Clinical assessment and grading of back pain in horses.

BACKGROUND: The clinical presentation of horses with back pain (BP) vary considerably with most horse's willingness to take part in athletic or riding purpose becoming impossible. However, there are some clinical features that are directly responsible for the loss or failure of performance.
OBJECTIVES: To investigate the clinical features of the thoracolumbar region associated with BP in horses and to use some of the clinical features to classify equine BP.
METHODS: Twenty-four horses comprised of 14 with BP and 10 apparently healthy horses were assessed for clinical abnormality that best differentiate BP from normal horses. The horses were then graded (0-5) using the degree of pain response, muscular hypertonicity, thoracolumbar joint stiffness and overall physical dysfunction of the horse.
RESULTS: The common clinical features that significantly differentiate horses with BP from non-BP were longissimus dorsi spasm at palpation (78.6%), paravertebral muscle stiffness (64.3%), resist lateral bending (64.3%), and poor hindlimb impulsion (85.7%). There were significantly (p < 0.05) higher scores for pain response to palpation, muscular hypertonicity, thoracolumbar joint stiffness and physical dysfunction among horses with BP in relation to non-BP. A significant relationship exists between all the graded abnormalities. Based on the cumulative score, horses with BP were categorized into mild, mild-moderate, moderate and severe cases.
CONCLUSIONS: BP in horse can be differentiated by severity of pain response to back palpation, back muscle hypertonicity, thoracolumbar joint stiffness, physical dysfunctions and their cumulative grading score is useful in the assessment and categorization of BP in horses.

INTRODUCTION

Back pain (BP) is a clinical condition that causes performance failure among athletic and riding horses [1]. BP in horses are of many aetiologies, although among the common causes are thoracolumbar musculoskeletal lesions, supraspinous desmitis, lameness, ill-fitting tack, and inadequate schooling [234]. Diagnosis of equine BP can be very challenging, because the condition is presented as a syndrome rather than with specific clinical signs. Thus, despite the availability of sophisticated clinical aids, definitive diagnosis of equine back injuries is most often made by eliminating other conditions.

Clinical manifestations in equine BP of primary and secondary causes may be similar. For example, lameness or neurological disorders involving the thoracolumbar and sacral region may be similarly presented with spinal muscular atrophy and abnormal gait [5678]. Among methods of diagnosis of equine BP in the field is by visual inspection, palpation, and manipulation of thoracolumbar spine [9] and examination at exercise. Currently, there is no suitable grading system that can be conveniently used by practicing veterinarians to grade and quantify BP/disorders in horses. A validated and practical grading system would allow veterinarians to categorize back disorders for better therapeutic interventions, assessment, and monitoring of the conditions. In this study, we hypothesised that the influence of clinical and spinal abnormalities on the manifestations of equine BP vary with type and severity of abnormalities. Thus, the aim of the study was to develop a grading system for equine BP based on thoracolumbar spinal abnormalities and clinical features.

MATERIALS AND METHODS

Animals

Twenty-four patrolling or endurance horses from ambulatory cases of the University Veterinary Hospital, Universiti Putra Malaysia (UPM) were used in the study. The horses comprised of 14 with BP and 10 apparent healthy horses as normal. The ages, sex, and breeds were recorded. The study was with horse owner consents and approved by Institutional Animal Care and Use Committee, UPM (UPM/IACUC/AUP-R016/2018).

Clinical examination

Complete clinical examination was conducted on all the horses at rest to determine the general conformation of the thoracolumbar region, including epaxial and pelvic muscle development, symmetry, and spinal curvature. The thoracolumbar region was then palpated with firm but gentle digital pressure along the dorsal thoracolumbar midline. Avoidance reaction (i.e., sinking) to palpation was used as indicator of pain. Digital palpation was repeated to ensure consistency of clinical findings. Response to thoracolumbar flexibility was assessed by spinal manipulation: lateral bending and ventral and dorsal flexion. Conformational variations, response to palpation and spinal manipulation among the 2 groups were noted. The horses were trotted on a hard surface and in small circles to determine gait. Oral examination of the horses was performed to rule out influence of dental pain on gait.

BP grading

The assessment of the horses for BP grading was then performed by an independent equine veterinarian who was blinded of the clinical category of the horses. Spinal abnormalities were graded on a scale of 0 to 5 based on the developed guide: degree of pain response to back palpation, back muscle hypertonicity, thoracolumbar joint stiffness, and physical dysfunction (Table 1). Lameness was graded using the American Association of Equine Practitioners lameness grading scale [10]. The score for each horse was calculated and compared between categories of horses. Using the cumulative grading scores of pain response to palpation, muscle hypertonicity, thoracolumbar joint stiffness, and physical dysfunction, BP horses with cumulative score of 0 was classified as normal, mild (1–4), mild-moderate (5–8), moderate (9–12), marked (13–16), and incapacitated (17–20).

