Unexpected high spinal block in obstetrics.

Sir,—Elliott, Voyvodic and Brownridge recently reported an unexpected subarachnoid block in a parturient, despite careful aspiration testing and prior administration of local anaesthetic via a catheter, presumably located in the lumbar extradural space. 1 We recently described five similar cases of high spinal block produced by the initial test dose alone after negative aspiration testing. 2 We have had three additional cases since our report, two in labouring parturients and one in an elderly woman undergoing ankle surgery. In common with our previously reported cases, patients were seated during insertion and testing of the catheter, and 18-gauge Tuohy needles with 20-gauge open-tip, single-orifice polyamide catheters were used (B. After uneventful catheter insertions, careful negative pressure aspiration produced no fluid. The test dose administered (3 ml of glucose-free 1.5% lignocaine with adrena-line 1 : 200 000) produced abrupt high sensory block and complete lower extremity motor block consistent with subarachnoid injection in each patient. High spinal block was accompanied by profound hypotension in both parturients. One developed a high motor block resulting in ventilatory failure requiring tracheal intu-bation, followed by emergency Caesarean delivery for persistent fetal bradycardia. In each case, CSF was aspirated easily through the catheter with subsequent negative pressure aspiration testing. As in our previous cases, none of the patients developed postdural puncture headache (PDPH), despite dural and arachnoid punctures. Several authors have suggested that " a subdural catheter can perforate the arachnoid matter...at any time " 1 and, " the test dose given through a catheter placed in the subdural space causes the arachnoid to tear ". 3 We agree, and provide the accompanying figure (fig. 1) as a unified explanation of the mechanisms underlying arachnoid rupture and the associated low incidence of PDPH. Although initial placement of the catheter is subdural, subsequent arachnoid rupture occurs because of sudden positive pressure generated in the low-compliance subdural space by injection of a small volume of fluid. This is consistent with findings common to all our cases, namely absence of CSF during placement of the catheter, inability to aspirate fluid before catheter injection, and free flow of CSF after injection and onset of spinal block. Elliott, Voyvodic and Brownridge did not state if CSF was aspirated after the onset of subarachnoid block, however, computed tomography demonstrated clearly subarachnoid catheter placement in their patient. Also, consistent with this mechanism is the absence of PDPH in all cases described by us (seven …