Incidence of dural sac puncture during neuraxial anesthesia in cats: an observational,retrospective study

Abstract

Objectives

The aim of this study was to determine the occurrence of dural puncture, indicated by cerebrospinal fluid (CSF) outflow, in cats receiving neuraxial anesthesia through a lumbosacral injection guided by a pop sensation method.

Methods

This was an observational, retrospective study. Cats that were scheduled for lumbosacral neuraxial anesthesia were included. Medical records were analyzed to investigate: (1) demographic data; (2) neuraxial anesthesia performed (epidural/spinal); (3) type of needle used, including gauge and length; (4) presence of CSF (yes/no) and/or blood (yes/no) in the hub of the needle; and (5) flicking of the tail during needle advancement (yes/no).

Results

A total of 94 medical records were analyzed. A 22 G 50 mm Tuohy needle was used in all cats scheduled for an epidural injection (n = 60), whereas a 22 G 40 mm Quincke needle was used in all cats scheduled for an intrathecal injection (n = 34). CSF outflow was detected in 55/60 (91.7%) cats in which a Tuohy needle was used, and 34/34 (100%) of the cats in which a Quincke needle was used (P = 0.15). Flicking of the tail was detected in 41/60 (68.3%) and in 24/34 (70.6%) injections with Tuohy and Quincke needles, respectively (P >0.99). Traces of blood, but not active blood outflow, were detected via staining of the first drops of CSF in 2/34 cats in which Quincke needles were used and in none of the cats in which Tuohy needles were used (P = 0.12).

Conclusions and relevance

This study shows that the lumbosacral approach for neuraxial anesthesia in cats may result in a dural sac puncture when 22 G Quincke or Tuohy needles are used. The pop sensation method should be deemed effective in predicting intrathecal but not epidural needle placement.

Introduction

Neuraxial anesthesia involves the administration of local anesthetics in the epidural or intrathecal space. Although both techniques are indicated to desensitize a specific body area, the injectate distribution, the onset and duration of effect, and the potential adverse effects of each method are different. Spinal anesthesia involves the injection of a small volume of preservative-free drugs, whereas epidural anesthesia allows for the injection of a larger volume of regular anesthetics and adjunctive drugs. ¹ In fact, inadvertently interchanging these techniques could lead to adverse scenarios ranging from technical failure to life-threatening hemodynamic events. ¹

In cats, neuraxial injections are usually performed via a lumbosacral access.^1,2 At this level, the spinal cord and surrounding meningeal membranes occupy the majority of the vertebral canal, and the distance between the ligamentum flavum and the dura mater is smaller than the length of the bevel of the recommended needles for both epidural and intrathecal approaches.^3,4 Commonly, final confirmation of needle tip epidural or intrathecal location relies on the absence or presence of cerebrospinal fluid (CSF) outflow, respectively.^1,2 However, unlike in human medicine in which the occurrence and adverse clinical consequences of dural puncture are well documented, ⁵ no reports evaluating the occurrence and outcome of this event have been published in the veterinary literature.

Therefore, the aim of this retrospective study was to document the occurrence of CSF outflow when performing both epidural and intrathecal lumbosacral injections guided by the pop sensation method in cats.

Materials and methods

Owner consent allowing the anonymous use of their animal’s medical information was obtained upon admission of the cats to the hospital. A search of the medical records database for cats with an American Society of Anesthesiologists physical status of I and II that underwent ovariectomy and received neuraxial anesthesia through a lumbosacral injection scheduled between June 2017 and June 2018 was performed. Records lacking information on the presence or absence of CSF outflow or the type of needle used were excluded.

Data extracted from medical records included: (1) demographic data (breed, age and body condition score (BCS) 1–9]6 of the included animals; (2) neuraxial anesthesia performed (epidural/spinal); (3) CSF (yes/no [Y/N]) and/or blood (Y/N) outflow in the hub of the needle; (4) flicking of the tail during needle advancement (Y/N); and (5) type of needle used including gauge and length. No data on injection or on its outcome was collected.

