Evaluation of cranial tibial and extensor carpi radialis reflexes before and after anesthetic block in cats

Abstract

Objectives

This study aimed to test the extensor carpi radialis and cranial tibial reflexes in cats before and after anesthetic block of the brachial and lumbosacral plexus, respectively, to determine whether they depend on a myotatic reflex arc.

Methods

Fifty-five cats with a normal neurologic examination that were referred for elective gonadectomy were divided into group 1 (29 cats) for testing the extensor carpi radialis reflex, and group 2 (26 cats) for testing the cranial tibial reflex. In group 1, the extensor carpi radialis reflex was tested after anesthetic induction and 15 mins after brachial plexus block with lidocaine. In group 2, the cranial tibial, withdrawal and patellar reflexes were elicited in 52 hindlimbs and retested 15 mins after epidural anesthesia.

Results

In group 1, before the anesthetic block, 55.17% of the cats had a decreased and 44.83% had a normal extensor carpi radialis reflex. After the block, 68.96% showed a decreased and 27.59% a normal reflex. No cat had an increased or absent reflex before anesthetic block. In group 2, prior to the anesthetic block, 15.38% of the cats had a decreased cranial tibial reflex and 84.62% had a normal response, whereas after the block it was decreased in 26.92% and normal in 73.08% of the cats. None of the cats had an increased or absent reflex. Regarding the presence of both reflexes before and after anesthetic block, there was no significant difference at 1% (P = 0.013).

Conclusions and relevance

The extensor carpi radialis and cranial tibial reflexes in cats are not strictly myotatic reflexes, as they are independent of the reflex arc, and may be idiomuscular responses. Therefore, they are not reliable for neurologic examination in this species.

Introduction

Spinal reflexes test the integrity of the sensory and motor components of the reflex arc and the influence of the descending motor pathways on that reflex.^1,2 The reflex arc is a basic response after a stimulus, and the neurologic examination evaluates, among others, spinal and cranial nerve reflexes. Spinal reflexes can help identify damage to upper or lower motor neurons, and lesions can be localized to a certain section of the spinal cord or even the brain.^3,4

Myotatic reflexes represent the basic reflex arc and involve only two neurons, one sensory and one motor. They are elicited by stimulation of the muscle spindles and Golgi tendon organs via stretching of the muscle fibers contained within the spindle. ⁵ They are thus obtained by percussion of the tendon of a tense muscle or the muscle belly. ⁶

In the thoracic limbs, the myotatic reflexes that can be evaluated are biceps brachii, triceps brachii and the extensor carpi radialis reflex.^7,8 These reflexes are useful when present, but their absence is of little value because they are not always observed in a healthy animal. ⁷

Some authors consider the extensor carpi radialis reflex the most predictable reflex in the thoracic limb.^1,2 It is mediated by the radial nerve and tests spinal segments C6–T2, ⁵ C7–T1^1,2,8 and C7–T2. ⁹ To obtain this reflex, the animal is placed in lateral recumbency with the limb relaxed and supported at the elbow by the examiner, with the carpus flexed. The belly of the extensor carpi radialis muscle is then struck with a reflex hammer immediately distal to the elbow. The normal response is a slight extension of the carpus.^1,2 This response becomes decreased or absent in injuries affecting the C6–T2 spinal segments and/or the radial nerve or its nerve roots, located in the brachial plexus, and could be increased in injuries cranial to the C7 spinal segment.^9–11

In the pelvic limbs, myotatic reflexes that can be evaluated are: dorsal sciatic, ⁹ patellar, cranial tibial and gastrocnemius,^7,8 where the patellar is the most reliable.^2,5,7,9 The patellar and the withdrawal reflex are considered the most significant spinal reflexes when altered.^7,8

To elicit the cranial tibial reflex, the belly of the homonymous muscle is struck with a reflex hammer, immediately distal to the proximal epiphysis of the tibia, and the adequate response is flexion of the tarsus.^1,2,9 It is mediated by the common fibular branch of the sciatic nerve, originating from spinal segments L6–S2, ⁹ L6–L7(S1) ⁵ and L6–L7.^1,2,8 This reflex becomes decreased or absent in lesions affecting these segments of the spinal cord, nerve roots, or sciatic and fibular nerves, and may become increased in lesions cranial to L6.^1,12–14

