Statistics: can we prove an association for a rare complication?

Abstract

Microcatheters for continuous spinal anesthesia were withdrawn from the market after an apparent increase in the incidence of cauda equina syndrome (CES) associated with spinal anesthesia after introduction of these catheters. The objective of this review is to evaluate the historical data on CES after spinal anesthesia and to compare the result to the recent data. The literature on the use of statistics and on complications associated with spinal anesthesia was reviewed. Poisson probabilities were calculated to assess the probability of seeing the recent cases of CES, given the historical data. The sample size required for a prospective study was calculated. Statistics cannot "prove" an hypothesis but can only support or fail to support it. Use and interpretation of the p value are discussed, as are possible problems with interpretation of the p value. The difference between causal and statistical inference is discussed. Probabilities for the occurrence of CES after microcatheter use were calculated. The sample size for a prospective study of this problem was calculated; a large sample is required. Statistics alone cannot support an association of microcatheters with CES after spinal anesthesia. Additional considerations suggest a possible association, but further study is required.