How Much Is Normal? CT Myelography Benchmarks for the Healthy Warmblood Cervical Spine

Computed tomographic myelography of the cranial cervical spine in Warmblood horses with no spinal pathology—Inter- and intravertebral ratios and distribution of contrast columns in neutral and flexed cervical spine

Maren Hellige; Caroline Schröder; Frauke Seehusen; Jessika-M. Cavalleri; Karl Rohn; Peter Stadler; Florian Geburek

Background

Computed tomographic myelography (CTM) and radiographic myelography (RxM) are commonly used to diagnose extradural spinal cord compression in horses, particularly in cases of suspected cervical vertebral compressive myelopathy. While flexion of the cervical spine is routinely employed to enhance lesion detection, normal reference values for spinal cord diameter, contrast column distribution, and sagittal ratios in unaffected horses—especially Warmbloods—are poorly defined. This lack of normative data complicates interpretation and increases the risk of false-positive diagnoses.

Methods

This terminal in vivo method-comparison study included 13 neurologically normal Warmblood horses with histological confirmation of absence of spinal pathology. All horses underwent RxM and CTM of the cervical spine (C1–C7) under general anaesthesia in both neutral and flexed positions. Measurements at C3–C4 included inter- and intravertebral sagittal ratios, sagittal diameter of the spinal cord, and heights of dorsal and ventral contrast columns at predefined intra- and intervertebral locations. Measurements were performed independently by two observers on three occasions to assess repeatability and observer agreement.

Results

Flexion of the cervical spine resulted in a significant reduction in spinal cord diameter at intervertebral locations on CTM, but not on RxM. Both CTM and RxM demonstrated significant decreases in dorsal and ventral contrast column heights at intervertebral locations during flexion. The ventral contrast column showed the greatest reduction, occasionally becoming undetectable in flexion on CTM despite absence of pathology. Inter- and intravertebral sagittal ratios did not differ significantly between CTM and RxM in either position, although individual CTM measurements in normal horses fell below previously proposed radiographic cut-off values. Intraobserver reliability was excellent, and interobserver reliability ranged from fair to good.

Limitations

The study population was small and consisted of mature Warmblood horses without neurological disease, limiting generalizability to younger horses or those with cervical instability. Only the C3–C4 region was evaluated. Radiographs were not calibrated for magnification, precluding direct comparison of absolute measurements between modalities. Additionally, the contrast agent used for intrathecal injection is not routinely recommended, although its use was justified by the terminal study design.

Conclusions

Flexion of the cervical spine significantly reduces spinal cord diameter and myelographic contrast column heights at C3–C4 in Warmblood horses without spinal pathology. Normal CTM measurements can overlap with values previously considered pathological on radiographs, indicating that established radiographic thresholds cannot be directly applied to CTM. These findings provide important reference data and highlight the need for CTM-specific diagnostic criteria to improve differentiation between normal variation and clinically relevant spinal cord compression.

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