Assessment of the cutaneous trunci reflex in neurologically healthy cats

Abstract

Objectives

The aim of this study was to perform the cutaneous trunci reflex (CTR) in neurologically normal cats using two different instruments and determine how body condition score (BCS), body circumference, age, sex and instrument type may affect this reflex.

Methods

Sixty-five cats without evidence of neurologic disease were prospectively enrolled. Cats were randomly assigned to have the reflex tested first using a pair of hemostatic forceps or the integrated Babinski tip of an MDF Babinski Buck Reflex Hammer. After 30 mins, the reflex was retested using the other instrument. Data collected included the reflex presence, reflex caudal border, reflex intensity (weak, moderate, strong) and reflex symmetry (unilateral or bilateral). The influence of BCS, body circumference, age and sex on these variables was statistically evaluated along with effect of the instrument used.

Results

The CTR was elicited bilaterally in 52 (80%) cats and unilaterally in 64 (98%) cats. In two cats, the CTR was only able to be elicited using the Buck Reflex Hammer, while in four cats, the CTR was only able to be elicited using hemostatic forceps. Body circumference, BCS, age and sex had no effect on the presence, caudal border, intensity or symmetry of the CTR, regardless of the instrument used. No difference in the bilateral presence of the CTR was noted based on the instrument used first (P = 0.53). When assessing the influence of the instrument on reflex presence, caudal border, intensity and symmetry, the hemostatic forceps elicited the reflex further caudally (P = 0.02) and usually bilaterally (P = 0.02).

Conclusions and relevance

The CTR could be elicited in the majority of cats with both instruments. However, hemostatic forceps elicited a reflex more caudally and bilaterally symmetrical than the Buck Reflex Hammer.

Introduction

The cutaneous trunci reflex (CTR) is most commonly performed as a means of localizing thoracolumbar spinal cord lesions. This reflex has been well described in normal dogs and has been shown to provide useful information in both localizing spinal cord lesions and as a predictor of recovery following disc extrusions.^1–3 However, there are limited data on the diagnostic utility of this reflex in cats, either healthy or those with myelopathic disease. ⁴

A previous study reported that visibly healthy and neurologically normal cats may not demonstrate a menace response with the same reliability as dogs. ⁵ Additionally, both neurologically normal and abnormal cats have been reported to lack a CTR, for which the significance is unknown. ⁴ To our knowledge, no investigation has been reported on the assessment of the CTR in healthy cats. The aims of this prospective descriptive study were to assess the CTR in healthy cats using two different instruments and to assess for any effect instrument, body condition score (BCS), body circumference, age and sex may have on the CTR presence, the caudal border of the reflex, reflex intensity and reflex symmetry.

Materials and methods

Inclusion criteria

This prospective study was approved by the University of Illinois Institutional Animal Care and Use Committee and informed client consent was obtained prior to examination in all cats enrolled. Criteria for enrollment included no history of previous or concurrent illness, specifically metabolic disease; no history or clinical evidence of neurologic or orthopedic disease; age ⩾2 years; and normal physical and neurological examinations. Age, BCS (Nestlé Purina Body Condition Scale), body weight and breed were recorded for all cats. Additionally, the body circumference (measured at the level of the 13th rib) was recorded in every cat. All examinations were performed in a quiet room with no other animals present. Examinations were performed by a board-certified veterinary neurologist (KF or DH)

