146 Med. Weter. 2015, 71 (3), 146-151 Artykuł przeglądowy Review Clinical anatomy of syringomyelia and Chiari malformation in dogs NORBERT CZUBAJ, MICHAŁ SKIBNIEWSKI, MARTA KUPCZYŃSKA, KAROLINA BARSZCZ, WOJCIECH SOKOŁOWSKI Department of Morphological Sciences, Warsaw University of Life Sciences – SGGW, Nowoursynowska 159, 02-776 Warszawa Received 19.11.2013 Accepted 05.03.2014 Czubaj N., Skibniewski M., Kupczyńska M., Barszcz K., Sokołowski W. Clinical anatomy of syringomyelia and Chiari malformation in dogs Summary Syringomyelia (SM) is a rare disorder characterized by the development of fluid filled cavities (syrinxes) in the spinal cord parenchyma which occurs secondarily to an obstruction of cerebrospinal fluid (CSF) outflow at the foramen magnum level. The precise pathophysiology of SM is still not fully understood. The most common predisposing cause in dogs is a Chiari-like malformation (CM). This disease is defined as a developmental failure of the occipital bone leading to overcrowding of the caudal cranial fossa. Abnormally small bony structures contain rhomencephalon structures (cerebellum and medulla oblongata) which are unchanged in size. This “mismatch” causes caudal displacement of encephalon structures via the foramen magnum, altering the cerebrospinal fluid flow. CM is thought to be associated with a higher grade of brachycephaly. The SM/ CM complex is very common in Cavalier King Charles Spaniel (CKCS) breed. Keywords: Chiari malformation, syringomyelia, occipital hypoplasia, brachycephaly Chiari-like malformation syndrome, or simply in Cavalier King Charles Spaniels (CKCS), Brussels Chiari malformation (CM), is a morphological abnor- Griffons and Yorkshire Terriers (4, 23, 40). mality of the caudal fossa of the cranial cavity. It is The aim of the work is a review of the research re- described as a mismatch in volume between encepha- sults concerning the influence of morphological traits lon structures and caudal fossa, which leads to dis- on the pathogenesis of the above mentioned diseases placement of rhombencephalon parenchyma towards and the presentation of the current knowledge about the vertebral canal (6). These changes are sometimes the hydrodynamics of the cerebrospinal fluid. described as the cerebellar herniation (7, 10, 24, 41). Consequently, some disturbances occur at the level of An outline of the morphology of the cranial cavity the foramen magnum in the flow of cerebrospinal fluid in dogs from the perspective of syringomyelia and (CSF). Aleteration of CSF flow dynamics may lead Chiari malformation to accumulation of tissue fluid in the spinal cord and The shape of the cranial cavity and the internal eventually cause the development of cavities (syrinxes) surface of bones forming it are “sculptured” by the in its parenchyma (21, 32, 33, 40, 47). This state is respective structures of the brain. Three main structures known as syringomyelia (SM) (8, 11, 22, 34, 36, 38). can be distinguished in its area, gradually declining The above mentioned disease entities are mainly diag- towards the back: rostral cranial fossa (fossa cranii nosed in people. They are more and more frequently rostralis), middle cranial fossa (fossa cranii media) diagnosed due to the development of the techniques and caudal cranial fossa (fossa cranii caudalis). Their of diagnostic imagining. One of them is MRI, which “inhabitants” are respectively: olfactory bulbs (bulbus allows the detection of relatively small changes with olfactorius) and the most frontal descriptive elements a symptomless course (14, 19). Despite numerous of the hemispheres, in the next – the remaining part clinical observations and experiments performed on of the telencephalon together with diencephalon and animals, the pathogenesis of SM is not fully explained mesencephalon, and in the last part – metencephalon (26, 27, 49). These disturbances are also observed together with myelencephalon, i.e. rhombencephalon in dogs, mainly in brachycephalic breeds and the so s. medulla oblongata (Fig. 1). The caudal cranial fossa called toy breeds. They were described, among others, is located over the basilar part of the occipital bone Med. Weter. 2015, 71 (3), 146-151 147 (pars basilaris ossis occipita- lis s. basiooccipitale). On the sides it is limited by the petrous parts of the temporal bones (partes petrosae ossis tempo- ralis), and from the back by the squamous part of the occipital bone (squama occipitalis). The bottom vault is formed by the osseous cerebellar tentorium (tentorium cerebelli osseum), and its symbolic frontal limit is the line running from the men- tioned structure to the rostrally situated dorsum sellae. On the ventral surface of the petrous part (facies ventralis partis Fig. 1. Medial aspect of sagittal section of a dog skull. 