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A lesson from the occult
Abstract Number: F-01
Abstract Type: Case Report/Case Series
Lessons from the occult
The lumbosacral cerebrospinal fluid (CSF) volume, as assessed by magnetic resonance imaging, is a major determinant of the intrathecal spread of local anaesthetics(1). Asymptomatic spina bifida occulta is relatively common in the general population (10-50%) and may be associated with enlarged lumbosacral dural sac dimensions. We report a case of low block following spinal anaesthetic in a parturient with spina bifida occulta and enlarged sacral dural sac dimensions.
A healthy 34-year-old lady presented for elective Cesarean Section for transverse lie.
She reported a small lower back anomaly present since birth that had remained completely asymptomatic. Physical examination revealed a subtle fat pad stretching from L2-L4. There were no other anomalies noted.
Lumbar spine X-ray revealed incomplete fusion of the posterior elements of the L3 and L4 vertebrae, consistent with underlying spina bifida occulta. MRI showed a tethered, low lying spinal cord extending to L4 and capacious lumbosacral dural sac dimensions.
Using spinal ultrasound the L5-S1 interspace was identified and marked. A spinal anaesthetic containing 13mg of bupivacaine, 15 mcg of fentanyl and 0.1mg of morphine was successfully inserted with aspiration of clear CSF prior to, and following injection. The patient was placed in the supine position and a wedge inserted under the right hip. After 15 minutes, including 10 minutes in 15 degree Trendelenberg position, the patient had Bromage 3 motor block bilaterally, while the sensory block level extended to T12, determined by pinprick. An epidural catheter was subsequently inserted at T12/L1 level and 12ml of 5mg/ml bupivacaine administered to achieve a sensory block level of T4 bilaterally. Surgery proceeded uneventfully and a healthy baby was delivered. The mother recovered well, and was discharged home on the fourth postoperative day, with neurosurgical follow up arranged.
We propose that the low block level following spinal anaesthesia was most likely related to enlarged lumbosacral dural sac dimensions. While this relationship has been reported in the general population, with a good correlation between lumbosacral dural sac dimensions on MRI and block height, the same data has not been available to date for the obstetric population. This report supports the theory that spinal canal dimensions are the primary determinant of block height following spinal anaesthetic administration in the pregnant population.
(1) Carpenter RL, Hogan QH, Liu SS, Crane B, Moore J. Lumbosacral cerebrospinal fluid volume is the primary determinant of sensory block extent and duration during spinal anesthesia. Anesthesiology 1998; 89: 24–9.