MORPHOMETRIC ANALYSIS OF THE CRANIAL CAVITY AND CAUDAL CRANIAL FOSSA IN THE DOG: A COMPUTERIZED TOMOGRAPHIC STUDY ISABEL GARCI ´ A-REAL, DVM, PHD, PHILIP H. KASS, DVM, PHD, BEVERLY K. STURGES, DVM, ERIK R. WISNER, DVM This investigation was initiated to develop a reliable and simple computerized tomography (CT) method for evaluating the morphometry of the cranial cavity (CC) and caudal cranial fossa in the dog. We obtained linear, area, and volume measurements of the entire CC and the caudal fossa compartment in 24 dogs, using transverse and reformatted sagittal and dorsal CT images. The ratios between the CC and the caudal fossa volumes were calculated, as were the ratios between the CC and caudal fossa midsagittal (MS) areas. There were statistically significant correlations between absolute volume data and certain linear measurements, a positive correlation between volume ratio and weight, and significant differences in absolute volume values and volume ratio between males and females. The volume ratio, MS area, and certain linear measurements included in this study can be considered as accurate parameters to evaluate the relative size of the CC to caudal fossa in the dog. Such quantitative measures may be useful in systematically characterizing Chiari-type malformations and other developmental anomalies in the dog. Veterinary Radiology & Ultrasound, Vol. 45, No. 1, 2004, pp 38–45. Key words: Arnold–Chiari syndrome, caudal cranial fossa, computerized tomography, dog, skull, morphometry. Introduction T HE WIDE VARIATION in skull size and shape among domestic dogs may necessitate the use of anatomic craniometry to characterize cranial and intracranial disease affecting specific canine breeds and crosses. 1 Advanced imaging techniques such as computerized tomography (CT) and magnetic resonance imaging (MRI) can now be used to perform precise morphometric studies in live animals. There are marked differences in skull shape between miniature/toy and brachycephalic dogs as opposed to larger mesaticephalic and dolichocephalic dogs. The major differences between breeds occur in the facial region of the skull, while the neurocranium is more consistent. 1 How- ever, the neurocranium as well as the occipital region of the skull are particularly prone to uneven or asymmetric growth. The cranial cavity (CC) is the part of the skull that contains the brain, with its coverings and vessels. 1 While the roof (calvaria) of the CC is largely undivided, the base is divided into the cranial, middle, and caudal fossa. 1,2 The caudal fossa is defined rostrally by the dorsum sellae and caudally by the foramen magnum, and it contains the midbrain, pons, and medulla oblongata ventrally and the cerebellum dorsally. 1,2 The edges of the petrosal crests and the osseous tentorium serve as points of attachment of the membranous tentorium cerebelli, which separates the cerebellum from the cerebrum. 1 A Chiari-type malformation has been recently described in Cavalier King Charles Spaniels. 3–6 There are no references to other canine breeds apart from a possibly affected Maltese poodle dog. 7 This malformation has been described in people, and is characterized by a downward herniation of the caudal part of the cerebellum and/or medulla oblongata into the spinal canal. 8 Based on the morphometry of the posterior fossa in humans using CT, a smaller posterior fossa was present in the Chiari patients when compared with control subjects, suggesting that, in people, an underdeveloped occipital bone induces over- crowding in the posterior cranial fossa, which contains a normal-sized hindbrain. 8,9 A small caudal fossa, a narrow foramen magnum, and cranial vertebral canal stenosis was described in seven Cavalier King Charles Spaniels with syringohydromyelia. 3 This condition is considered to be a consequence of this type of malformation in both humans and dogs, due to overcrowding of the foramen magnum Address correspondence and reprint requests to Erik R. Wisner, DVM, at the above address. This study was supported by the Center for Imaging Science, University of California, Davis, CA, and Del Amo Grant, Universidad Complutense de Madrid. Received November 11, 2002, accepted for publication on April 10, 2003. doi: 10.1111/j.1740-8261.2004.04006.x From the Department of Patologı´a Animal II (Garcı´a-Real), Uni- versidad Complutense de Madrid, 28040 Madrid, Spain, and Department of Surgical and Radiological Sciences (Garcı´a-Real, Sturges, Wisner) and Department of Population Health & Reproduction (Kass), School of Veterinary Medicine, 2112 Tupper Hall, University of California, Davis, CA 95616. 38