Horse Health Article - Neck Pain in Horses
There has been an increase in research into the equine neck over the last few years, leading to an increased clinician-focus on pain originating from this region. This article will explore a few of the conditions that we are starting to observe as equine clinicians.
Cervical Vertebral Stenotic Myelopathy (CVSM) - “Wobblers Syndrome”
CVSM or “Wobblers” is a condition characterised by compression of the spinal cord in the cervical spine leading to neurologic disease. It can be present from birth, however is more commonly noted either at weanling age (due to symmetrical overgrowth (Type I) of the articular processes of a vertebra in the mid-cervical region), or in older horses due to asymmetric bony proliferation of one of the articular processes (i.e. due to osteoarthritis) or soft tissue lesions (Type II).
Wobblers are diagnosed by their clinical signs, which in older horses usually includes abnormal gait, tripping/toe dragging, neck stiffness, or stumbling/falling. More severe cases may show obvious ataxia (weakness) or spasticity in the pelvic limbs.
CVSM is diagnosed by high-powered standing radiography (X-rays.) The image quality provided by a standing machine is important due to the increased soft tissue density within the cervical region, requiring a high-powered machine to penetrate through these tissues to obtain a detailed image.
There are 5 findings that are indicative of CVSM (though not all are required):
“Flare” of the caudal epiphysis of the vertebral body
Abnormal ossification of the articular processes
Malalignment between adjacent vertebrae
Extension of the dorsal laminae
Degenerative joint disease of the articular process
Radiologists will also often use a sagittal ratio measurement to detect CVSM. Other more advanced diagnostic imaging techniques include myelography (injecting a contrast ratio into the CSF) followed by X-rays to show the site and extent of compressive lesions.
Osteoarthritis of the articular facet joints of the cervical spine
At each vertebral junction there are two articular facet joints, where one vertebra articulates with the next. The cranial articular processes of the lower vertebra are lateral and ventral to the caudal articular processes of the upper vertebra, forming a diagonal joint line.
The movement at each articular facet joint is a shearing motion, consisting of dorsal-ventral flexion/extension as well as lateral bending coupled with axial rotation, due to the angulation of the joint. The equine cervical spine is subject to the some of the greatest degrees of motion within the spinal column as a whole.
Due to the rapid advancement in imaging technology, equine veterinarians are increasingly diagnosing osteoarthritis of the cervical articular facet joints. While the biomechanics and kinematics of the equine cervical spine likely have a role in the significant load distributed across these joints, there is also growing concern that these findings may be related to breed or discipline - though the research is still investigating these theories.
Signs of cervical osteoarthritis may include: stiffness of the neck, inability or difficulty to laterally flex in one/both directions, difficulty in flexion of the neck/lowering neck carriage, muscle atrophy, or pain upon palpation over specific cervical segments. Sometimes, horses with articular facet joint arthritis may present with forelimb lameness that is unable to be localised to the limb through nerve blocks.
Cervical osteoarthritis of the articular facet joints is diagnosed via imaging, usually through radiography - though ultrasonography may also be used to image the synovial capsule of the facet joints. The gold-standard for assessing the cervical spine is CT, which allows high-detail 3D imaging of each of the facet joints to gain a complete perspective of the extent of arthritic change.
Common ridden signs of neck pain include: stiffness, difficulty working to one side (e.g. struggles to bend to one side) and issues obtaining/maintaining a working neck frame.
Cervical fascial pain
Fascia is a connective tissue within the body, which forms “cling wrap” overlying every structure in the entire body - from layers in between skin, muscles, organs - to lying within each muscle fibre, tendon fibre, and even between cells. There are multiple layers of fascia with several complex relationships throughout the body, however fascia of the musculoskeletal system can more simply be referred to as “superficial fascia” i.e. the fascia under the skin overlying major muscle groups, and the “deep fascia” wrapping around individual muscles and forming attachment points for muscular tissue to their origins/insertions.
The deep fascia is becoming increasingly understood to be the greatest pain-sensitive tissue in human literature due to its rich innervation. Compensatory changes in the fascia due to abnormal biomechanics (and pain signalling) can lead to the fascia becoming more dense/fibrous, leading to further pain.
My own clinical experience through treating is that the vast majority of horses are experiencing abnormal fascial changes and adhesions around the cervical spine, which is likely a huge source of pain and discomfort in this region.
Cervical muscular pain
There are several major muscle groups located within the equine cervical spine. The brachiocephalicus muscle is a huge focus for my practice - as it is generally a major focal point of myofascial pain.