Table 1

Grading of equine back pain based on the spinal structural and functional abnormalities

Parameter	Abnormality score (grade)
	0 (absence)	1 (mild)	2 (mild-moderate)	3 (moderate)	4 (severe)	5 (incapacitated)
Pain response	No clinical evidence	Occasional, mild reaction to grooming or firm pressure only	Mild-moderate reaction to firm pressure	Moderate reaction with avoidance response to moderate pressure	Marked reaction with strong avoidance response to mild pressure	Marked generalized reaction
Muscle hypertonicity	No clinical evidence	Mild hypertonicity without fasciculations to firm pressure	Variable hypertonicity with pressure	Moderate unilateral or bilateral hypertonicity with fasciculations to moderate pressure	Marked hypertonicity with spontaneous fasciculations to pressure	Marked, generalized, and persistent muscle spasm
Lameness	No lameness observed	Inconsistent lameness at the trot that is difficult to discern under any circumstance	Lameness is difficult to observe at a walk or trot in a straight line, but consistently apparent under special circumstances	Consistent lameness at the trot and under all circumstances	Obvious lameness with marked asymmetry of gait at the trot without manipulation	Minimal or no weight bearing on the affected limb and reluctant to move
Thoracolumbar joint stiffness	No clinical evidence	Mild segmental stiffness	Stiffness from restricted muscle function	Moderate dorsoventral and unilateral bending	Marked reduced dorsoventral and bilateral bending	No flexibility
Physical dysfunction	Full functional activity	Able to walk and trot with mild restriction	Noticeable dysfunction during certain activities	Performs activities with much effort	Able to stand comfortably but resents activities	Unable to stand comfortably or perform at all levels

Total BP severity was calculated as follows:

Data analysis

The statistical analyses were conducted using GraphPad Prism version 8.0.2 (GraphPad Software, USA). Descriptive statistic was used to summarize the data and the scores for each abnormality expressed as mean ± standard deviation. The association between BP and clinical findings was determined using the Fisher's exact test. Differences between mean scores were analysed using the nonparametric Mann-Whitney U test. The relationship between spinal abnormalities in BP was determined using the Spearman's correlation. The p value < 0.05 was considered significant.

RESULTS

Breeds and frequency of BP

Among the horses 13/24 (54.2%) were geldings and 11/24 (45.8%) mares. The mean age of horses with BP was 14.9 ± 3.2 years while normal horses in the study was 12.3 ± 3.3 years old. Among horses with BP, the predominant breed was Warmblood (7/14, 50%) followed equally by thoroughbreds and Polo ponies (3/14, 21.4% each), and Criollo (1/14, 7.1%).

Clinical observations

The clinical findings of horses with and without BP in the study are shown in Table 2. All horses with BP showed pain response to digital pressure on the back. Among other most common signs of BP were poor hindlimb impulsion, muscle spasm to palpation, resistance to lateral bending, and back muscle stiffness. Six horses with BP showed poor epaxial muscle development (Fig. 1). Other changes/abnormalities observed to be associated with BP the horses include mark of ill-fitted saddle (Fig. 2) and swelling on the back (Fig. 3).

Table 2

Clinical features and signs in horses with BP

Clinical features/signs	Horse		p value
	BP (n = 14)	Non-BP (n = 10)
Pain response to back digital pressure	14 (100.0)	0	< 0.000
Poor development of epaxial muscles	6 (42.9)	1 (10.0)	0.172
Moderate-to-good epaxial muscle development	8 (57.1)	9 (90.0)	0.172
Warm area at back region	4 (28.6)	0	0.114
Focalised back swelling	3 (21.4)	0	0.239
Paravertebral muscle stiffness	9 (64.3)	1 (10.0)	0.013
Longissimus dorsi muscle spasm at palpation	11 (78.6)	0	0.000
Resistance to lateral bending	9 (64.3)	0	0.000
Poor hindlimb impulsion	12 (85.7)	1 (10.0)	0.001
Forelimb lameness	1 (7.1)	0	> 0.999
Hindlimb lameness	3 (21.4)	1 (10.0)	0.615
Bad attitude/aggressive behaviour	2 (14.2)	1 (10.0)	> 0.999
Ill-fitted saddle mark	3 (21.4)	3 (30.0)	0.665

Values are presented as number (%).

BP, back pain.

Fig. 1

Poor development of paravertebral muscles in horse with back pain. Note the prominence at the summits of the thoracolumbar spinous processes (arrow).

