Preliminary Assessment of Cranial Cervical Dislocation in Stranded Cetaceans Using Multislice Computed Tomography
Cranial cervical dislocation (CCD) has been reported in humans,4 domestic,1,10 and wild animals,3 but rarely in marine mammals. The only known formal literature describing CCD in cetaceans was associated with violent struggling of entangled individual.6 Other possible causes of CCD in cetaceans include hyperextension injury secondary to vessel collision,9 epimeletic behavior,2,7 or postmortem change. In human medicine, plain lateral cervical radiograph is recognized as a conventional radiographic assessment of CCD. As cross-sectional imaging technologies have become more readily available, the standard of cervical spine assessment shifts from plain radiographs to multidetector computed tomography (MDCT). Kot and his team5 suspected high incidence of CCD in stranded cetaceans using postmortem MDCT. This preliminary study aimed to investigate the applicability of MDCT on the assessment of CCD,and to propose a standardized method to diagnose CCD in stranded cetaceans.
A total of 18 carcasses of 3 stranded cetacean species (Neophocaena phocaenoides, Sousa chinensis, and Tursiops aduncus; 6 male, 9 female, 3 unknown sex; juvenile-adult; 105–259 cm in length; condition code ranged from 2–4) were included in the study. All carcasses underwent whole-body MDCT scan using Toshiba 16-row multislice CT scanner AlexionTM (Toshiba Medical Systems, Tochigi, Japan), with slice thickness of 1 mm. Volumetric data were reconstructed and reformed for multiplanar reconstruction using an open medical image viewer OS X (Horos Project). CCD was evaluated using the ratio of basion-dorsal atlas arch (BD) to the opisthion-ventral atlas arch (OV) adopted from Powers and his team.9 Neonate, calf, and carcasses with collapsed skull were excluded from the study. All measurements were performed on the midsagittal plane of the cervical region. All atlantooccipital membranes were observed for their intactness.
The preliminary results indicated that CCD was identified in 9 out of 18 carcasses using MDCT. The ratio of BD/OV was considered as one of the discriminators for CCD in humans,9 which should also be considered applicable to cetaceans in terms of the atlantooccipital joint anatomy. All carcasses with the ratio of BD/OV less than 1.2 exhibited signs of CCD. Rupture of atlantooccipital membrane in conjunction with CCD was observed only in 3 carcasses, indicating ligamentous instability might not be the single causation of CCD in cetaceans. Superimposed cervical bony structures secondary to initial improper carcass positioning could be individually identified using MDCT multiplanar reconstruction technique, which allowed correct identification of bony landmarks for BD and OV at true midsagittal plane of cervical region. To conclude, MDCT may be an efficient means to assess CCD, with the ratio of BD/OV as a standardized method to diagnose CCD in stranded cetaceans.
Investigation is ongoing to establish a specific cutoff value to diagnose CCD from a larger sample size.
This project was financially supported by the Hong Kong Research Grants Council [Grant number: UGC/FDS17/M07/14]. The authors would like to thank the Agriculture, Fisheries and Conservation Department of the Hong Kong SAR Government for the continuous support in this project. Sincere appreciation is also extended to veterinarians, staff, and volunteers from Ocean Park Hong Kong, Ocean Park Conservation Foundation Hong Kong, and Tung Wah College for paying great effort on the stranding response and necropsy in this project. Special gratitude is owed to technicians from Hong Kong Veterinary Imaging Center for operating the CT and MRI for this research.
* Presenting author
+ Student presenter
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