Sep 27, 2014
Muscles need anchors to function… But what happens if you are born with malformed cervical vertebrae that lack a crucial anchoring point?
This is the case for a large number of thoroughbred horses – and the subject of a 15 year study by equine researcher, Sharon May-Davis. I initially wrote about Sharon’s work on this topic at the beginning of 2014, but with Sharon’s findings now published in the Journal of Equine Veterinary Science, it’s about time we tried to answer some of the common questions that have arisen. Be warned – much of what we do know just raises even more questions! But first of all, what exactly is different about the anatomy in these horses’ necks?
2015 UPDATE: A second paper authored by Sharon and yours truly has also been accepted for publication – to read the abstracts and purchase the papers, follow these links:
The Occurrence of a Congenital Malformation in the Sixth and Seventh Cervical Vertebrae Predominantly Observed in Thoroughbred Horses
- Congenital malformation predominately noted in Thoroughbred and derivative horses.
- Reported absence of the caudal ventral tubercle of C6 in 19 of 50 Thoroughbreds (38%).
- Transposition of the caudal ventral tubercle from C6 onto C7 in nine of 19 Thoroughbreds.
- Variations to equine anatomy should be reported so to better understand function.
- Compromised structural integrity leads to dysfunction and locomotive ramifications.
Variations and Implications of the Gross Morphology in the Longus colli Muscle in Thoroughbred and Thoroughbred Derivative Horses Presenting With a Malformation of the Sixth and Seventh Cervical Vertebrae
Sharon May-Davis & Catherine Walker
- Longus colli relocates in malformed C6 and C7 Thoroughbred and derivative horses.
- Asymmetrical biomechanical forces alter articular process joints.
- As a cybernetic muscle, its afferent or efferent neural signals may influence posture.
- Reported postural changes—forelimbs: base wide (bilateral), one forward (unilateral).
- Reported forelimb ataxia, paresis, limb flaccidity, decreased limb tone, and reflex.
Images & text © Cat Walker 2014.
Feel free to share the link to this article, but no reproduction without permission, please!
How is the anatomy of the vertebrae changing in these horses?
The horse’s sixth cervical vertebra (C6) is unique in its shape, in order to allow for particular muscle attachments. As you can see below, C5 has a transverse process with two prominences – the green arrow to the right is the part that is palpable on C3, C4 and C5 as a series of bumps when you run your hands down the sides of the horses neck.
C6, however, is a bit different. It has three branches – we’ll call #1 the cranial ventral tubercle, and #3 thecaudal ventral tubercle (cranial = closer to the head; caudal = closer to the tail; ventral = underneath). And further down the neck at C7, you should normally find a singular transverse process.
This caudal ventral tubercle (let’s just call it the CVT) is the bumpy bit of interest. Not only can it go missing from one or both sides of C6 – it can also end up on the ventral surface of C7 instead. When this happens, the transposed CVT always ends up on the side of C7 that corresponds to the absence on C6. Sometimes, both CVTs can end up on C7! And, just to make matters trickier, even the horses affected on both sides may not necessarily show a symmetrical presentation. As Sharon says,
“On a whole, the abnormal cervical vertebrae appear asymmetric, twisted and misshapen when compared to the normal C6 and C7 counterparts.”
Crunching the numbers… Just how common is this thing?
Well, that’s where Sharon’s research comes in. Out of the thoroughbreds, 38% exhibited the malformation of C6 in some form. In addition, a warmblood x thoroughbred, a quarter horse x thoroughbred and an Anglo (a.k.a. thoroughbred derivatives) were also found to have the malformation. No other horses of identifiable breeding exhibited the malformation – the only other was one nondescript horse of pony/galloway size. While this doesn’t eliminate the possibility of other breeds exhibiting the malformation, it DOES give us a concerning statistic in the thoroughbred group. What’s more, C7 was also affected in 50% of the horses with an abnormal C6. I found those numbers hard to wrap my head around at first. Yet, over the 15 years of this study, the numbers consistently hovered around that 4/10 mark as the sample size increased.
So this is just an issue in the thoroughbred industry?
No – far from it, in fact. While Sharon’s paper identifies this prevalence in thoroughbreds, it’s very important to note that it was also confirmed in several other horses with a thoroughbred parent. Think about it – how many sporthorse breeds have been heavily influenced and refined by the thoroughbred? How many thoroughbred mares go on to breed crossbred performance horses? Combine that population with all of the ex-racehorses that go on to have equestrian careers, and chances are you’ve sat on at least one of these horses without knowing it. And while asymmetrical horses might do okay around a racetrack, what about when you try to straighten what can’t truly be straightened in the dressage ring?
How can this go unnoticed?
