THE UN-BALANCED HORSE

Updated: Apr 27, 2021


Practical considerations for ECVM affected horses

 

In nature, horses use their neck as a natural balancing entity. Ever seen a horse turning flat like a motorcycle but still keeping balance? They can do so by placing their weight into the inside front limb while cantilevering with the head and neck to the outside. Furthermore, nerves passing through the lower neck ensure good proprioception so the horse can keep good control over its limbs while doing so. But what if the neck is compromised?

Unfortunately, the cervical region is sometimes overlooked as a source of pain and dysfunction in general equine veterinary practice. However, from 2017-2020 I have had the pleasure to work with 152 horses worldwide of which roughly 40% presented with lower neck problems to a certain extent. I am fully aware that these numbers are not enough to create a data sample. I also acknowledge the fact that more cases with lower neck problems presented to me once word got out I had experience with these issues and this might have affected the representativeness of the numbers.


Nevertheless, the high prevalence should not be ignored as numbers continue to rise. I owe it to these horses to share their stories in an effort to spread awareness and potentially help others. In this article I want to elaborate on one of the common radiological findings in the lower neck that I have encountered so far: Equine Vertebral Complex Malformation. (I will discuss osteoarthritis in a separate article).

I will first explain its morphology and possible variations. Then, I will continue to discuss its clinical relevance and potential ramifications. I will conclude the article with various practical management and training considerations for affected horses.


INTRODUCTION

Equine Complex Vertebral Malformation, or ECVM, is a congenital malformation found in the 6th cervical vertebrae (C6) and potentially extends to the 7th cervical vertebra (C7) as well as the first and second sternal rib(s) (T1 – T2) in horses. ECVM has a hereditary aspect and although genetic research is being conducted in the USA, the gene(s) responsible are unfortunately yet unknown (Equus-Soma 2021).

ECVM was brought to light by renowned anatomist Sharon May-Davis. She discovered her first introduction to the congenital malformation 20 years ago through the bones of a Thoroughbred named Presley. In her original paper (2013) she explained this condition as follows:

‘‘In the 6th cervical vertebra (C6), either a unilateral or bilateral absence of the caudal ventral tubercle (CVT) was noted in the presence of C6 malformation, the 7th cervical vertebra (C7) presented either as normal, or, with a unilateral or bilateral transposition of the CVT from C6 onto the ventral surface of C7 with an arterial foramen. This transposition onto C7 was noted to be present on the corresponding side as the absent caudal ventral tubercle on C6.’’


Put simply, ECVM presents in the absence of either one (asymmetric malformation) or both (symmetric malformation) caudal ventral tubercles of C6. C7 can either be normal, or it can show transposition of the missing CVT(s) of C6 unto C7. The malformation can potentially also extend to the first and second sternal rib(s) that in turn affects the curvature of the sternum and its associated structures.


Left: normal C6 & C7. Right: unilateral malformation C6 with transposition to C7



While it was first discovered in Thoroughbreds, it has now also been identified in Andalusians, Arabs, Crossbreds, Friesians, Irish Sport Horses, Quarter Horses, Riding Ponies, Standardbreds, and Warmbloods.

In addition, the occurrence of ECVM is not geographically isolated, with studies identifying the congenital malformation in Europe, USA, UK, Australia, and Asia.

HISTORY

Interestingly, the occurrence of ECVM is not completely new. In the late 1900’s a malformation of the cervico-thoracic junction was reported in Holstein Dairy cows (Agerholm et. al 2001). The condition was termed CVM (Cervical Vertebral Malformation) and became a global affliction due to artificial insemination (May-Davis 2020).

The congenital malformations found in horses were not too dissimilar from CVM observed in cows, hence the name ECVM came about. Caution is warranted to not confuse ECVM with CVM in horses, the latter referring to Wobbler's syndrome, which is a completely different condition.


MORPHOLOGIC VARIATIONS & RAMIFICATIONS Potential variations of the malformation include:

Malformation of C6 only Malformation of C6 and C7 Malformation of C6, C7 and T1 Malformation of C6, C7, T1 & T2

Illustration: Malformation of T1. Adapted from May-Davis 2016.


From this listing, it becomes clear that the sixth cervical vertebra (C6) is the only constant factor. ECVM thus begins with alterations to the ventral caudal tubercles at C6 and then potentially progresses through the successive vertebrae and ribs.