There have been few papers on spinal reflexes in domestic animals. One study evaluated, quantified and standardized the occurrence of spinal reflexes in calves, ³ and another evaluated and quantified reflexes in young sheep. ⁴ A more recent study in dogs observed that the extensor carpi radialis and cranial tibial reflexes were still present in dog cadavers after transection of the radial and sciatic nerves, thus casting doubt on their usefulness in the neurologic examination in that species. ¹⁵ In light of the latter finding, our objective was to characterize the extensor carpi radialis and cranial tibial reflexes in cats, before and after anesthetic block of the brachial and lumbosacral plexus, respectively, to determine whether they are dependent solely on a myotatic reflex arc in this species. Our hypothesis was that the extensor carpi radialis and cranial tibial reflexes would still be elicited even after anesthetic block of the associated nerves.

Materials and methods

This experiment was authorized under license number 056/2012, protocol number 23082.018974/2012, by the Ethics Committee in the Use of Animals of the Federal Rural University of Pernambuco (UFRPE).

A pilot study using six cats was initially performed to compare the spinal reflexes before and after anesthesia, in order to identify the best instant for evaluation. The cats underwent the same anesthetic protocols and surgical procedures as the cats later selected for the research.

For the main research, 55 cats were selected from the Veterinary Hospital at UFRPE, regardless of sex, age and breed, who were submitted to elective orchiectomy and ovariohysterectomy. They were divided randomly into two groups: group 1 (29 cats) for testing the extensor carpi radialis, and group 2 (26 cats; 52 pelvic limbs) for testing the cranial tibial reflex.

After a clinical examination, a neurologic examination was performed, ⁵ in order to exclude animals with any neurologic dysfunctions. The presence and quality of the reflexes before anesthesia were observed, particularly the withdrawal and extensor carpi radialis reflexes in the thoracic limbs, and the withdrawal, cranial tibial and patellar reflexes in the pelvic limbs. All neurologic examinations were performed and reflexes tested by the same investigator (MLF).

The anesthetic protocol consisted of intramuscular xylazine 1 mg/kg and intramuscular ketamine 10 mg/kg. For analgesia, 2 mg/kg tramadol and 1.1 mg/kg ketoprofen were administered subcutaneously. The local blocks were performed by the same anesthesiologist (THTF) according to a previously described method. ¹⁶

In group 1, a brachial plexus block in one of the thoracic limbs was performed with the aid of an electroneurostimulator, ¹⁷ using lidocaine 2% at 3.5 mg/kg (0.17 5ml/kg) diluted in an equal volume of saline (NaCl 0.9%). The extensor carpi radialis reflex was tested by percussion of the extensor carpi radialis muscle belly immediately distal to the elbow with the relaxed limb supported at the elbow at a 90º angle and with flexion of the carpus. This was done immediately after general anesthesia and 15 mins after the local anesthetic block. ¹⁸ Anesthetic block was considered successful when there was no muscle contraction in the elbow when testing the withdrawal reflex, as well as absent nociception after recovery from general anesthesia. For the withdrawal reflex, a normal reflex would require flexion of the entire limb after noxious stimuli, in this case, pinching of the toe. Nociception was tested by pinching the bone of a toe with a forceps and deemed absent when there was no indication the cat was aware of the stimulus.

In group 2, immediately after anesthesia, the cranial tibial, flexor and patellar reflexes were tested, and then again 15 mins after anesthetic block via epidural injection of lidocaine 2% (0.22 ml/kg). For the epidural injection, the cats were placed in ventral recumbency (‘sphinx’ position) and a 22 G (25 × 0.7 mm) needle inserted into the lumbosacral space (L7–S1). Location of the epidural space was confirmed by the presence of negative pressure that aspirated a drop of anesthetic placed on the hub of the needle and also by the absence of backflow of cerebrospinal fluid. Cats were kept in the sphinx position for 5 mins after injection. For the cranial tibial reflex, the belly of the cranial tibial muscle was struck with a reflex hammer immediately distal to the proximal epiphysis of the tibia, with the limb relaxed and supported with the knee at a 90º angle, and the hock in slight extension. The efficacy of the regional block was confirmed by absent patellar and withdrawal reflexes.

