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Introduction
The current incidence of subdural catheter placement during attempted epidural anesthesia in the acute pain setting is unknown. Classification systems and clinical criteria of subdural injections have been suggested.1 2 Nevertheless, subdural injection or catheter placement remains especially difficult to identify clinically and can occur despite negative aspiration of cerebrospinal fluid and a negative test dose. Subdural placement may lead to inadequate analgesia because the dura-arachnoid interface varies in size and local anesthetics tend to distribute posteriorly, sparing anterior nerve roots.3 In addition to ineffective analgesia, subdural infusions of local anesthesia may lead to serious morbidity such as cardiovascular and respiratory depression.1 2 4 Capitalizing on a unique epidural catheter service that relies exclusively on fluoroscopic imaging with real-time contrast injection, we aimed to calculate the incidence of subdural catheter placement in a contemporary academic acute pain medicine practice.
Methods
We conducted a retrospective review of patients who underwent fluoroscopically guided thoracic epidural catheter placement at Dartmouth-Hitchcock Medical Center in New Hampshire from July 1 2014 to August 19 2020. Data were extracted from our electronic medical record system.
We routinely place preoperative thoracic epidurals under fluoroscopic guidance for thoracic and abdominal surgeries.5 A standard 17 gage Tuohy needle is used to access the epidural space in the low-thoracic and lumbar regions, usually between T12 and L1 or L1 and L2. Under live fluoroscopy, a 19 gauge radiopaque epidural catheter (Arrow TheraCath) with a single orifice is then threaded up to the targeted thoracic levels based on analgesic needs. By injecting 2 mL of iohexol contrast (240 mg/mL) through the catheter and acquiring real-time images, we are able to immediately identify the location of the catheter, and make corrections accordingly. Other potential unexpected locations of the epidural catheter include intrathecal, subdural, intravascular and subcutaneous.
Results
In 2472 epidurals, the overall incidence of an inadvertent subdural catheter placement was 5.3 per 1000 (95% CI 2.8 to 9.0)(table 1). All subdural catheters (figure 1) were removed and then successfully replaced at a different level, either above or below the previous level into the epidural space (figure 2).
Patient and procedural characteristics by subdural catheter status
A classic contrast study for subdural catheter insertion. (A) Posterior–anterior fluoroscopic image demonstrating the linear spread of 2 mL of Omnipaque contrast consistent with a subdural location. The catheter (triangle) projects over the spinal canal. dense undulating linear contrast extends cephalad and caudal bilaterally conforming to the expected peripheral location of the thecal sac (long block arrows). note the multiple dermatomes to which the contrast extends. Subtle focal contrast outlines the proximal dural sleeves (short block arrows) on the left. Absence of nerve root filling defects would be seen in a subarachnoid injection. Diffuse hazy contrast opacification projects over the midline, which can be appreciated more on the lateral image. (B): lateral fluoroscopic image demonstrating the linear spread of contrast in the subdural space. Dense linear contrast is ventral to the posterior epidural space extending cephalad and caudal. Smaller dense linear contrast is seen anteriorly with typical ‘railroad track’ (long block arrows) appearance of a subdural injection. There is a sharp margin posteriorly indicating absence of epidural extension. Subarachnoid contrast would maintain a dependent position without anterior extension and be less dense due to cerebrospinal fluid dilution.
A classic contrast study for epidural catheter insertion. (A) Frontal projection demonstrates contrast surrounding the thecal sac in the epidural space (long block arrow). The central expected location of the thecal sac is less dense than the periphery. The lateral margins of the contrast are seen to thicken and extend toward each neural foramina. The epidural catheter can be easily identified toward the bottom of the image (triangle). Note the circular fat globules (small arrow), which are indicative of epidural space contrast distribution. (B) Lateral projection demonstrates linear contrast opacity (long block arrow) in a dependent location within the spinal canal, dorsal to the expected location of the thecal sac. There is no ventral contrast. There is contrast seen extending into the neural foramina at multiple levels (short block arrow). Ill-defined opacities are seen ventral to the linear contrast in the dorsal epidural space representing contrast lateral to the thecal sac. Again, the epidural catheter (triangle) can be seen at the bottom of the image.
Discussion
Our retrospective review demonstrates that inadvertent subdural catheter placement during attempted epidural placement is more common than previously thought. Unintentional subdural anesthesia can have severe even fatal clinical implications.2 Prior reports of subdural catheter incidences were extracted from clinical evidence alone, small samples or single injections instead of with live imaging.1 6 Because of the dedicated use of fluoroscopy and contrast injections, we were able to definitively calculate the incidence and address the malposition prior to surgery.
Our study is limited in that our practice of live fluoroscopy and prone positioning is unique and may introduce variables that impact on subdural rates that are not realized with a more conventional practice. The potential space between the ‘dura-arachnoid interface’ as described by Reina et al, does not exist uniformly and is not necessarily a continuous space.7 It can be created and extended by traction in dura sacs of cadaver samples or by catheter manipulation in vivo.7 Therefore, technical considerations related to needle, catheter and loss of resistance techniques may influence rates of subdural catheter insertion.
We conclude that subdural catheter insertion is an event that can be identified in real time and has predicted incidence of 2.8–9.0/1000 in a tertiary care academic practice.
Ethics statements
Patient consent for publication
Ethics approval
Approval was obtained from the Dartmouth-Hitchcock Health Human Research Protection Program.
Footnotes
Correction notice This article has been corrected since it published Online First. The provenance and peer review statement has been included.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests Brian Sites is the Editor-in-Chief of Regional Anesthesia & Pain Medicine.
Provenance and peer review Not commissioned; externally peer reviewed.