The latest numbers show that ECVM presents in a ratio of 4:2:1 (!) meaning 4 out of 10 horses will present malformation of C6. Two of those will show transposition to C7 and one of those will also show malformation of the ribs. (May-Davis 2020)


Additional consequences observed with ECVM include altered sternum curvature, facet joint and vertebral canal symmetry, variations in musculature, altered nerve pathways, scoliosis, and trachea malformation. Let’s have a closer look to some of these additional ramifications.


ECVM influences musculature through alteration of attachment sites. Nature is a genius architect that designs anatomy in such a way that it allows for optimal bodily function of any creature. Within the cervical spine, C6 is the only vertebrae designed with two caudal ventral laminae. So why is that? The answer is that it provides anchor points for stabilizing paravertebral muscles such as M. Longus Colli.



Known as a V-shaped cybernetic muscle, the Longus Colli provides dynamic intersegmental stability as well as postural control. Rombach (2014) highlighted the importance of paravertebral stabilizers stating:

“Effective stabilization by the deep paravertebral muscles may reduce the risk of osteoarthritis and development of neck pain in horses, which is a limiting factor for performance of such animals.”


Ex-vivo studies showed that horses affected with ECVM display altered muscle attachment of M. Longus Colli on C6:


‘‘This study showed that the function of the L. Colli muscles had been severely compromised in the presence of the congenital malformation in C6 and C7 and furthermore, that mechanical forces placed an asymmetric load at the points of attachment. Thus, with impeded function the L. Colli muscle has faltered in its role as an intersegmental stabilizer, subsequently lead to vertebral instability, degenerative joint changes, and asymmetrical articular processes.’’ - May-Davis 2014

Furthermore, since the Longus Coli is cybernetic in nature any dysfunction also results in neurological ramifications as it ‘‘would lead to the brain receiving incorrect neural messages due to abnormal paired left and right tension in the muscle and as a direct consequence, the horse would adjust its posture accordingly.’’ – May-Davis 2014


Apart from altered muscle attachment, ECVM also has a potential direct effect on neurology due to changes to the intervertebral foramen - the canals through which spinal nerves pass. In some horses, the foramen present asymmetrical whereas in others they might be severely narrowed. Unpublished field research also shows C6-C7 vertebral canal symmetry in ECVM horses is often compromised. These changes may result in nerve compression.


Additionally, ex-vivo specimens as well as radiographic evidence, also suggests a potential ink between ECVM and scoliosis in the vertebral column.


Ilustration: a horse with unilateral malformation of C6, C7 and the first rib. The horse also presented a wavey sternum and scoliosis in the neck and lumbars.



Finally, ex-vivo studies also revealed that ECVM horses often show malformation of the trachea, however further research is needed to investigate potential causality and clinical relevance of this feature.


Malformation of the trachea in an ECVM affected horse.



METHODS OF ASSESSMENT A standardized method of ‘’assessment and diagnosis in live horses would allow for scientifically robust, quantitative studies into the relevance of this syndrome’’ - Gee et. al 2020


The cervical region is sometimes overlooked as a source of pain and dysfunction in general equine veterinary practice and there is a high likelihood ECVM is frequently under diagnosed as ‘’standard veterinary neurological and clinical examinations may well miss what is suspected to be an intermittent and dynamic instability in some cases.’’ – Gee et. al 2020


For those reasons, radiographic examination is the only viable method of assessment to determine ECVM. Correct orientation and positioning of the machine are critical to reliably diagnose ECVM radiographically. Gee. et al (2020) have developed a protocol that includes a left and right 30◦ dorsal to ventral views in conjunction with laterolateral views of C6 and C7: ‘’A lateral 30 degrees dorsal to ventral oblique highlights the lamina of C6 against the radiolucent trachea (…) The left and right 30 ◦ dorsal to ventral views, in conjunction with the laterolateral views of C6 and C7, are necessary for a diagnosis or exclusion of ECCMV. The potential issues arising from these osseous malformations and their subsequent altered muscular attachments can be appreciated on the vertebrae in cranial and ventral views. ‘’


Above: normal C6 C7. Bottom left: unilateral malformation C6. Bottom right: transposition C7.