The reflexes were scored by the evaluator (MLF) as absent (0), decreased (+1), normal (+2) and increased (+3) according to the following criteria: absent (0), no extension of the carpus (extensor carpi radialis reflex) or no flexion of the hock (cranial tibial reflex); decreased (+1), partial extension of the carpus (extensor carpi radialis reflex) or partial flexion of the hock (cranial tibial reflex); normal (+2), full extension of the carpus (extensor carpi radialis reflex) or full flexion of the hock (cranial tibial reflex); increased (+3) full extension of the carpus (extensor carpi radialis reflex) or full flexion of the hock (cranial tibial reflex), but faster than normal.

Using the Wilcoxon non-parametric test with GraphPad InStat version 3.05 for Windows (GraphPad Software Inc), the reflexes were then analyzed regarding their presence before and after anesthetic block.

Results

Prior to anesthetic block, 55.17% (n = 16) of the cats had a decreased (+1) and 44.83% (n = 13) had a normal extensor carpi radialis reflex. After the anesthetic block, 68.96% (n = 20) of the cats had a decreased and 27.59% (n = 8) had a normal reflex. The extensor carpi radialis reflex was not increased (+3) before or after brachial plexus block, and it was absent only in one cat after anesthetic block. In the neurologic evaluation before anesthesia, all cats showed a more obvious extensor carpi radialis reflex, without it actually being considered increased (+3). After anesthesia, 44.83% remained normal (+2) and 55.17% became decreased (+1). There was no statistical difference regarding the presence of the extensor carpi radialis reflex before and after anesthetic block at 1% (P = 0.031).

Regarding quality of the cranial tibial reflexes before anesthetic block in group 2, 15.38% of the cats had a decreased cranial tibial reflex (+1) and 84.62% a normal one (+2), while after the block, 26.92% had a decreased (+1) and 73.08% a normal (+2) reflex. Reflexes maintained the same intensity before and after anesthetic block. There were no absent or increased responses. Regarding the reflexes being present before and after anesthetic block, there was no statistical difference at a 1% significance level (P = 0.031).

Discussion

In the pilot study, all six cats were very tense, which hindered evaluation of the reflexes before anesthesia. The evaluation was easier to perform after anesthesia, when relaxation allowed the reflexes to be elicited without interference from muscle tension; therefore, these were the conditions selected for the main research.

Animals must be relaxed and preferably in a lateral recumbency so that there is an adequate reflex response, because when restrained, they may become tense and/or excited, which interferes in the responses.^5,19

Within seconds of the lidocaine injection for the brachial plexus block in group 1, all cats lost muscle contraction near the elbow and of the extensor muscles in the carpus and digits, indicating that needle placement had been in the desired location, ²⁰ and thus confirming the efficacy of the anesthetic block. Absence of the withdrawal reflex was also considered in evaluating the efficacy of the block as it is the only reliable reflex in the thoracic limb. ⁷ Upon return from anesthesia, this reflex was still absent in all limbs where an anesthetic block had been performed.

The decreased responses before the anesthetic block in cats without neurologic dysfunction supports previous statements in the veterinary literature that the absence of the extensor carpi radialis muscle may be irrelevant, as this reflex may not be present in all healthy animals, meaning the withdrawal is the only reliable reflex in the thoracic limb. ⁷

This high frequency of decreased responses lead us to disagree with authors who cite the extensor carpi radialis reflex as the most predictable reflex in the thoracic limb.^1,2 However, it is worth mentioning that this statement was made regarding animals that were not given an α₂-adrenergic agonist, which may be responsible for this discrepancy. Xylazine, an α₂-adrenergic agonist, inhibits the release of noradrenaline into the synaptic cleft, thus modulating the activity of the sympathetic nervous system and resulting in characteristics such as myorelaxation.^21,22

However, the extensor carpi radialis reflex remained after anesthetic block in all cats, regardless of whether previously decreased or normal, except in one cat, in which the reflex became absent. In this case, we cannot rule out the possibility that the percussion of the muscle to elicit the reflex was not performed correctly, seeing as it is a difficult reflex to perform and interpret.^5,9

Therefore, it remained the same even after the anesthetic block of the radial nerve, which is the nerve responsible for this reflex. This leads us to believe that the extensor carpi radialis reflex does not depend solely on a reflex arc, and thus would not be a myotatic reflex. It may be solely or additionally a muscular response. It has been suggested that the mechanical movement of the carpus during percussion of the muscle may be confused with a myotatic response that generates extension of the carpus via muscle contraction. ⁹

Xylazine is a sedative agent which causes a dose-dependent depression by stimulation of the α₂-adrenergic receptors, both in the central and peripheral nervous system, producing sedation, hypnosis, muscle relaxation, ataxia and analgesia. ²¹ As xylazine was part of the anesthetic protocol, it is probable that the reflexes may have been decreased because of the muscle relaxation or depression of the LMNs related to that muscle brought on by the xylazine. Presence of the reflexes, though decreased, after the block suggests that there was, indeed, muscle relaxation due to anesthesia.

