Flex the Neck, Shrink the Columns: CT Myelography Normals at C3–C4 in Warmbloods

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

Radiographic myelography (RxM) and computed tomographic myelography (CTM) are used to detect extradural spinal cord compression in horses, yet normative data describing how contrast columns and cord dimensions change with neck position—particularly in nonaffected Warmbloods—are limited. Established radiographic criteria (for example, ≥50% dorsal column reduction in flexion) risk false positives without species-, level-, and modality-specific reference values. This study set out to provide such reference measurements at C3–C4.

Methods

In a terminal, in-vivo method-comparison study, 13 neurologically normal Warmblood horses underwent RxM and CTM in neutral and maximally flexed positions, followed by histopathology confirming absence of spinal disease. On mid-sagittal images, investigators measured intravertebral and intervertebral sagittal diameter ratios, the dorso-ventral spinal cord diameter (sc), and heights of the dorsal (dcc) and ventral (vcc) contrast columns at predefined intravertebral (C3/C4 cranial, mid, caudal) and three intervertebral points. Measurements were repeated by two observers to assess intra-/interobserver repeatability (ICC, CV).

Results

Neck flexion (~24° in both modalities) significantly decreased CTM spinal cord diameter at intervertebral locations (inter2: p=0.02; inter3: p=0.007), while RxM cord diameters showed no significant positional change. Flexion reduced dorsal contrast column height at intervertebral points in both modalities (CTM inter2 p=0.004, inter3 p=0.01; RxM inter2 and inter3 p=0.005) and reduced ventral contrast column height at multiple sites (CTM C3-cranial p=0.04, inter2 p=0.003, inter3 p<0.001; RxM C3-cranial p=0.007, C4-cranial p=0.003, inter3 p<0.001). In some CTM flexed scans, the ventral column at the caudal intervertebral point was absent (0 mm). Despite these flexion effects, in this normal cohort the dorsal column remained ≥4 mm across positions and modalities. Inter/intravertebral sagittal ratios were broadly similar between RxM and CTM in neutral (e.g., CTM intervertebral mean 0.76; intravertebral C3 0.58, C4 0.56), with a nonsignificant tendency toward a lower intervertebral ratio on CTM vs RxM in flexion (0.62 vs 0.69; p=0.05). Measurement repeatability was excellent within observers (ICC ≥0.77 RxM; ≥0.85 CTM) and good (CT) to fair-good (RxM) between observers; CVs were low–moderate.

Limitations

The sample was modest and restricted to Warmbloods and to the C3–C4 level. RxM measurements lacked a magnification correction factor. Ioversol (used intrathecally) is not the conventionally recommended contrast agent, though the protocol was terminal. Findings at other cervical levels or in clinical CVCM cases were not assessed.

Conclusions

In neurologically normal Warmbloods, neck flexion consistently narrows dorsal and ventral myelographic columns at C3–C4 and reduces CTM spinal cord diameter at intervertebral points. A dorsal column ≥4 mm was typical in unaffected horses, underscoring that stringent thresholds (e.g., ≤2 mm) may better avoid false positives. Interpreting CTM vs RxM requires modality-specific expectations, and normative, position-dependent reference data—such as those provided here—should inform clinical decision-making for suspected dynamic compression. Further work in confirmed CVCM cases is warranted to establish diagnostic cut-offs.

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