Good quality radiographs taken from the right angles allow for detection of ECVM in live horses and provides a platform for further studies into its clinical relevance. Including these radiographs into purchase exams will bring about more transparancy to breeders as well as prospect buyers into the prevalence and relevance of the condition.


CLINICAL RELEVANCE Ever since Sharon May-Davis documented and quantified the condition in 2013, the clinical relevance of ECVM is widely debated. A 2016 paper by De Rouen et al. concluded that:

“The alteration of the attachment site for regional musculature due to anomalous C6 may lead to altered biomechanical forces resulting in perceived pain or reduced range of motion” (…)

‘’Anomalous C6 vertebrae in our population were associated with a higher likelihood of cervical pain (P = 0.013). Authors propose that morphologic variations in the C6 ventral laminae could be linked to other developmental abnormalities such as vertebral canal stenosis, might affect regional biomechanics, and should therefore be considered clinically relevant in horses. Future, controlled prospective studies are needed to test this theory.’’


In 2019, Veraa et. al came to a different conclusion stating that: ‘‘Homologous morphologic variation is common in the caudal cervical spine of Warmbloods (…) Radiographic presence of such variation does not necessarily implicate the presence of clinical signs.’’

However, the control group consisted of young horses presented for pre-purchase examination and are thus less likely to be symptomatic. Furthermore, the criteria of the radiographic findings differed. The subjectivity of the neurological examination and detection of subtle clinical signs and behavioral abnormalities challenges interpretation of these results.

In contrast to these results, a more recent study conducted by Becatti et. al (2020) did find a significant link between abnormalities of the ventral lamina of C6 and clinical signs of cervical disease such as pain and ataxia:


‘’In agreement with De Rouen et al, horses with abnormalities of the ventral lamina of the sixth cervical vertebra (AVL-C6) had 4.73 greater odds of showing neck pain, characterized here as clinical signs of inability to eat from the ground and/or straddling the forelimb excessively and/or neck fixing and/or forelimb lameness (…) Horses with AVL-C6 had 8.2 times higher odds of showing clinical signs of spinal ataxia (…) In addition, it has also been demonstrated that horses with AVL-C6 had an intravertebral sagittal ratio less than 0.5 at the C6 site, suggesting that AVL-C6 may be associated with other simultaneous regional developmental abnormalities.'' There is thus increasing evidence suggesting that ECVM has potential clinical and functional ramifications that might affect the horse's overall well-being and locomotive function.


The main limiting factor of all studies conducted so far is the time frame of observation. The complexity of ECVM requires observations for longer periods of time, but unfortunately, to date, there are no longitudinal studies assessing the progression of ECVM throughout the horse’s life.


However, there is plenty of empirical evidence from horse owners and professionals working in the field that have been able to observe the life of ECVM affected horses over an extensive time. Common clinical symptoms that have been reported include:

1. Asymmetry

ECVM causes structural asymmetries (osseous and soft tissue) that can’t be overcome. Structural asymmetry has high clinical relevance as ‘’logic dictates that asymmetric form comes with asymmetric function.’’ – May-Davis

Evidence also shows a high prevalence of scoliosis in affected horses, making the condition even more complex to quantify.

Furthermore, empirical evidence also reports a high prevalence of additional asymmetries found in the teeth (crooked jaw) and feet of affected horses.

Horses with unilateral ECVM tend to stand with uneven front limb posture, predominantly one foot forward. The ipsilateral frog may atrophy, and affected horses may have high-heel low-heel syndrome due to uneven loading of the feet.


Horses with bilateral ECVM tend to stand base wide in front with feet point to the outside. These horses may toe land and behave ataxic over uneven ground or going down the hill. In general, ECVM affected horses tend to jump with uneven forelimb position.

Clinical manifestations of asymmetry might present through resistance behavior such as contact problems, difficulty turning and bending to one side, difficulty accepting the bit, bucking, rearing etc. Physical symptoms include uneven muscle development, head/neck tilt, compensation patterns, reduced load capacity and lameness’s. These asymmetries can affect saddle fitting.

Since straightness is an important aspect when it comes to athletic performance, ECVM is a potential risk factor for the performance horse.


2. Gait issues

ECVM horses are often reported to display gait abnormalities that include unexplained (intermittent) front limb lameness’s and neurological abnormalities. These abnormalities might either be present from a young age or develop later through life.

