The subdural space: the third place to go astray.

Subdural placement of the tip of the Tuohy needle or epidural catheter may account for many unexpected complications of attempted epidural blockade, for example, 'unexplained' headache, false-negative aspiration test down needle or catheter, false-negative test dose, unilateral block, delayed total spinal and neurological sequelae, as well as profound block of delayed onset that is characteristic of subdural blockade. Cases are reported in support of this hypothesis.