Assessment and scoring of the CTR

Prior to examination, all cats were randomly assigned by a coin toss to either have the CTR tested first using hemostats or the integrated Babinski tip of an MDF Babinski Buck Reflex Hammer (Buck Reflex Hammer) (Figure 1). The caudal border of the CTR was assessed by pinching the skin 1–2 cm lateral to the dorsal midline at the level of the iliac crests (approximately sixth lumbar vertebra) moving forward cranially on the ipsilateral side, approximately 0.5 cm at a time until the reflex was elicited or reaching approximately T1. The same procedure was performed on the opposite side of the body. The caudal border was defined by the corresponding dorsal spinous process at the level at which the response was elicited. If the border was in between two dorsal spinous processes, the border was then defined by the more cranial spinous process. When assessing the bilateral presence of the CTR, if asymmetry in the caudal border was noted between sides, the cutoff was recorded for each side and the caudal border was defined as the more cranial of the two. The intensity of the CTR was scored as grade 1 (weak = very minimal twitching of the epaxial muscles noted), grade 2 (moderate = movement of the muscles is clear, twitching of the muscles observed in proximity to the site of the stimulation) or grade 3 (strong = very obvious twitching with movement of the muscles over the entire dorsum) based on visual observation of the muscle contraction. Approximately 30 mins after the initial examination, the CTR was repeated using the other instrument.

Figure 1.

MDF Babinski Buck Reflex Hammer with integrated tip (arrow)

Data analysis and statistics

A sample size of 65 cats was calculated using the kappa statistic to test for agreement between two raters. This sample size was found to achieve 80% power to detect a true kappa value of 0.85 in a test of H0: kappa = 0.60 vs H1: Kappa ≠ 0.60 when there are two categories with frequencies equal to 0.50 and 0.50. This power calculation was based on a significance level of P <0.05.

Statistical analysis was performed using SAS Version 9.4 (SAS Institute). A P value of <0.05 was considered statistically significant. Continuous variables were tested for normality using skewness, kurtosis and Shapiro–Wilk’s tests. For normally distributed data, the data were described using the mean ± SD. For data that were non-normally distributed or ordinal, the median and interquartile range (IQR) were used. Categorical data were presented as frequencies and percentages.

The effect of BCS, body circumference, age and sex on the presence of the CTR bilaterally and reflex symmetry was assessed for each technique using univariable logistic regression. The effect of BCS, body circumference, age and sex on reflex intensity and caudal border of the reflex was assessed using Fisher’s exact tests; for this analysis continuous variables were categorized as either above or below the median. McNemar’s tests were used to assess if the instrument used affected bilateral presence of the reflex, reflex symmetry or caudal border of the reflex. A Wilcoxon sign-rank test was used to assess if the instrument used affected the intensity of reflex.

Results

Signalment

Sixty-seven cats were presented for examinations for specific enrollment in this study. Two cats were excluded from the study. One cat was found to have spinal hyperpathia on examination and the other cat’s temperament did not allow examination. Therefore, a total of 65 cats were enrolled. The age of the cats ranged from 2 to 14 years (median ± IQR, 5.7 ± 3.5 years) and the study population consisted of 30 spayed female and 35 castrated male cats. Breeds included domestic shorthair (n = 51), domestic mediumhair (n = 1), domestic longhair (n = 4), Maine Coon (n = 4) and one cat from each of following breeds: Siamese, Russian Blue, Bengal mix, Sphynx and Egyptian Mau. Body weight ranged from 3.0 to 6.7 kg (median 4.8 kg; IQR 1.6 kg). BCS ranged from 5 to 8 (median 6.0; IQR 2.0). Body circumference ranged from 30 to 59 cm (median 42.5 cm; IQR = 5.5 cm). Data obtained for all enrolled cats are provided in the supplementary material.

Presence of the CTR

The reflex was elicited bilaterally using both instruments in 52 cats, and was present unilaterally in 64 cats, regardless of the instrument used. In one cat, the CTR could not be elicited with either instrument. The CTR was only present using hemostats in four cats and using the Buck Reflex Hammer in two cats. Using the hemostats, the reflex was present on the left side in 57 cats and present on the right in 59 cats. When using the Buck Reflex Hammer, the CTR was present on the right in 57 cats and on the left in 57 cats.

BCS (P = 0.7), body circumference (P = 0.06), sex (P = 0.84) or age (P = 0.32) had no effect on the presence of the CTR using hemostats. No effect of BCS (P = 0.7), body circumference (P = 0.63), sex (P = 0.54) or age (P = 0.67) was found on the presence of the CTR using the Buck Reflex Hammer.

