Anesth Analg 1999;89:1267
© 1999 International Anesthesia Research Society
OBSTETRIC ANESTHESIA
Dilute Infusion for Labor, Obscure Subdural Catheter, and Life-Threatening Block at Cesarean Delivery
Douglas J. Forrester, MD*,
Suresh K. Mukherji, MD
,
David C. Mayer, MD*, and
Fred J. Spielman, MD*
Departments of
*Anesthesiology and
Radiology, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
Address correspondence and reprint request to Douglas J. Forrester, MD, Department of Anesthesiology, The University of North Carolina at Chapel Hill, CB# 7010, 223 Burnett-Womack Bldg., Chapel Hill, NC 27599-7010.
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Introduction
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Subdural cannulation occurs in 0.82% of epidural catheter attempts (1). Presentation may include delayed, extensive sensory block (2–7), grossly asymmetric block (2–5), sparing of sacral dermatomes (2,4,5,7), and varying degrees of motor block (2–7). The trend toward using smaller concentrations of local anesthetic for labor analgesia may make epidural evaluation more difficult. We present a case of a profound subdural block from a catheter that had previously provided a relatively normal labor block with diluted bupivacaine/fentanyl/epinephrine.
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Case Report
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A healthy 33-yr-old woman received a labor epidural, placed on a single pass at the L3-4 interspace, with a 17-gauge Tuohy-Schliff needle, while she was in the sitting position. Needle entry was bevel cephalad, with no rotation. The catheter (closed-tip) was threaded to 4 cm beyond the Tuohy; aspiration and a spinal test dose were negative. Using bupivacaine 0.04%, fentanyl 1.7 µg/mL, and epinephrine 1:600,000, we initiated the block with 15 mL in divided doses and then an infusion of 15 mL/hour. Sensory level was T-8 on the left and T-10 on the right, and the patient remained comfortable without further boluses. After 6 h, cesarean delivery was recommended for failure to progress.
In the operating room, the patient moved herself onto the table; examination revealed an appropriately dense sensory block and moderate left lower extremity weakness. The catheter position was unchanged at the skin, with negative aspiration. Lidocaine 2%, fentanyl 5 µg/mL, and epinephrine 1:200,000 was used as a 3-mL test dose (negative at 5 min) and then incrementally dosed. After 10 min and a total of 15 mL, there was evidence of a high block, followed by apnea. Rapid sequence induction was complicated by unsuccessful intubation and difficult mask ventilation. Diaphragmatic movement resumed after 10 min, and, after an oral fiberoptic intubation, the cesarean proceeded under the residual block and general anesthesia. Blood pressure and heart rate were normal throughout; Apgar scores were 7 and 8 at 1 and 5 min, respectively.
Although the patient’s strength was sufficient for tracheal extubation in 90 min, complete lower extremity recovery took approximately 10 h. Neurologic recovery was complete, and the patient recalled only the initial mask ventilation and awake fiberoptic intubation.
Repeated attempts to aspirate from the catheter were negative, and the patient did not develop a postdural puncture headache. After patient consent, a computed tomography scan revealed subdural location of the catheter and the injected dye (3 mL Omnipaque; Nycomed, Inc., Princeton, NJ), with no subarachnoid spread (Figure 1).
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Figure 1. Lumbar axial computed tomography (CT) images after contrast injection through catheter ("cathetergram"). In our patient (A) contrast material (arrows) circumferentially outlines the thecal sac (T), strongly suggestive of subdural confinement of the contrast material. For comparison (B) is a "cathetergram" from a patient with a normally functioning epidural catheter. Unlike our case, the contrast material appears along the nondependent anterior portion of the thecal sac (straight arrows), with sparing of the dependent posterior portion (curved arrow), and is typical of contrast located in the epidural space.
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Discussion
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Although motor block is minimized, the trend toward using smaller concentrations of local anesthetic for labor epidural analgesia may obscure dangerous catheter malpositions. We believe this case represents such an event and demonstrates the unpredictable nature of subdural blocks.
We suspect that, with the aid of a dural rent from the Tuohy, our catheter threaded into the subdural space at the time of placement and that the dilute concentration of bupivacaine/fentanyl/epinephrine was inadequate to discriminate between an epidural and subdural location. In this regard, previously used labor analgesia techniques (e.g., 0.25% bupivacaine initiation and 0.125% infusion) may have raised suspicions of catheter malposition and, thus, prevented the near catastrophe that occurred in the operating room.
Alternative explanations for our case include delayed subdural migration (8) and sudden subdural to subarachnoid conversion from arachnoid membrane rupture with the local anesthetic bolus for surgery (9,10). Delayed migration through the dura, but not arachnoid, seems less likely, given laboratory findings showing that penetration of intact dura by an epidural catheter is relatively difficult, but that penetration of the arachnoid occurs easily (11). Additionally, unlike an open-tip catheter, in which presumably less forward migration would be necessary to functionally change the catheter tip position, significant forward advancement would seem necessary to locate the holes of our closed-tip catheter within the subdural space; however, catheter position at the skin remained unchanged. A subdural catheter with subsequent arachnoid rupture, though explaining the profound block at cesarean, appears inconsistent with our negative aspirations and radiologic findings.
Our literature review suggests that this subdural block exhibited unusually high cephalad spread and exceptionally prolonged motor block. The apparent cranial nerve anesthesia, judged by complete apnea and lack of response to an oral fiberoptic intubation, can be explained anatomically by the fact that the subdural space, unlike the epidural space, extends intracranially. The etiology of the prolonged lower extremity motor block in our case is unclear. Except for a single case that was not radiologically documented (12), our review indicates that the motor component of subdural blocks typically resolves in two to three hours, much like an epidural.
In summary, a catheter, later proven as subdural, provided a normal labor block with dilute local anesthetic, but dosing for cesarean delivery resulted in total paralysis and prolonged motor block. Although delayed migration cannot be ruled out, it appears likely that the dilute concentration of local anesthetic for labor obscured the presumed, hazardous, subdural location.
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References
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Accepted for publication July 6, 1999.
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Introduction
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