Are Horses Born with Neck Vertebrae Variants Prone to Health Issues?

Do neck vertebrae variants (not like other vertebrae) that a horse was born with cause health issues? In some cases, yes. However, recent research found that congenital (born with it) variants of the ventral laminae of the transverse processes of the sixth and seventh cervical vertebrae are of unlikely clinical relevance.

This area refers to the bony projections on the underside of the transverse processes of the C6 and C7 vertebrae in the neck. The transverse processes are bony projections on either side of a vertebra that provide attachment points for muscles and ligaments. The laminae refer to the flattened plates of bone normally on the underside of the sixth cervical vertebra.

Equine Complex Vertebral Malformation

A term that has been increasingly used in the equestrian world is equine complex vertebral malformation. This refers to congenital (present at birth) variations in the shape of the sixth and seventh cervical vertebrae (see above images). Specifically, the variants can be the absence of one or both protuberances on the ventral (underneath) aspect of the sixth cervical vertebra (correctly called the ventral laminae of the transverse processes) and the presence of one or both ventral laminae on the ventral aspect of the seventh cervical vertebra.

It has been suggested that such variants are associated with the development of osteoarthritis of the articular process joints and a variety of clinical problems.

Some have proposed that the acquisition of X-rays of the neck should be a routine part of pre-purchase examinations. It also has been suggested that horses with the congenital variants of the ventral laminae of the transverse processes of the sixth and seventh cervical vertebrae (transposition of one or both ventral laminae from the sixth to the seventh cervical vertebra) should not be recommended for purchase.

Several small-scale studies have been conducted in Europe to determine the prevalence of these radiological variants in different populations of horses, with equivocal results with respect to their likely clinical significance.

Recent Research

In a recent collaborative study¹ between UC Davis in California and the Animal Health Trust in the UK, a prospective study was performed in Warmblood horses. Most horses have seven cervical (neck) vertebrae, numbered from the head caudally (moving toward the tail). Irrespective of their clinical history, all horses underwent radiographic examination of the caudal neck vertebrae (fifth cervical to second thoracic vertebrae).

The radiographs were interpreted based on a detailed, predetermined protocol. The assessors were unaware of the clinical category of the horse as either case or control.

Cases were horses with incoordination and weakness consistent with compression of the spinal cord in the neck region, neck-related forelimb lameness, or neck pain and/or stiffness. Control horses were either those undergoing a pre-purchase examination or those with problems that had been proven to be unrelated to the neck.

Findings

The ventral profile of the seventh cervical vertebra was abnormal in 24% of horses. Congenital transposition of one or both ventral laminae was observed in a similar proportion of horses to that seen in previous studies.

Cases were significantly less likely to have congenital variants than control horses.

There was no association between the presence of a congenital variant of the seventh cervical vertebra and the presence of modelling (alteration in shape and size) of the articular processes of the sixth cervical to second thoracic vertebrae.

There was no association between the presence of congenital variants of the seventh cervical vertebra and any other radiological findings (for example, subluxation of adjacent vertebrae). It therefore seems unlikely that the congenital variants were of clinical relevance in this population of horses.

There was no association between the presence or absence of a congenital variant and the type of case. This reinforces the observation that these congenital variants are unlikely to contribute in any way to neck pain or stiffness, neck-related forelimb lameness, or incoordination and weakness.

Cases were more likely to have severe modelling of the articular processes between the sixth and seventh cervical or seventh cervical and first thoracic vertebrae compared with control horses. Severe alteration in size and shape of the articular processes may result in restricted movement, pain, nerve root compression contributing to forelimb lameness, or spinal cord compression resulting in incoordination.

Asymmetry in the development of specific muscles ventral to the vertebrae has been documented in association with congenital variants of the sixth and seventh cervical vertebrae. It has been suggested that this may result in vertebral instability predisposing to osteoarthritis of the articular process joints. However, there are many other large, symmetrically developed muscles surrounding the neck vertebrae on all sides in horses with these congenital variants. These muscles are likely to provide adequate neck stability.

Final Words

In conclusion, congenital variants of the caudal cervical vertebrae occurred less frequently in cases compared with control horses. The absence of any relationship with other radiological abnormalities that do have the potential to cause limited range of motion, pain, or spinal cord or nerve dysfunction indicates that, in the populations of horses studied, the presence of congenital variants of the ventral laminae of the transverse processes of the sixth and seventh cervical is of unlikely clinical relevance.

The authors therefore suggest that the term ‘equine complex vertebral malformation’ should be avoided. It is potentially misleading. Horses in the current study ranged from 3 to 22 years of age, thus the results are of relevance to senior horses.