The caudal border was recorded using both instruments. When using the hemostats, the caudal border was identified in 60/65 cats. The CTR was present only unilaterally (left side) in 2/65 cats and bilaterally in 58/65. When using the Buck Reflex Hammer, the caudal border was identified in 60/65 cats with the following findings: 6/65 unilateral only (three left only; three right only) and 59/65 had the caudal border identified bilaterally. In one cat where the reflex was present, the level of the caudal border was not recorded for either instrument.

There was also no effect on the symmetry of the reflex based on BCS (P = 0.03), body circumference (P = 0.25), sex (P = 0.40) and age (P = 0.34) when performing the reflex with hemostats. BCS (P = 0.55), body circumference (P = 0.07), sex (P = 0.67) and age (P = 0.66) also had no effect on the symmetry of the CTR when performed with a Buck Reflex Hammer.

Intensity of the CTR

The intensity of the CTR was not significantly influenced by BCS (P = 0.18), body circumference (P = 0.22), sex (P = 0.68) or age (P = 0.34) when using the hemostat. Similar findings were noted when using the Buck Reflex Hammer, with BCS (P = 0.42), body circumference (P = 0.48), sex (P = 0.98) and age (P = 0.46) not having an effect on the intensity of the CTR. When comparing the technique used, the overall intensity of the CTR was found to be moderate when using the Buck Reflex Hammer (grade 2), while the overall intensity when using hemostats was strong (grade 3; Table 1). This difference was not found to be statistically significant (P = 0.07)

Table 1.

Results of reflex intensity by technique

Variable	Buck			Hemostat
	Right	Left	Maximum intensity	Right	Left	Maximum intensity
Median intensity (IQR)	2.0 (2.0)	2.0 (2.0)	2.0 (2.0)	3.0 (1.0)	3.0 (1.0)	3.0 (1.0)

IQR = interquartile range

Caudal border of the CTR

Neither body circumference (P = 0.66), BCS (P = 0.05), sex (P = 0.11) nor age (P = 0.40) had any effect on the caudal border of the reflex when using a hemostat. When using a Buck Reflex Hammer, BCS (P = 0.49), body circumference (P = 0.40), sex (P = 0.54) and age (P = 0.19) also had no effect on the caudal border of the CTR (Table 2).

Table 2.

Results of caudal most aspect of reflex by technique

Caudal border	Buck	Hemostat
T12	1	1
T13	2	0
L1	1	0
L2	1	4
L3	3	3
L4	9	7
L5	7	3
L6	23	32
L7	13	10
NA*	5*	5*

^*Caudal border not noted for one cat although the reflex was present

Effect of instrument on the CTR

The type of instrument used to perform the CTR did not have any significant effect on the bilateral presence of the reflex (P = 0.10) or reflex intensity (P = 0.07). The hemostats were used to assess the CTR first in 35/65 cats. When using hemostats first, 30/35 cats had the CTR present bilaterally, and 29/35 cats had the CTR present bilaterally using the Buck Reflex Hammer second. In 30/65 cats, the Buck Reflex Hammer was used first, followed by the hemostats. When using the Buck Reflex Hammer initially, the CTR was present bilaterally in 26/30 cats, while 28/30 cats had bilateral presence using the hemostat second. The instrument used first had no impact on the bilateral presence of the CTR (P = 0.53). However, the type of instrument used to elicit the CTR had a significant effect on the level of the caudal border (P = 0.02) and the symmetry (P = 0.02) of the reflex. The CTR was symmetrically present in 38 (58%) cats when using a hemostat vs 28 (43%) cats when the reflex was tested using the Buck Reflex Hammer. Using hemostats, the caudal border of the reflex was found to be more consistently located at the level of L6 or L7 in 42/60 and at L5 or cranial in 18 cats. The caudal border was located at L6 or L7 in 36/60 cats upon assessment with the Buck Reflex Hammer and at L5 or cranial in 24 cats.

