Failure of extradural blockade in obstetrics. A new hypothesis.

Three-thousand and eleven consecutive obstetric extradural blocks were evaluated retrospectively. The distance from the skin to the extradural space was correlated with the incidence of unilateral blockade. There was a significant correlation (P less than 0.001) between an increasing distance and increased incidence of unilateral block. We propose that this is because of the topography of the extradural space, and because of the deviation of the tip of the needle from the midline (when using a midline approach).