Well, it’s not that we haven’t known it existed. It is described in that old faithful of anatomy texts, Sisson & Grossman’s ‘The Anatomy of the Domestic Animals’, and has been noted in the equine literature as a normal anatomical variation. Previously, it was estimated that this could be found in around 5% of thoroughbred and warmblood horses. What hasn’t been reported previously is this increased prevalence in thoroughbreds. Sharon’s research also delves into the anatomical changes in more detail, and her findings call into question whether this is, in fact, as inconsequential as previously believed, or whether this malformation may have functional and biomechanical implications in affected horses.
But if it has been reported, how has it not been identified more often, even as an incidental finding?
For starters, the base of the horse’s neck is not an easy area to obtain clear radiographs, due to the bulk of the musculature. In addition, the main areas of clinical interest are usually the canal through which the spinal cord passes, and/or the facet joints. In comparison, the undersides of the vertebrae don’t get much of a look-in, as the focus is on the more dorsal structures of the vertebrae. If we don’t place any significance on a finding like this, it’s only natural that we are not going to pay so much attention to it. There are anomalies upon anomalies to be found in every biological system of every species, so why would anyone choose to concentrate on an anomaly that isn’t considered important if they have no reason to believe otherwise?
On dissection, it would be easy to miss unless you knew what you were looking for. You can’t see the abnormal shape of the bones until the deepest ventral muscle, the longus colli, is removed. Between time constraints and whether or not a subject’s skeleton is retained and cleaned, the structures in question are not always going to be examined in detail. As for clean bones and articulated skeletons? Again, it depends if you know what you are looking for or not. I have come across multiple teaching or display specimens exhibiting forms of this malformation, either presenting it as the normal anatomy, or without any comment. “How on earth is this possible?!” I hear you ask? Well, it’s a simple mistake to make.
While an asymmetrical presentation is more obvious, a bilateral presentation throws a spanner in the works. Unless you are rock solid in your spinal anatomy, it is easy to view the skeleton of a bilaterally affected horse and assume this is the normal shape. The part of C6 that goes missing is the feature that distinguishes it from C3, C4 and C5. So, when this distinguishing feature is absent from both sides of C6, the shape becomes very similar to that of C5. While there are some giveaways (such as the length – C6 is shorter than C5), it is easy to confuse them in a bilateral horse. So easy, in fact, that this appears to be the case with some of the aforementioned display skeletons – the fifth and sixth cervical vertebrae seem to have swapped places during articulation.
Equine skeletons are a valuable commodity for teaching purposes, and as such, students don’t always get access to many different examples. If veterinary and science students are learning ‘normality’ from a classroom skeleton that is abnormal, how are they going to know otherwise? Just as we see in the nuchal ligament paper recently published by Sharon and her colleague, Dr Janeen Kleine, misconceptions or gaps in knowledge can be passed down through generations like an anatomical game of telephone. This isn’t a reflection on those who missed something or didn’t have it quite right, so much as a reminder that anatomy textbooks and research are a constant work in progress, and shouldn’t be taken as gospel. A good student learns to think and question, and a good teacher encourages them to do so. As Sharon says time and time again, ‘the textbook is on the table.’ Words of wisdom from two other veterinary researchers and lecturers I hold in high regard also spring to mind. To quote Kerry Ridgway, ‘today’s information is tomorrow’s misinformation.’ And the parting statement in each of Bob Bowker’s lecture handouts? ‘The unanswered questions aren’t nearly as dangerous as the unquestioned answers…’ Hear, hear.
So, is it a clinical problem, or not?
Current thinking says no – it’s an incidental finding. However, many riders and professionals associated with horses confirmed on dissection challenge this view, with observations often including (but not limited to):
- Altered posture/stance
- Crookedness and/or a blocked feeling through the base of the neck that is not resolved through training or physical therapy
- An apparent lack of balance and/or spatial awareness, or a history of falls
While anecdotal at this stage, these observations suggest that there may indeed be a functional impact to consider. Adding weight to this is the fact that, like Sharon, some of us are beginning to identify these horses quite accurately before they are confirmed (ETA: this happened yet again only this week – to none other than yours truly!). Many of the clues are subjective – without radiographic evidence in a live horse, you can only make an educated guess. However, if the pattern of dysfunction or asymmetry is consistent enough to make riders and professionals suspicious, it seems there may be some clinical ramifications to investigate after all.
Where to from here, then?
It is the horses – who keep challenging our concept of normality with what they reveal on dissection – that are rewriting the anatomy books. And when horses with this malformation are accompanied by histories and observations so consistent that they start sounding like a broken record, it seems it’s time for us to investigate whether this variation in anatomy is causing a variation in function. Deep down, I know what the impact on the affected horses I have worked with, trained and loved was. I know others who feel the same way. But that simply isn’t good enough in a scientific setting. What Sharon has achieved by following this research through, is a foundation from which we can start to pursue answers to some of the most important questions. It only seems appropriate to close with this quote from the preface to the fifth edition of Sisson & Grossman’s text:
Got questions after reading this article?
Sound off in the comments section and we’ll do our best to answer them!