All animals were administered an anesthetic protocol standard for our institution. General anesthesia was induced, the trachea was intubated, and all animals were instrumented and monitored using a multi-parametric monitor. After initial stabilization, the lumbosacral region was clipped and aseptically prepared. In all cases, needle advancement was guided by the pop sensation,^1,2 and neuraxial injections were performed by two experienced anesthetists. Cats were positioned in sternal recumbency, with the pelvic limbs extended cranially. The lumbosacral space was identified by palpation of the seventh lumbar vertebra spinous process and the wings of the ilium. The needle (ie, Tuohy or Quincke) was inserted perpendicularly to the skin via the lumbosacral junction and advanced towards the target space (ie, epidural or intrathecal), until the presence of a distinct pop sensation, indicative of ligamentum flavum piercing was perceived. Subsequently, the needle’s stylet was removed, and the presence or absence of CSF and blood in the hub was noted.

Statistical analysis

Data were assessed for normality using the Shapiro–Wilk test. The occurrence of events (Y/N) was analyzed using Fisher’s exact test. The age, weight and BCS of cats receiving intrathecal or epidural injections were compared using an unpaired Mann–Whitney U-test. Differences were considered significant with a P value <0.05. Statistical analysis was performed using GraphPad Prism Version 8.0.

Results

A total of 94 medical records (100% of the initially evaluated records) were included. No records were excluded. All cats were domestic shorthairs. Of the 94 cats, 60 (63.8%) and 34 (36.1%) were scheduled for epidural or intrathecal anesthesia, respectively. The median (range) weight, age and BCS of the cats that were administered epidural and intrathecal injections, respectively, were 4.2 kg (3.7–5.3) and 3.9 kg (3.7–4.7) kg, 14 months (9–19) and 14 months (10–19), and 5 (4–5) and 5 (4–5). No differences were found between groups regarding weight (P = 0.08), age (P = 0.88) and BCS (P >0.99). A 22 G 50 mm Tuohy needle was used in all the cats scheduled for epidural injection, whereas a 22 G 40 mm Quincke needle was used in all the cats scheduled for intrathecal injection.

CSF outflow was detected in 55/60 (91.7%) cats and in 34/34 (100%) of the cats in which epidural and spinal injections were performed, respectively (P = 0.15). The total occurrence of CSF outflow was 91.2%.

Tail flicking was observed in 41/60 (68.3%) cases and in 24/34 (70.6%) of the cases in which epidural and spinal injections were performed, respectively (P >0.99). Tail flicking was seen in 69.1% of cases overall. CSF outflow was observed in all of the cases in which tail flicking was present.

Traces of blood, but not active blood outflow, were seen to stain the first drops of CSF in 2/34 cats that received a spinal injection and in none of the cats that received an epidural injection (P = 0.12).

Discussion

A retrospective analysis of the anesthetic records of 94 cats in which a pop sensation-guided lumbosacral neuraxial injection was performed demonstrated an occurrence rate of dural puncture (confirmed by the univocal observation of CSF outflow) of 91.2%. When epidural needle placement was attempted, the use of a 22 G Tuohy needle did not serve to protect the dura mater and thus minimize the incidence of accidental dural puncture (ADP).

Dural puncture involves passing a needle through the dura mater into the CSF-filled subarachnoid space that surrounds the spinal cord and nerve roots. Although this event is the primary outcome of an intrathecal injection, it is a complication reported to occur in up to 2.5% of epidurals in humans. ⁵ In this report, ADP was observed in 91.7% of the cases in which epidural injection was attempted. Interestingly, to our knowledge, scientific reports of ADP in cats are scarce, ⁷ and therefore it remains to be determined whether this is a matter of a low incidence, underdiagnosis or a tendency to under-report.

The traditional epidural anesthesia technique in cats involves the introduction of a 22 G spinal needle through the lumbosacral space guided by either loss of resistance or pop sensation methods. ¹ A recent recommendation is to use Tuohy needles as they are more protective of the dura mater. ⁴ In this study, no significant difference was observed in the occurrence of dural puncture when 22 G Quincke or Tuohy needles were used for intrathecal or epidural anesthesia. It is therefore evident that factors other than the type of needle used, such as the angle of needle introduction, the length of the needle’s bevel and the anatomy of the species, play a more central role in the occurrence of dural puncture when performing lumbosacral neuraxial anesthesia in cats. A rational explanation for the occurrence of dural puncture may be that the lumbosacral epidural space in cats is only 0.4 ± 0.2 mm, ⁴ which is smaller than the length of the bevel located in the epidural space when a perpendicular angle of needle entry is used. Thus, standard epidural technique may pose a risk of ADP and nerve tissue damage if the dura mater is not displaced ventrally during introduction of the needle (Figure 1).

