Table 1.
Pathogenesis of Chiari-like malformation and syringomyelia: summary of the existing knowledge base—skull changes.
| Anatomical feature | Study finding(s) | Possible implication |
|---|---|---|
| Brachycephaly | Brachiocephalic breeds have early closure of the spheno-occipital synchondrosis. In CKCS closure is even earlier (49, 50) | Premature closure of the spheno-occipital synchondrosis will result in a short cranial base (basicranium). |
| CKCS have shorter cranium in relation to width compared to other brachycephalic dog breeds (51). | ||
| Griffon Bruxellois with CM have shortened basicranium and supraoccipital bone, with a compensatory lengthening of the dorsal cranial vault, especially the parietal bone (52) | Basiocranial shortening results in compensatory changes in the rostral cranial fossa which results in a head shape with rostrocaudal doming and is broad in relationship to the length (reduced cephalic index) | |
| Association between increased cranial height and SM in CKCS, Griffon Bruxellois and Affenpinscher (38, 40) | ||
| CKCS with broader and shorter skulls and increased rostro-cranial doming are at increased risk of developing SM (53) | ||
| Association between acute angulation at spheno-occipital synchondrosis (Sphenoid flexure) and SM (40) | May be associated with premature closure of spheno-occipital synchondrosis. This angulation occurs in rodent models where the spheno-occipital synchondrosis is damaged (1, 54) | |
| Rostral forebrain flattening, short basioccipital bone associated with CM pain | CM pain is associated with increased brachycephaly | |
| Increased risk of SM with increased proximity of dens to basioccipital bone and/or increased airorhynchy with small more ventrally rotated olfactory bulbs (39) | There are two SM phenotypes: one typified by extreme brachycephalism and one by craniospinal junction deformation | |
| Occipital Crest | Association between reduced occipital crest and SM in CKCS, Affenpinscher and Chihuahua (40) | Suggests insufficiency of the supraocciptal bones and possibly the intraparietal bone |
| Frontal Sinus | Association between small frontal sinuses and SM in small breed dogs (55) | Suggests that SM may be related to rostrotentorial skull changes rather than being confined to a hind skull abnormality. |
| Caudal cranial fossa volume | CKCS with CM and SM have a shallower and smaller volume caudal cranial fossa compared to CKCS with CM only and other control breeds (56, 57) | Smaller caudal cranial fossa volume predisposes caudal cranial fossa overcrowding |
| CKCS have a strong relationship between hindbrain volume and volume of the rostral part of the caudal cranial fossa and a weak relationship between hindbrain volume and volume of the caudal part of the caudal cranial fossa. In Labrador retrievers and other small breed dogs this relationship is reversed (56, 58) | Small breed dogs and Labrador retrievers compensate for variations in hindbrain volume by modifying growth of the occipital skull. In the CKCS, increased cerebellar size is not accommodated by increased occipital bone development and the tentorium cerebelli compensates by developing / remodeling in a rostral direction | |
| Occipital bone volume | No difference in volume of the occipital bones between CKCS (with and without SM) and French Bulldogs (59) | Does not support theory of occipital bone hypoplasia |
| Jugular foramina | CKCS with CM and SM have narrowed jugular foramina in comparison with CKCS with CM only (52, 60) | Venous narrowing at the jugular foramina associated with reduced skull base can lead to elevated venous pressure and impaired CSF absorption |
| Venous sinus volume | CKCS with CM and SM have reduced venous sinus volume in comparison with CKCS with CM only (61) | Reduced venous sinus volume could result in intracranial hypertension and impaired CSF absorption |