What causes the classic appearance of HPNE?

JAVMA 2019

January 01, 2025

Marc Kent, Eric N. Glass, Rachel B. Song et al.

Background
The meningovertebral ligament (MVL) is a fibrous structure found within the ventral epidural space, anchoring the dura mater to the vertebral bodies. While its presence and clinical implications are well-documented in humans, the MVL has not been described in veterinary literature. This study aimed to determine whether dogs have an MVL and assess its potential impact on pathological lesions within the ventral epidural space.

Methods
Cadaveric specimens of six neurologically normal dogs, along with tissue samples and MRI data from two dogs with vertebral neoplasms and two dogs with hydrated nucleus pulposus extrusion (HNPE), were evaluated. Techniques included gross and histological examinations, cytological analyses, and imaging (MRI and CT) to identify and characterize the MVL. Surgical observations were used to validate findings in dogs with HNPE.

Results
Neurologically Normal Dogs: All six dogs exhibited an MVL spanning from the ventral dura mater to the dorsal surface of vertebral bodies. The MVL appeared as thin fibrous bands, most robust in the cervical spine (C3–C6) and weaker in thoracic and lumbar regions.

Pathological Lesions:
-Vertebral Neoplasms: Both neoplasms (osteosarcoma and plasma cell tumor) exhibited a bilobed shape in the ventral epidural space, with the cleft between lobes aligned with the MVL's midline attachment.

HNPE: In both cases, extruded material formed bilobed shapes, confirmed as nucleus pulposus via cytology. Surgical and imaging findings validated the MVL's role in shaping these lesions.

The MVL created an anatomic barrier, dictating the bilobed configuration of lesions regardless of the pathological process.

Limitations
The study was descriptive and relied on a small sample size. Routine histological processing often disrupted the fragile MVL, making its detailed structural characterization challenging. Functional implications of the MVL, such as its role in biomechanics or disease progression, were not addressed.

Conclusions
The MVL is a distinct anatomical structure in dogs, influencing the morphology of lesions in the ventral epidural space. Its presence explains the bilobed shape of vertebral neoplasms and HNPE lesions observed in imaging and surgical contexts. Further research is needed to explore its biomechanical and clinical significance, particularly in disease progression and surgical planning.

Photograph of a transverse section of the vertebral column obtained at the cranial aspect of the body of C5 in a 6-year-old Labrador Retriever with a plasma cell neoplasm (red dotted line) that extended from C5 into the ventral epidural space. Notice that the portion of the neoplasm within the ventral epidural space has a bilobed shape, which was attributed to the presence of the MVL (arrow) that extended from the ventral external surface of the dura mater to the dorsal longitudinal ligament (arrowhead). Bar = 0.5 cm. Inset—Postcontrast transverse T1-weighted MRI image with fat saturation obtained at the same level as the tissue specimen in the photograph. Notice the strong homogeneous contrast enhancement of the neoplastic tissue, which accentuates the bilobed shape of the neoplasm within the ventral epidural space and facilitates visualization of the MVL and dorsal longitudinal ligament.

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