Electron Microscopy Reveals Abnormal Otoconia in a Hypercalcemic Mouse Model of Primary Hyperparathyroidism

HYPOTHESIS: Mice with chronic hypercalcemia have smaller otoconia and altered surface morphology when compared with controls. BACKGROUND: Serum calcium dysregulation may contribute to otoconial degeneration and has been linked to benign paroxysmal positional vertigo (BPPV), but direct evidence is scarce. METHODS: Four female transgenic mice modeling hypercalcemic primary hyperparathyroidism (PHPT) and 4 female age-matched wild-type mice were studied. Utricular otoconia were extracted under sodium cacodylate solution, gold-coated, and imaged by Zeiss Sigma scanning electron microscope. Otoconia were hand-measured using ImageJ to quantify major axis, minor axis, and area. Statistical analysis included linear mixed-effects modeling and Mann-Whitney testing across 3 size categories (small, medium, and large). Fractured otoconia were quantified to assess structural stability. RESULTS: Transgenic PHPT mice exhibited smaller otoconia than control mice across the major axis (5.15±0.05 µm vs. 9.09±0.16 µm), minor axis (2.79±0.02 µm vs. 4.55±0.07 µm), and area (13.24±0.28 µm2 vs. 39.98±1.55 µm2). The linear mixed-effects model showed a significant interaction between group (control vs. transgenic mice) and otoconial size category. Subgroup analysis showed the greatest between-group differences among large otoconia for major axis (P=0.039), minor axis (P=0.038), and area (P=0.032). Otoconia fracture frequency was low but there was a 4.4-fold increase in cracks within the hypercalcemic mice. CONCLUSIONS: Mice with chronic hypercalcemia and PHPT have smaller otoconia when compared with normal mice, with increased frequency of fractured otoconia, consistent with a potential role of chronic calcium dysregulation in otoconial degeneration and potentially BPPV.