Effect of instrument on CTR lateralization

When assessing the effect of the instrument used on the lateralization of the CTR, when using only hemostats, the reflex was more commonly present on the left vs the right side (two cats vs no cats), and in four cats the reflex was not present on either side. In these four cats, the CTR was able to at least be unilaterally elicited using the Buck Reflex Hammer. However, this finding was not statistically significant (P = 0.16). There were also no statistically significant differences in the side of the reflex with the Buck Reflex Hammer (P = 1.0). In regard to the Buck Reflex Hammer specifically, in three cats, the reflex was present only on the right, was present only on the left in three cats and was not present on either side in five cats.

Discussion

The cutaneous trunci muscle is a thin sheet of skeletal muscle beneath the skin extending over the dorsum and lateral walls of the abdomen. ¹ In the cat, its origin is the ventral cranial end of the latissimus dorsi muscle, the bicipital arch in the axillary region and the linea alba. The insertion is in the dorsal thoracic region, but the muscle extends as far caudally as the lumbosacral region and ventral sides of the tail. ⁶ The cutaneous trunci muscle is mainly composed of type II (fast, glycolytic) fibers and does not contain muscle spindles. ¹ The CTR occurs as a contraction of the cutaneous trunci muscle and is observed as a twitching of the skin. In order to evoke the CTR in mice and rats, a noxious stimulus is required, whereas in other species, such as cats, a light touch is sufficient enough. ⁷

The afferent innervation for this reflex is provided by the cutaneous branches of spinal nerves supplying the skin of the thorax and abdomen.^1,8 The cutaneous branches of the spinal nerves enter the dorsolateral fasciculus and synapse on long interneurons and ascend within the fasciculus proprius to the motor neurons within the cervicothoracic intumescence.^1,2,6 The efferent innervation is via the lateral thoracic nerve, which arises from the C8–T1 spinal cord segment within the cervicothoracic intumescence. ⁹ The major function of the reflex is thought to be a defensive mechanism to protect the skin from irritating stimuli such as insect bites or foreign objects.^1,6 It has also been thought to play a role in functions such as forced expiration, vomiting, coughing and defecation.^1,6

The CTR has been well characterized in the dog. The sensory field in regard to the caudal and lateral borders has been established in the normal dog and it has been consistently elicited in nearly 100% of neurologically normal patients.^1,2 The usefulness of the CTR in assisting in localization of thoracolumbar spinal cord lesions in dogs, as well as predicting recovery in dogs with acute thoracolumbar myelopathies secondary to intervertebral disk disease has also been demostrated.^2,3

Currently, there are very limited studies investigating the CTR in cats. A recent retrospective study on the assessment of the CTR in 182 cats with neurologic disease found the CTR was only able to be elicited in 64.8% of the patients. ⁴ A significant relationship between cats exhibiting spinal pain and the CTR outcome was found, in which the reflex was more frequently elicited in cats with spinal pain. ⁴ However, this study was retrospective in nature and the CTR was performed by a variety of clinicians, which may skew the results. Prospective studies in cats with neurologic disease in which the reflex is performed by the same individual are warranted to further determine the reliability of this reflex in cats.

The results of the current study show that BCS, body circumference, age and sex had no effect on the presence, caudal border, intensity or symmetry of the CTR, regardless of the instrument used. The effect of age on the CTR in the dog has been previously evaluated. ¹ Similar to the findings in the current study, age did not appear to affect the caudal border of CTR in the dog. Interestingly, studies in dogs have shown that age may have an impact on the strength/intensity of the CTR,^1,10 which was not shown in the cats in the current study. This age-related change in the contractile strength of the CTR in the dog was speculated to be due to a change in the sensory perception of the reflex or possible decrease in muscle strength. ¹ Additionally, studies on human skin have found a significant reduction in intradermal nerve fiber density with increasing age and age-related muscle atrophy may also affect reflex intensity.^11,12 Metabolic disorders may also lead to decreased skin innervation, such as diabetes in humans. The lack of a correlation of age and the CTR in this study is therefore quite interesting. Potential explanations of the difference may be that all cats enrolled were considered healthy, with no known history of metabolic disease and none of the cats in this study were considered underconditioned nor had any evidence of overt muscle atrophy on physical examination.