Contrary to what was reported in calves and lambs,^3,4 in which species the extensor carpi radialis was considered a reliable reflex to be tested, our research emphasizes that this reflex is not appropriate for the neurologic assessment of cats, as other authors have previously suggested.^5,7

As some of the cats had a decreased reflex before the anesthetic block without neurologic dysfunction, the absence of the cranial tibial reflex (not seen in the cats of the present study) would be irrelevant because it may not be present in healthy animals. ⁷ The only reliable reflexes in the pelvic limbs would thus be the flexor and the patellar. Some authors state that in some systemic diseases, the patellar may be the only absent reflex, and in general normoreflexic situations, it can be the only one decreased (eg, hypothyroidism).^7,8

Because they are considered the most reliable,^7,8 the withdrawal and patellar reflexes were used to confirm a successful epidural anesthetic block of the pelvic limb, by testing the sciatic and femoral nerves, respectively.

The cranial tibial, unlike the patellar and withdrawal reflexes, remained unchanged, even after the anesthetic block of the axons (at the level of the cauda equina) of the common fibular branch of the sciatic nerve. This confirms in cats what has been previously described in dogs, ¹⁵ that is, that the cranial tibial reflex does not depend solely on a reflex arc, and may not be a response mediated by myotatic reflexes, possibly being only a muscular response. It has been suggested that care should be taken not to mistake a mechanical response resulting from muscle percussion with the tibial cranial reflex, as they are hard to differentiate. ⁹

Based on our results, the cranial tibial reflex should not be trusted during neurologic examinations in cats. Also, its presence in situations where it should be absent or decreased, such as injury or functional block of its anatomical components, makes its interpretation confusing and interferes with lesion localization.

As it is not a reliable reflex, the suggestion made by some authors of examining only the cranial tibial reflex in situations where the patellar reflex is abnormal or when there is suspected injury to the sciatic nerve becomes uncertain or inappropriate.^1,9 As it is still present after anesthetic block, its use for neurologic evaluation in cats is not justified.

A decreased cranial tibial reflex in the cats before the block has been similarly observed in a less evident and less constant manner in calves and lambs.^3,4 In the present study, we determined this reflex was not adequate for neurologic evaluation of healthy cats, as supported by some authors,^2,5,7,11 and disagreeing with statements that it is the most constant and easily elicited reflex. ⁸

In human neurology, the stimulus for eliciting a reflex should be rapid muscle distension obtained by percussion of the tendon, thus generating a true myotatic reflex.^23,24 When the muscle is struck directly, the response is considered idiomuscular, which is reliant on intrinsic muscle characteristics and does not constitute a reflex.^25,26 In small animal neurology, the extensor carpi radialis and cranial tibial reflexes are elicited by direct percussion of the muscle belly.^1,2,9 Therefore, if we consider the definition of a true reflex as given in human neurology, the extensor carpi radialis and cranial tibial would not be, strictly speaking, a myotatic reflex, but rather an idiomuscular response, which is consistent with our results.

Regardless of how the reflex arc was interrupted, whether by neurotomy or anesthetic block as in the current study, ¹⁵ the extensor carpi radialis and cranial tibial reflexes remained, invalidating their use in the neurologic exam. Future research could include the use of an electromyograph at the motor root to evaluate the response to percussion, before and after interruption of the reflex arc, to determine whether the observed response is solely muscular or if there is a neurologic component.

We were unable to identify in the revised literature the first author to include the extensor carpi radialis and cranial tibial reflexes in the neurologic examination of cats and dogs. Earlier books mention only the flexor reflex in the thoracic limbs, and the patellar and flexor reflexes in the pelvic limbs.^27–29 Would they, along with literature from over 30 years ago, ³⁰ have been correct in only evaluating these three reflexes?

Conclusions

The extensor carpi radialis and cranial tibial reflexes are not strictly myotatic reflexes as they do not depend on the functional integrity of the reflex arc to occur. It is possible that these reflexes are solely idiomuscular responses. Therefore, they are not reliable reflexes to test during the neurologic examination of cats.