In 2018, Dyson et al. found ‘’increasing evidence that nerve root injury may cause forelimb lameness.” In this study, 3 of the 25 horses in the study had overt ECVM. The altered nerve pathways associated with ECVM might thus play a role into the development of (intermittent) lameness in affected horses.


Horses with unilateral ECVM may walk with a stilted action in the forelimb. When the ribs are also affected, there may be abduction of the front limb as well.


Furthermore, horses with ECVM often show less stability, poor coordination, and proprioception. Affected horses may stumble often and some cases might display stringhalt or an ataxic gait.

Canter problems are often reported. Horses may show a four-beat or non-suspension canter and struggle with flying changes and/or collection.


In general, it is often reported that affected horses fail to perform to expectation. Age might play a factor as it seems most symptoms worsen after +9 years of age.


3. Behavioral abnormalities

ECVM horses are often described as predictably unpredictable. Behavioral abnormalities can be related to pain, general discomfort and/or nerve compression.

The range of behavioral signs can vary within the individual. It might be random or even dangerous behavior. Some might be erratic; some might be stoic. Some might be spooky; Some might be clumsy; some might be generally nervous. The behavior is not necessarily present all the time. The horse can be fine for 95% of the time, with only sudden behavioral abnormalities that appear seemingly for no reason. Therefore, longitudinal observation is so important.


ECVM affected horses can tend to bite or scratch themselves on the shoulder. Furthermore, they can be girthy and/or may be problematic with rugging. Extreme hormonal behavior of mares has also been reported.


An anatomical explanation of stress-related behavior can lie in ex-vivo findings of the adrenals in ECVM affected horses. The adrenals are responsible for hormone production, specifically cortisol. The dissection of numerous ECVM affected horses revealed enlarged adrenals that often display adenomas. This suggests the horse has had a high level of (internal) stress.


4. Presence of co-existing pathologies

It seems that the complexity of many ECVM cases that are reported to be difficult or even impossible to manage lies in the presentation of multiple co-existing pathologies.


Unfortunately, studies into the causal relationship between ECVM and additional pathologies are limited. Rombach (2014) suggested that horses with altered lower cervical biomechanics are more prone to degenerative changes and pain. However, studies trying to identify the existence of a causal relationship between C6 abnormalities and osteoarthritis concluded that: ‘‘there was no association between the morphology of C6 and articular process osteoarthritis.’’ – DeRouen et. al 2016 (…)

‘’This study supports the findings obtained by DeRouen et al. (…) Considering that there were no differences in the severity of radiographic abnormalities related to osteoarthritis of the articular process joints in horses with and without AVL-C6’’ – Beccati et. al 2020

So even though both conditions have been found together upon dissections, a direct causal relationship cannot be assumed.


However, empirical evidence of 10 ex-vivo specimens and 40+ case study reports of horse owners reveal a high prevalence between ECVM affected horses and various co-existing pathologies, including scoliosis. When asking horse owners about the background of their ECVM horse they often begin the answer with ‘’it’s a long and complicated story….’’ After which a long list of performance problems and other pathologies follows.


The question is thus: can ECVM be linked to the development of any additional pathologies? Do they arise out of long-term compensation for structural asymmetry and/or altered nerve pathways? Or are they the result of exceeding the load capacity of these horses by asking maximal athletic performance?

Radiological detection of ECVM often occurs only at a later stage in life when performance problems are already noted. But are these problems because of ECVM or because of non-related pathologies?


To answer this question, early detection of ECVM is crucial so that young, unridden horses can be monitored over time to see how ECVM manifests throughout life and if, through correct management and training, additional pathologies can be prevented or not.


PERSONAL EXPERIENCE Now that I have extensively elaborated on ECVM as a complex phenomenon, it is time to share some personal data and experience that I have gained through doing whole horse dissections, being a horse owner and as a professional rehabilitation specialist world-wide.


I have been working with ECVM in the field since 2017 and have had hands-on experience with 46 confirmed cases – of which 3 are horses owned by myself and the other 43 owned by clients in Australia, Taiwan, New Zealand, The Netherlands, and United Kingdom.

There was a high variety in age, sexes, breed, and discipline. All horses found themselves between 4-19 years old, with the majority being between 7-12 years old. Most owners reported a worsening of the clinical symptoms after the age of 10.

