Table 1.
Chi = Chihuahua; Affens = Affenpinscher.
Fig 1.
T1w sagittal MRI of a five year old Chihuahua without SM with a framework of 19 measurements (11 lines and 8 angles) with three ratios used to ‘map’ the hindbrain and craniocervical junction.
Key: (a) dorsum of spheno-occipital synchondrosis. (b) basion of basioccipital bone. (c) rostral edge of the dorsal lamina of the atlas. (d) junction between the supraoccipital bone and the occipital crest. (e) most dorsal point of intersection of the cerebellum with the occipital lobe circle. (f) centre of’ best fit’ occipital lobe circle placed on the cranial baseline (abi) and extending to encompass the occipital lobes. The centre of the circle is f and its diameter (f-diam) indicates the maximun height of the caudal calvaria dorsal to the spheno-occipital synchondrosis. (g) point at which the optic nerve deviates into the optic canal. (i) intersection point of the extended cranial baseline (ab) caudally with extended line dc ventrally to form angle 3. This indicates the relative positions of the supra and basioccipital bones to the atlas. (j) most rostral aspect of the dens of the axis bone. (k) extended line from point b along the best fit line of the ventral medulla oblongata to where it changes angle to the spinal cord (degree of medullary kinking). 8 angles measured are (1) afg, (2) fac, (3) dib, (4) jcb (5) aeb (6) ebd (7) bdi (8) dbk. * trait used in previous GB study [9].
Fig 2.
Scatterplots for canonical Discriminant Functions Analysis for total group cohort.
Discriminant analysis is used to determine the minimum number of dimensions needed to describe differences between the group for (1) Breed and (2) SM. These significant variables identified are allocated a weight within each discriminant function (S2 Table) and the two functions plotted against each other to illustrate group seperation. The 11 traits can distinguish with 100% accuracy for CKCS, 92.8% for Chihuahuas and 90% for Affenpinschers. Separation in SM status using 3 traits yields 75.7% accuracy for SMO, 73.9% for SM2 but 0% for SM1 predictive success for each group.
Fig 3.
Scatterplots for canonical Discriminant Analysis for three breeds, Chihuahua, Affenpinscher and CKCS.
Chihuahua: Left to right, there is less risk of SM left to right as angle 3 decreases and angle 4 increases (atlas further caudal from the supraoccipital and basioccipital bones) Affenpinscher: Left to right, increased risk of SM with decreasing line cj and increasing f-diam) CKCS: Left to right, increasing risk of SM with increasing line id and ratio f-diam: ai (i.e. dogs that are more brachycephalic).
Table 2.
Significant variables for SM status identified in ANOVA after Tukey correction.
* Traits identified as significant in previous GB study. L = angle. Significant p values ≤ 0.05 for SM affectedness are highlighted in bold.
Fig 4.
Descriptive boxplots of variables associated with SM status significant for both Chihuahua and CKCS cohorts.
The mean values for SM0, SM1 and SM2 have been linked with a colour coded line for each breed. In addition, a thin grey line has been drawn between mean values SM0 directly to SM2. Unless indicated by red bar/s, the significance is between SM0 and SM2.
Fig 5.
Descriptive boxplots of significant variables associated with SM status in the Affenpinscher, Chihuahua and CKCS.
The mean values for SM0, SM1 and SM2 have been linked with a colour coded line for each breed. In addition, a thin grey line has been drawn between mean values SM0 directly to SM2. Unless indicated by red bar/s, the significance is between SM0 and SM2.
Fig 6.
Four pairs of TW1 mid- sagittal MRI of exemplar Chihuahua, Affenpinscher and CKCS with and without SM and their morphometric overlays.
The pairs of dogs have been matched as far as possible in age and size of the occipital circle (f-diam) which is typically larger in Affenpinschers and CKCS dogs with SM. AOO = atlanto-occipital overlap. Note the reduced occipital crest in all 3 dogs with SM (a) and marked ventriculomegally in SM dogs 2b and 3b. The images in each pair have been morphed with each other to provide a dynamic illustration (S1–S3 Morph Movies).
Fig 7.
Schematic framework of selected CM traits (red lines) and ‘normal’ traits (blue lines) to illustrate underlying unifying pathophysiological processes.
Key: (a) dorsum of spheno-occipital synchondrosis. (b) basion of basioccipital bone. (c) rostral edge of the dorsal lamina of the atlas. (d) junction between the supraoccipital bone and the occipital crest. (f) centre of “best fit” occipital lobe circle placed on the cranial baseline (abi) and extended to encompass the occipital lobes. The centre of the circle is f and indicates the height of the caudal cranial fossa. (g) point at which the optic nerve deviates into the optic canal. (k) extended line from point b along the best fit line of the ventral medulla oblongata to where it changes angle to the spinal cord (degree of medullary elevation/ kinking). The red (SM affected) lines are superimposed on those for representing normal (blue) and aligned along the skull base at fixed point ‘a’ dorsum of spheno-occipital synchondrosis (star). The basisphenoid, possibly the presphenoid, is flexed dorso-caudally (‘sphenoid flexure’) from the point of the synchondrosis (*) and the cranial cervical vertebrae especially the odontoid process that becomes angled ventrally and closer to the basioccipital bone with medullary elevation (‘cervical flexure’). Dogs may have one or more of the features which predispose to SM.