Figure 1

Proposed sequence of theoretical events that might explain the incidence of cerebrospinal fluid (CSF) outflow observed during the introduction of a 22 G Tuohy or Quincke needle to perform neuraxial anesthesia via a lumbosacral approach in cats. This infographic shows the size of needles and structures of interest, and it is drawn in real-size scale. (a) The needle is advanced until its tip contacts the ligamentum flavum. (b) The tip of the needle pushes the ligamentum flavum ventrally, which collapses the epidural and subarachnoid spaces. (c) As soon as the resistance of the ligamentum flavum is overcome, the tip of the needle punctures the dura mater and reaches the subarachnoid space, allowing for CSF outflow. (d) The dural sac is most likely depressurized by the lack of CSF through the hole of the needle after its removal. The laceration of either the meningeal structures and/or nerve tissues will initiate an inflammatory repair process

In this report, dural puncture and tail flicking were observed in 94.7% and 69.1% of cases, respectively. According to Valverde, ¹ these events are commonly seen in cats when the needle enters the epidural space and pricks the spinal cord or cauda equina.

Whether intended or accidental, the disruption of the meningeal layers and the potential damage to nerve tissue is associated with consequences that range from an acute inflammatory response to more serious chronic pain syndromes, which, up until now, have only been reported in humans.^5,8,9 Further studies are required to determine the occurrence rate of such events in cats and their association with dural puncture and nerve tissue damage.

Post-dural puncture headache (PDPH) is considered to be the main adverse effect of ADP in humans and its frequency can reach up to 81% when a large-bore needle is used. ⁹ CSF leakage into the epidural space and the consequent depressurization of the dural sac is thought to be the cause of PDPH. ⁵ In adult humans in a supine position, CSF pressure in the lumbar region ranges between 5 and 15 cm H₂O, and usually remains below 4 cm H₂O after dural sac puncture. ¹⁰ The rate of CSF loss through the hole produced in the dura mater after a subarachnoid injection is generally higher than CSF production (0.35 ml/min), particularly when needles greater than 25 G are used. ¹⁰ In cats, the pressure of the CSF in the lumbar region in sternal recumbency was reported to reach 11.8 ± 0.6 cm H₂O, ¹¹ and therefore we could expect a similar physical outcome, especially when relatively large bore needles are used. Although no clinical consequences of intended dural puncture have been reported in cats, it seems that the use of a needle gauge <22 G may be more protective of neural structures, minimizing the risk of adverse clinical effects. The use of a 25 G needle for spinal anesthesia in cats was proposed by Sarotti et al, ² but it was associated with a procedure failure rate of 27%. Further studies are warranted to weigh the efficacy of the technique vs the protection of the neural structures and to investigate the short- and long-term clinical consequences of dural puncture.

Several authors proposed the sacrococcygeal approach as an alternative access point to perform epidural injections in cats.^3,4 This rational alternative is based on the fact that neither the dural sac nor the spinal cord are present at this level in the cat. If the preservation of nerve structures and meningeal membranes is crucial when performing epidural anesthesia, a sacrococcygeal access seems to be a more suitable technique than the traditional lumbosacral approach.

The study has some limitations. First, only one size of Quincke or Tuohy needles were used, and this could have been a determinant in the occurrence of dural puncture. Secondly, only the subjective ‘pop sensation’ method was tested. However, as the length of the bevel of the needle used requires ventral displacement of the dura mater for epidural location, it appears that the confirmation method might not be the main determinant factor of the outcome. Finally, given the retrospective nature of this study, inaccuracy with the data collection cannot be discounted. Therefore, further prospective studies that evaluate other methods or other needle sizes are warranted.

Conclusions

This study shows that the lumbosacral approach for neuraxial anesthesia in cats may result in a dural sac puncture when 22 G Quincke or Tuohy needles are used. The pop sensation method should be deemed effective in predicting intrathecal but not epidural needle placement. Alternatively, a sacrococcygeal approach could be considered when planning an epidural injection, to avoid the risk of ADP in cats.