Subdural block – a complication of epidural anesthesia
Abstract
A 62-year-old, 185 cm tall, ASA-I male patient was scheduled for right hip bipolar hemiarthroplasty under epidural anesthesia. Epidural space was identified at L2-3interspace in the sitting position, by loss-of-resistance to air technique using 18-gauge Tuohy needle. There was no problem during needle placement and it was not rotated. Epidural catheter (multiport, closed tip) was inserted through the needle with minimal resistance and fixed at 10 cm mark at skin. After negative aspiration for blood and CSF, 3 ml lignocaine (1.5%) with epinephrine (1:200000) was injected as test dose followed by 8ml (0.5%) bupivacaine.
After 15 minutes, patient could still lift his left leg against gravity and move his right leg though unable to lift it due to injury. His blood pressure decreased from baseline 112/70 to 60/48 mmHg and heart rate from 100 to 80/ minute. His mentation was appropriate and he had no difficulty in breathing or phonation. 100% oxygen, crystalloids and mephentermine 3 mg was administered. Blood pressure transiently rose to 85/60 mmHg, but fell agin to 62/50 mmHg, so dopamine infusion was started. The level of sensory block was at T2 dermatome. SpO2 remained 99-100% throughout.
We suspected a subdural block. Surgery was not possible in absence of motor blockade and alternative anesthesia was required. We considered it safe to wait for the effect of subdural block to subside before administering general anesthesia to avoid the risk of further myocardial depression in presence of an already high sensory and sympathetic block. The case was postponed. After removing the epidural catheter the patient was shifted to postoperative room with continued fluids and dopamine infusion. After 2 hours, patient had complete recovery of sensory loss, with BP 110/80 mmHg without vasopressor support.
An unintended subdural drug deposition may occur with an incidence of 0.006%1 to 17%2 during epidural anesthesia. Predisposing factors include rough handling, epidural needle rotation in epidural space3 and previous back surgery.4 The onset of subdural block is usually slow (15-20 min). The presentation of clinical signs is varied and depends on the spread of local anesthetic. The sensory block is usually high and disproportionate to the volume of drug injected5; but in some cases it may be inadequate or completely absent.2 Sympathetic and motor functions are usually not affected due to the relative sparing of the ventral nerve roots.6 According to Lubenow’s diagnostic paradigm,4 a negative aspiration and an unexpected extensive sensory block are considered as major criteria. While minor criteria include sensory or motor nerve blockade with delayed onset of greater than ten minutes, a variable motor blockade and sympatholysis out of proportion to the dosage of local anesthetic administered. If both major and at least one minor criterion are met, then a subdural blockade should be considered. Our case met both major and minor criteria confirming the diagnosis of a subdural block.We did not do a radiological confirmation of catheter position as it has risks with no therapeutic benefit.
In case of accidental subdural catheter placement there are no established guidelines for management. Agarwal et al3have suggested that the catheter should be relocated to another space. Patient should be reassured, monitored and provided supportive treatment. Prior to planning a subarachnoid block in these patients, enhanced cephalad spread of local anesthetic should be anticipated.7 While administering general anesthesia, succinylcholine should be used cautiously as it may induce severe bradycardia in patients with high sympathetic block.8
To conclude, subdural drug deposition is a possible complication of neuraxial block. Its early recognition and prompt management can avert a catastrophe. In the event of an inadequate or partial effect, anesthesia must be supplemented with extreme caution especially in patients with high sympathetic and sensory block. In elective, non-emergent surgery, it may be prudent to wait for the effect of subdural block to wear off before administering alternative anesthesia.
