Epidural catheter knot formation is a very rare complication of indwelling epidural catheters and has an estimated incidence of 0.0015% . In a case report and literature review by Brichant et al. , 18 cases of epidural catheter knot formation were identified. The epidural catheter was placed at the lumbar level in most cases, and an epidural catheter placed at the thoracic level formed a knot in only one case. To our knowledge, this is the second case report of knot formation in an epidural catheter placed at the thoracic level.
The reason why knot formation is less frequent with thoracic epidural catheters is not clear; however, one possible reason is that the catheter advances differently in the epidural space in the thoracic and lumbar regions. Muneyuki et al.  reported that an indwelling thoracic epidural catheter was less likely to curl, bend, or kink in epidural space than an indwelling lumbar epidural catheter, and a greater amount of the catheter can be inserted without coiling. They suggested that this difference in catheter travel is caused by the difference in the angle of insertion of the needle. In the lumbar region, the epidural needle impinges on the dura at a right angle, whereas in the thoracic region, the needle is inserted at an obtuse angle to the epidural canal, which may make it easier for the catheter to be inserted straight.
The catheter in the epidural space in the current case was observed to be coiling on postoperative abdominal plain radiography, which may have led to knot formation when the catheter was attempted to be removed. Several authors recommend that the length of catheter placement in the epidural space should be limited to minimize the risk of complications (e.g., catheter dislodgement, intravenous cannulation, or knot formation) [4,5,6]. To prevent catheter loop formation, indwelling lumbar epidural catheters should not be placed into the epidural space beyond 5 cm [5, 7]. As for the thoracic epidural, it has been reported that catheters tend to insert straighter compared to inserting into the lumbar region, and inserting up to 10 cm without forming a loop is possible . However, a more recent study reported that the thoracic epidural catheter forms a loop at 4.9–7.4 cm, depending on the angle of approach . In fact, in the present case, a loop was formed after 7 cm of catheter placement, and knot formation occurred during removal. This case suggests that loop and knot formations occur even when the length of the indwelling thoracic epidural catheter is less than 10 cm.
Pulling gently with a constant force to prevent catheter breakage is necessary when removing an epidural catheter. Although catheters were successfully removed in most of the reported cases of knot formation, applying traction on the catheter resulted in catheter breakage in about 30% of cases . In our case, the catheter came out gradually by pulling gently, but if excessive force is required to remove the epidural catheter and the catheter is stretched, visualization of the catheter by plain X-ray or computed tomography should be considered to check for knot formation.
In conclusion, a knot formation of an epidural catheter placed at the thoracic level was experienced. Limiting the length of catheter placement may prevent knot formation. Moreover, visualization of the catheter by plain X-ray or computed tomography should be considered if the epidural catheter is difficult to remove.