The findings of this study show hemostatic forceps appear to elicit a reflex that is more caudal and symmetrical when compared with the Buck Reflex Hammer. This may simply be user related, as the clinicians performing the assessments for this study routinely use hemostats when eliciting CTR in clinical patients. Therefore, they were likely more comfortable and skilled with using hemostats than the Buck Reflex Hammer. Additionally, the amount of skin and/or fur displacement may affect the CTR. It has been shown that in the dog, the maximum CTR response is elicited by grasping and compressing 2–3 mm of skin.^1,10 It is possible that the surface area of the integrated Babinski tip was too small to trigger any sort of response.

In this study, the CTR was able to be elicited in 80% of the cats. This finding differs from dogs, in which the CTR was able to elicited in 100% of healthy dogs. ¹ A recent study investigated the presence of the CTR in cats with neurologic disease and found the reflex was only able to be elicited in 65% of patients and 25% of those patients had myelopathic disease. ⁴ The differences noted might be due to the fact that the CTR is routinely performed as part of the neurologic examination, and those patients undergoing a neurologic assessment typically have some degree underlying neurologic disease. Additionally, the previous study was retrospective in nature and therefore multiple clinicians were not only performing the CTR, but also a variety of methods to elicit the response may have been used. Therefore, it seems reasonable that we would elicit the CTR more frequently in the healthy cats as the current study was prospective in nature with two board-certified neurologists performing the reflex using a precise methodology. One reason why we may not have been able to elicit the reflex in all healthy cats could be associated with stress. Examination-related stress and anxiety in the cat may lead to alterations in various parameters such as blood pressure, heart rate, respiratory rate, temperature and menace response.^13–15 In order to decrease stress as much as possible, all cats were examined in a quiet area away from other animals. Additionally, 30 mins lapsed in between testing the two different instruments in the same cat. Future investigations could evaluate the behavior of cats to determine if there is an association with increased fear and/or stress and the presence or absence of the reflex.

There are some limitations to this study that should be considered when interpreting the results. Part of the inclusion criteria was no history of any concurrent illness, specifically metabolic disease; however, complete bloodwork was not required. Without bloodwork, we cannot completely exclude the presence of underlying endocrine disease such as diabetes mellitus or hyperthyroidism, which could potentially impact the CTR. Additionally, more than one clinician was performing the CTR. However, the two individuals performing the observations were boarded veterinary neurologists that have extensive training and experience in performing this reflex. Another limitation is that this study did not determine inter-observer agreement in regard to the presence of the reflex and its characteristics as the neurologists did not perform the CTR on the same cats at different times. As discussed above, there was a difference found in the caudal border and symmetry of the reflex when comparing hemostats with the Buck Reflex Hammer, which may reflect the observers’ possible inexperience using this instrument. The side that was stimulated first was also not randomized nor was it noted if the reflex was only elicited on one side, or if the side stimulated was first or second. These data may have shown some significant changes in regard to the results – the CTR in cats may become more difficult to elicit as more stimuli are applied. Lastly, the grading scale used to determine the CTR strength was highly subjective. Objective means of measuring the contractile strength of the muscles often requires invasive electrodiagnostic testing, such as electromyography or somatosensory evoked potentials,^16,17 and would not be considered ethical practice for the purpose of this study.

Conclusions

This study demonstrated that the CTR can be elicited in the majority of healthy cats, regardless of the instrument used. Additionally, BCS, body circumference, age and sex did not have an effect on the presence of the CTR in healthy cats. However, hemostatic forceps appear to elicit a CTR that is further caudal and more symmetrical compared with a Buck Reflex Hammer. Further studies are warranted in both neurologically normal cats and cats with myelopathic disease to determine the usefulness of the CTR in the feline patient. Additionally, studies to determine inter-observer agreement on assessing this reflex and evaluating technique in regard to instrumentation may be of benefit.