Fig. 2

Mark of ill-fitted and narrow saddle on a horse with back pain.

Fig. 3

Focal swelling (red ring) in a horse with back pain.

Physical and function abnormalities in grading horses with BP

The severity and frequency of spinal abnormalities in horses with BP are presented in Table 3. Horses with BP showed significantly (p < 0.05) higher mean overall abnormality score than those of normal horses. All horse with BP showed significant pain response to palpation, muscular hypertonicity, thoracolumbar joint stiffness, and physical dysfunction in comparison with normal horses. Lameness was not a significant finding in horses with BP. However, there was a positive strong correlation between pain response to palpation and the muscular hypertonicity (r = 0.9027, p < 0.05), thoracolumbar joint stiffness (r = 0.9098, p < 0.05), lameness (r = 0.7763, p < 0.05), and physical dysfunction (r = 0.9361, p < 0.05) in horses with BP. In general, horses with BP in this study showed mild to moderate physical and function abnormalities. A few horses showed marked pain response to palpation. Based on the cumulative scores of abnormalities horses with BP were categorized into mild, mild-moderate, moderate and severe (Table 4).

Table 3

Number of horses and frequency of physical and function abnormalities in horses with BP

Abnormalities	BP grade						Overall mean severity
	Horses with abnormality
	0 (normal)	1 (mild)	2 (mild-moderate)	3 (moderate)	4 (marked)	5 (incapacitated)	Back pain (n = 14)	Normal (n = 10)
Pain response to palpation	0 (0)	4 (4)	4 (2)	15 (5)	12 (3)	0 (0)	2.50 ± 1.16*	0
Muscular hypertonicity	0 (0)	7 (7)	8 (4)	9 (3)	0 (0)	0 (0)	1.79 ± 0.81*	0.20 ± 0.42
Thoracolumbar joint stiffness	0 (1)	5 (5)	6 (3)	15 (5)	0 (0)	0 (0)	1.79 ± 1.05*	0
Lameness	0 (0)	0 (0)	4 (2)	6 (2)	0 (0)	0 (0)	0.71 ± 1.20	0.20 ± 0.63
Physical dysfunction	0 (4)	6 (6)	4 (2)	3 (1)	4 (1)	0 (0)	1.21 ± 1.19*	0.20 ± 0.42
Total							8.00 ± 3.57	0.60 ± 1.27

Horses with abnormality are presented as total severity (number of horses). Total severity = Number of horses × BP grade. Overall mean severity are presented mean ± SD. Too few normal animals showed abnormality, thus, score breakdown is not included.

BP, back pain.

*Means for horses with back pain significantly different from normal horses at p < 0.05.

Table 4

Category of back pain in horses based on cumulative score for abnormalities

Categories	No. (%)	Cumulative score (mean ± SD)
Mild	4 (28.57)	3.25 ± 0.96
Mild-moderate	6 (42.86)	6.83 ± 1.17
Moderate	3 (21.42)	11.33 ± 0.58
Severe	1 (7.14)	14.00 ± 0.00*

*Value for one horse only.

DISCUSSION

In horses with BP, assessment of abnormal conditions is only based on observations and clinical findings; thus, it is therefore imperative that clinical abnormalities are clearly identified for the practitioner to accurately assess the clinical conditions to ensure precise management and treatments. Digital palpation and back manipulation are commonly used to determine BP. Though this method is subjective, it is still the most commonly used method in clinical practice to determine BP in horses [911]. Other method, like pressure algometry, has also been used in the assessment of BP [121314]. This method is more objective; however, there are confounding factors, such as individual differences in pain sensation and response that could compromise the accuracy and specificity of BP assessment using the method. Furthermore, pain perception can be influenced by time of day, rate and duration of machine pressure applied, and avoidance responses by the subjects from previous painful experiences [121516]. In horses, the assessment of pain using pressure algometry is also dependent on the ability of the operator to recognize pain response.

In the current study, we classified the severity of BP in horse based on clinical abnormalities: response to palpation, muscular hypertonicity, thoracolumbar joint stiffness, lameness, and physical dysfunctions. Though, there is overlap in the clinical abnormalities observed in both horses with BP and those without BP, this is not surprising considering the that other ailments may show clinical signs similar to BP [171819]. However, the clinical abnormalities fact observed in BP horses are often associated with thoracolumbar dysfunction. Furthermore, the percentage of occurrence of each clinical feature/abnormality was significantly higher in the horse with BP except for moderate-good epaxial muscle development. These clinical abnormalities include paravertebral muscle stiffness, longissimus dorsi spasm at palpation, poor epaxial muscle development, resistance to lateral bending, and poor hindlimb impulsion [91120].

