Ever heard of HIZ?

Characterization of annulus fibrosus lesions on magnetic resonance imaging in dogs affected by intervertebral disc disease, a descriptive case series

Yael Merbl,Sukhmeen Kaur,Tiffany G. Kei,Elle Ryan,Philippa J. Johnson

Background
Intervertebral disc disease (IVDD) is a leading cause of myelopathy in dogs, often diagnosed through MRI. This study aimed to characterize high-intensity zone (HIZ) lesions within the annulus fibrosus (AF) in dogs with IVDD. HIZ lesions, identified as hyperintense signals on T2-weighted (T2W) MRI, are associated with annular trauma and degenerative processes. While well-described in humans, their significance in dogs is less understood. The study focused on morphological features and clinical relevance of these lesions.

Methods
The study was a retrospective case series conducted at Cornell University, reviewing MRI records from 2011 to 2022. Inclusion criteria required diagnostic-quality MRI scans demonstrating T2W HIZ lesions in the dorsal AF. Data were collected on patient signalment, clinical presentation, and lesion characteristics, including shape, orientation, and location within the AF. Statistical analysis compared lesion dimensions across subgroups using Kruskal-Wallis tests.

Results
Cohort: 41 lesions in 39 dogs were analyzed. Mixed-breed dogs were the most common (25.6%), with a median age of 7.5 years and weight of 23 kg.

Lesion Types:
-With extrusion: 18 lesions were associated with acute non-compressive nucleus pulposus extrusion (ANNPE) or hydrated nucleus pulposus extrusion (HNPE).
-Without extrusion: 23 lesions were identified independently.

Morphology: Lesions were most common in the lumbosacral region and varied in shape (e.g., round, wedge, linear) and orientation (e.g., vertical, oblique). Lesions without extrusion frequently caused spinal pain or chronic paresis/plegia.

Detection: Lesions were most visible on sagittal T2W MRI, with limited visibility on other planes. Median lesion sizes varied but did not differ significantly between subgroups.

Limitations
The study’s retrospective nature limited standardization and population homogeneity. Small sample size, lack of a control group, and potential underreporting of HIZ lesions were additional constraints. The absence of histopathological data limited understanding of lesion etiology.

Conclusions
HIZ lesions in dogs were identified as potential contributors to spinal pain and chronic neurological deficits, especially in the absence of extrusion. Their morphology and distribution suggest biomechanical stress as a key factor. The study emphasizes the need for further research, including larger cohorts and histological evaluations, to clarify their clinical significance and prevalence in the canine population.