On the aspect of gender, roughly 70% were mares and 30% geldings. No stallions have been presented to me yet. There was a high variety of breeds including: Andalusians, Arabs, Crossbreds, French Trotters, Standardbreds, Thoroughbreds and Warmbloods, the latter one making up for most of my data with roughly 55%.


In terms of discipline, roughly 5% were horses competed in medium to advanced eventing. 20% were horses competed in medium to international dressage levels. 10% were horses competed in low level to medium jumping. 65% were horses that were either being reschooled, for example an ex-racehorse, horses trained in classical dressage or for the desire to keep them healthy and balanced while varying multiple elements such as trail rides, liberty training, working equitation, carriage driving etc.


Roughly 60% showed a malformation of just C6 – with a high predominance of unilateral malformation - and 40% showed a malformation of C6 and C7 transposition. 22 cases were also radiographed on the thoracolumbar spine and/or the limbs. In roughly 85% of these cases additional pathologies including scoliosis were found.


All horses showed clinical symptoms. Some horse owners reported the symptoms started at a later age when higher performance was asked, but some also reported the horse had never been totally ‘’right’’. Interestingly most owners reported that symptoms worsened when the horse was asked to be ridden.


The most observed symptoms included poor proprioception, asymmetry and poor posture, behavioral oddities, spookiness, being girthy, hoof problems and difficulty with contact, bending and collection.


Out of the 46 cases I have lost 7 of which 2 ended up on the dissection table. These were all complex, multifaceted cases that unfortunately had to be freed from their body to find peace.


In my years of experience, I have never come across an ECVM affected horse without (mild) clinical symptoms. However, since there is a high subjectivity between evaluating clinical symptoms, it might well be that other professionals do not list these as relevant or consider them as within ‘’normal range’’. Due to the lack of consensus as to when a symptom is ‘’clinically relevant’’ the discussion will remain.


However, there is consensus that straightness is an important aspect for the trained horse in any discipline. Straightness enhances balance which in turn promotes performance and decreases the risk of injury. Crookedness on the other hand promotes imbalance and increases the risk of strain and injury due to uneven loading. Therefore, it is my opinion that ECVM should always be considered as clinically relevant. I owe this statement to the 46 amazing horses and their owners who allowed me in their lives. Does this mean ECVM is always career ending or a death-sentence? No, I think that is too bold of a statement as each horse is an individual and multiple factors must be considered. ECVM affects individuals differently. The environment the horse is placed in, the work it does, it's character, its overall health, it's fitness and athletic ability all affect

the severity of symptoms and when they occur.


The fact remains the condition exists and that it affects many horses and their owners worldwide. Due to the high prevalence of ECVM there is an increasing demand as to how to manage these horses – if possible – to the best extent.


Led by the horses who crossed my path as well as dissection findings, I have experimented with various management and training solutions that I will discuss in the next section.

MANAGEMENT & TRAINING

When it comes to management training of ECVM affected horses it is important to realize that the condition is structural in nature meaning it can’t be overcome, but only managed. The condition will remain, but conscious management could contribute to prevent or minimize any potential discomfort experienced by the horse.

From my experience, there are four key-factors when it comes to establishing the potential possibilities towards successful management and training of ECVM affected horses. These include: 1. Early on detection

2. Degree of malformation

3. Clinical symptoms 4. Performance goals


First, early detection is key so that the trainer / owner of the horse knows what is going on inside and can adapt management timely.


Second, the degree of malformation influences the complexity. So far, it seems that horses with C6 malformation only are somewhat easier to manage than those that show transposition to C7 and malformed ribs as well.


Third, the presentation of clinical symptoms. If symptoms are mild, chances for successful management and performance are higher compared to those horses that show severe clinical symptoms – for example dangerous explosive behavior. In the latter case, these horses might not be safe to train.

Fourth, the performance goals should be considered. As mentioned earlier, jumping is usually a risk-factor due to asymmetry and poor coordination.


In general, it can be said that the higher the athletic demands placed on the horse, the greater the risk of compensation and associated behavior. Can ECVM affected horses perform at top sport levels? Perhaps, as it is commonly known that horses can go a long way by compensating. However, this always comes at a high cost eventually – usually around the age of +9.


Training should be about functionality, not about spectacle. There should be something in it for the horse, something to gain, instead of having to suffer or compensate.