the marriages among cousins varied between 19.3 and 37.1%, depending on the specific town on the island. The high incidence of urolithiasis and/or hypercalciuria among the resident communities of Iceland, Fiji, Puerto Rico and the Gomera islands predisposes us to believe that isolation may be a risk factor in developing kidney stones. In the past, the socio-economic conditions on the Canary Islands were very poor. Good hygienic and nutritional conditions were deficient and, as has been the case for similar places throughout the world, very few people reached maturity. For example, infant mortality in the orphanage in the town of La Laguna on the island of Tenerife in the 19th century was 809–886 per 1000 [7]. Therefore, at a time when antibiotics or vaccines were not available, the majority of the people reaching maturity would have been those showing the most resistance – from an immunologic standpoint. Our hypothesis is that children with hypercalciuria from La Gomera are probably descendants of those “immunologically strong” people. Therefore, insularity and its associated consanguinity may not be the cause of hypercalciuria but the means of perpetuating the immunologic circumstance of the subjects that survived. In an investigation along this line, Rudan et al. have investigated the susceptibility to nephrolithiasis in some villages on three Croatian islands. The average inbreeding coefficient (F) of V. Garcia Nieto (*) Unidad de Nefrologia Pediatrica, Hospital Universitario Nuestra Senora de Candelaria, Carretera del Rosario, s/n, 38010 Santa Cruz de Tenerife, Spain e-mail: vgarcia@comtf.es Tel.: +34-922-600675 Fax: +34-922-600562