Celeste Lazaris (Balance Through Movement Method) theorises that the brachiocephalic muscle acts as an antagonist to the thoracic sling, functioning as the main muscle for advancing and internally rotating the limb when the neck is fixed (shortening the attachments between the humerus and the occipital bone.) While the horse utilises this muscle, the thoracic sling is unable to engage to “take over” the role as primary mover of the forelimbs. The brachiocephalic muscle also has deep fascial attachments overlying “nerve highways” emanating from the cervical spinal segments, and as discussed previously, when this fascia becomes dense and fibrous, there can be a “compression” of these neurovascular pathways which may have a significant role in cervical pain.
Other signficant muscles of the neck include the splenius and semispinalis muscles (chronically atrophied and underdeveloped in a large number of horses), which function to laterally bend the neck and stabilise the horse’s neck posture against gravity (i.e. should carry the weight of the neck in conjunction with the nuchal ligament) when the neck is relaxed i.e. maintaining the head in partial extension against gravitational force.
The deeper muscles of the neck include the longus colli, multifidus cervicis, and intertransversarius muscles. Each has a role in stabilising the neck ventrally (longus colli), laterally (intertransversarius) and dorsally (multifidus cervicis.) Horses with osteoarthritic changes of the articular facets will often have concurrent focal atrophy of the multifidus cervicis, due to the fascial attachments of this muscle onto the joint capsules of each facet joint.
Above: Figure 3 from “Characterization of the Caudal Ventral Tubercle in the Sixth Cervical Vertebra in Modern Equus ferus caballus” (May-Davis et al. 2023) showing complete absence of the Caudal Ventral Tubercle of a C6 vertebra
Equine Cervical Vertebral Malformation (ECVM) or “Transposition of the Caudal Ventral Tubercle”
This condition is becoming hotly contested amongst equine veterinarians and professionals alike. Documented in the early veterinary literature as an incidental radiographic finding, the incidence of this findings as well as corresponding clinical implications has gained traction following the research of Sharon-May Davis, an Australian anatomist.
There have since been a few investigations into this condition in the veterinary literature, though different conclusions are being drawn. Anthony DeRouen et al. (2016) through a study conducted at the University of California Davis found an increased likelihood of cervical pain in horses with an anomalous C6, compared to those without. These findings were corroborated in a recent study by Beccatti et al (2020) showing an increase in the likelihood of cervical pain in horses with abnormalities of the ventral lamina of C6.
Interestingly, the 2016 study by DeRouen et al. showed no association between articular facet joint osteoarthritis or intervertebral sagittal ratios in horses with anomalous C6 when compared to horses without. Veraa et al. (2019) also noted that the presence of an anomalous C6 does not necessarily implicate the presence of clinical signs.
It is important to consider that the unilateral or bilateral absence or transposition of the caudal ventral tubercle of C6 (ECVM) may contribute to altered biomechanics in the lower cervical region. This is due to the altered attachment of the longus colli muscle, an important stabiliser of the ventral cervical spine which attaches directly onto the caudal ventral tubercles of C6.
The absence or transposition of one or more caudal ventral tubercles can be diagnosed via radiographic imaging (X-rays) using obique views to highlight the ventral margin of C6.
Though the presence of this malformation is not necessarily in and of itself pathologic, horses with this finding should potentially have particular attention paid to increasing the biomechanic stability of the lower cervical spine through appropriate conditioning.
If you think your horse may have neck pain, please contact the clinic to book an appointment to have your horse investigated. You can email us at hello@capeequine.com.au or contact us by phone at 0467 485 976.
Reference List:
Story, M.R. et al. (2021) ‘Equine cervical pain and dysfunction: Pathology, diagnosis and treatment’, Animals, 11(2), p. 422. doi:10.3390/ani11020422.
DeRouen, A., Spriet, M. and Aleman, M. (2016) ‘Prevalence of anatomical variation of the sixth cervical vertebra and association with vertebral canal stenosis and articular process osteoarthritis in the horse’, Veterinary Radiology & Ultrasound, 57(3), pp. 253–258. doi:10.1111/vru.12350.
Beccati, F. et al. (2020) ‘Radiographic findings and anatomical variations of the caudal cervical area in horses with Neck Pain and ataxia: Case–control study on 116 horses’, Veterinary Record, 187(9). doi:10.1136/vr.105756.
Veraa, S. et al. (2019) ‘Caudal cervical vertebral morphological variation is not associated with clinical signs in Warmblood horses’, Equine Veterinary Journal, 52(2), pp. 219–224. doi:10.1111/evj.13140.
May-Davis, S. et al. (2023) ‘Characterization of the caudal ventral tubercle in the sixth cervical vertebra in modern Equus ferus caballus’, Animals, 13(14), p. 2384. doi:10.3390/ani13142384.
Reed, S.M. and Toribio, R.E. (2022) Equine neurology. Philadelphia, PA: Elsevier.