Abstract

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 Alexion^TM (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.

Acknowledgements

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

Literature Cited

1.  Cerda-Gonzalez S, Dewey CW, Scrivani PV, Kline KL. Imaging features of atlanto-occipital overlapping in dogs. Vet Radiol Ultrasound. 2009;50:264–268.

2.  Liliane L. Epimeletic behavior of free-ranging rough-toothed dolphins, Steno bredanensis, from Brazil. Mar Mamm Sci. 1992;8:284–287.

3.  Fraga-Manteiga E, Eatwell K, Smith S, Mancinneli E, Schwarz T. Traumatic atlanto-occipital subluxation and cranial cervical block vertebrae in a golden eagle (Aquila chrysaetos). Vlaams Diergeneeskd Tijdschr. 2013;82:211–215.

4.  Imaizumi T, Sohma T, Hotta H, Teto I, Imaizumi H, Kaneko M. Associated injuries and mechanism of atlanto-occipital dislocation caused by trauma. Neurol Med Chir (Tokyo). 1995;35:385–391.

5.  Kot BCW, Chi J, Fernando N, Gendron S, Heng HG, Martelli P. Can necropsy go bloodless: applicability of virtopsy as a routine procedure in stranded cetaceans in the Hong Kong waters. In: 21st Biennial Conference on the Biology of Marine Mammals Proceedings, San Francisco, CA; 2015.

6.  Kuiken T, Simpson VR, Allchin CR, Bennett PM, Codd GA, Harris EA, Howes GJ, Kennedy S, Kirkwood JK, Law RJ. Mass mortality of common dolphins (Delphinus delphis) in south west England due to incidental capture in fishing gear. Vet Rec. 1994;134:81–89.

7.  Moore JC. Bottle-nosed dolphins support remains of young. J Mammal. 1955;36:466–467.

8.  Panigada S, Pesante G, Zanardelli M, Capoulade F, Gannier A, Weinrich MT. Mediterranean fin whales at risk from fatal ship strikes. Mar Pollut Bull. 2006;52:1287–1298.

9.  Powers B, Miller MD, Kramer RS, Martinez S, Gehweiler Jr. JA. Traumatic anterior atlanto-occipital dislocation. Neurosurgery. 1979;4:12–17.

10. Tanabe M, Kelly R, De Lahunta A, Duffy MS, Wade SE, Divers TJ. Verminous encephalitis in a horse produced by nematodes in the family Protostrongylidae. Vet Pathol. 2007;44:119–122.