Using the palpation method, horses with BP showed various degrees of pain response from mild to marked, with mild-moderate response being the most frequent. The variation in pain response to palpation is presumably dependent on the severity of the back disorder. Predisposing factors to equine BP are mostly associated with horse activity, age, use, management, diseases, and disorders. The age and the long duration of physical activities over the active lifespan of the horse could result in degenerative changes of the skeletal structure leading to spinal injury-associated BP [2122]. Present study showed otherwise with both BP horses and normal horses share similar age, sex, and breed. Therefore, these factors do not predispose horses to BP [123].

The movement and stability of the thoracolumbar spine involve spinal epaxial musculature, ligaments, and intervertebral joints [24]. Thus, the muscle hypertonicity and spasm associated BP in this study could be attributed to muscle fatigue, constant weight shifting because of pain and stress, and uncoordinated muscle contraction which may be due to high work demand. The BP horses showed variable muscle hypertonicity ranging from mild to moderate. Similarly, their pain responses to palpation were variable with a few showing marked responses. The reduced intervertebral joint motion and lateral bending is however considered a sequel to prolonged muscle spasm and stiffness of spinal musculature [2526]. Although, poor musculature can also be due to disuse or neurogenic atrophy, however, the poor development of epaxial muscle observed in this study is a reflection of disuse atrophy of spinal musculature due to reduced muscle activity seen in thoracolumbar pain [232728].

Furthermore, the abnormal gait or reduced hindlimb impulsion seen when the horse is ridden may not necessary be due to lameness but could because of weight shifting behaviour display by the horse to accommodate for the BP [17]. This is true for BP horses in this study as majority displayed reduced hindlimb impulsion. Nevertheless, forelimb or hindlimb lameness may occur concurrently with BP [293031]. Since the head is in continuum with the axial skeleton, abnormal gait will affect head motion pattern leading to asymmetrical thoracolumbar kinematics. In such cases, thoracolumbar asymmetry can be corrected by alleviating lameness-induced BP, allowing for increased axial rotation and flexion-extension range of motion, and lateral bending of the spine [32]. Very few horses showed signs of lameness in this study, which led to the conclusion that lameness may not be a significant cause of equine BP.

Among causes of equine BP are conformational abnormalities and degree of back mobility [33]. Conformation abnormalities, for example in the thoracolumbar region, predispose the horse to weakness, soft tissue injury, leading to poor performance may not necessarily be associated with BP, if motion is not affected [34]. Incidentally, the horses in this study did not show any conformation abnormality.

Using the clinical abnormalities, it is possible to grade thoracolumbar pain based a scale of 1 to 5. Horses with BP had significantly higher scores than those without for all parameters except lameness. Horses with BP responded painfully to back palpation that ranged from mild to marked response. The variability in pain response is presumably dependent on the severity of the back disorder. Thoracolumbar joint stiffness and muscular hypertonicity, to a lesser extent, were also significant in horses with BP. Furthermore, there was a strong significant correlation between the degree of pain response, muscular hypertonicity, thoracolumbar joint stiffness, lameness, and physical dysfunction. According to this method of assessment, it is proposed that pain response, muscular hypertonicity, and thoracolumbar joint stiffness can reliably be used to grade the severity of equine BP.

In this study, based on BP-associated parameters, the horses showed different degrees of severity in abnormalities. The reason for the variation in abnormalities in equine BP may be associated with duration of the BP and individual horse tolerances. Some experienced athletic horses with BP may mask their pain by compensation and still perform adequately, while those sensitive to BP resist work [2535]. It is important to note that because of differences in temperament, the relationship between clinical features or abnormalities and the true perceive pain behaviours may not be as straightforward in horses as in other animals and humans [3637]. Even in normal horses, response to digital palpation or pressure along the back region can vary, since some horses, known as “cold-backs,” are very sensitive to touch and react abnormally.

Using the cumulative scores of abnormalities, this study showed that horses with BP could be classified according to severity. In order of frequency, the horses suffered from mild-moderate, mild, moderate, and severe BP (Table 4). Since muscular hypertonicity, thoracolumbar joint stiffness, physical dysfunction impact performance, the grading system could be used as a measure of response to treatment and recovery to full function.

In conclusion, the study shows that among clinical abnormalities, pain response to back palpation, back muscle hypertonicity, thoracolumbar joint stiffness, and physical dysfunctions are useful in determining BP in horses. Noticeably, BP in horse can be differentiated by severity of presentation of these clinical abnormalities. Despite the small sample size of the horses in this study, the data showed that cumulative grading score of abnormalities is useful in the assessment and categorization of BP in horses.