1 – rostral cranial fossa, 2 – middle petrosae) there is a petrooc- cranial fossa, 3 – caudal cranial fossa cipital fissure (fissura petrooc- cipitale). That structure is in direct contact with the jugular foramen (foramen jugulare) located on the external surface of the base of the skull (Fig. 2). Through that foramen passes the internal carotid artery (a. carotis interna) and internal jugular vein (v. jugularis in- terna) which is the main drain- age of blood from the brain. The back border of the caudal cranial fossa is formed by the foramen magnum. From the top, that structure is limited by the squamous part of the occpital bone and on the sides Fig. 2. Ventral aspect of a dog skull. 1 – jugular foramen by lateral parts of the occipital bone (partes laterales ossis occipitalis s. exoccipitale). the vertebral canal and the differences connected with The lower border of the foramen magnum is formed it in the course of the disease. It should be stressed that by the basilar part of the above mentioned bone. The the gradation of pathology within types I–III worked foramen magnum marks the arbitrary border between out by Hans von Chiari was universally accepted and the medulla oblongata and spinal cord. The develop- is still used. However, many authors do not accept mental disturbances concerning the presented osseous type IV. In 1894, German pathomorphologist Julius structures forming the caudal cranial fossa and fora- Arnold described a case of an infant with the type II men magnum results in the misfitting of the brain’s defined by Chiari. On account of that, for a long time anatomical elements and appearance of the discussed the above discussed pathological state was described disease entities. as the Arnold-Chiari syndrome (37, 50). The more poorly expressed type I characterizes the Chiari and Chiari-like malformation displacement of medulla oblongata into the vertebral In 1981 an Austrian pathomorphologist Hans von canal and the herniation of the cerebellar tonsils (ton- Chiari published a monographic description of human silla cerebelli) into the foramen magnum. It appears brain developmental disturbances which were charac- mainly in 20-30-year-old people, thus it is called the terized by the displacement of the elements of rhomb- Chiari malformation of adults (2, 25). Symptoms indi- encephalon towards the vertebral canal. He classified cating type II malformation are present already at birth. the observed changes as type I, II and III disturbances, Anatomically a herniation of the vermis of the cerebel- and five years later he also described type IV (13). The lum, pons and fourth ventricle (ventriculus quartus) division was based on the analysis of the degree of her- into the foramen magnum and the medulla oblongata niation of particular rhombencephalon structures into into the vertebral canal are observed. Most often that 148 Med. Weter. 2015, 71 (3), 146-151 pathology coexists with other developmental disorders, They also noted that the characteristic dorsal incisure such as rachischisis (13, 37). Type III is characterized increasing the foramen magnum in all cases was cov- by a displacement of the entire cerebellum into the ered by a connective tissue layer. At present, it is still foramen magnum and vertebral canal. The described debated whether dorsal notch is a pathology or only by Chiari type IV consists in cerebellar hypoplasia with an expression of an anatomical variability within the rhombencephalon located within the posterior cranial foramen magnum (16). fossa which topographically corresponds to caudal Apart from the above described changes, the base of cranial fossa in animals (13). the skull becomes shortened in the brachycephalic dogs It is assumed that the most often observed type I, also (40). This is probably connected with the premature called classic, results from the hypoplasia of the basilar closure of the junctions between basilar parts of the part of the occipital bone which consequently causes presphenoid, basisphenoid and occipital bones (44). the volume decrease of the posterior cranial fossa. It At the same time some changes take place within the leads to the “overfilling” of the space of the fossa and cranial vault. A compensatory growth of the parietal squeezing of the cerebellum into the foramen magnum bone which forms space for the developing brain can (1, 25, 29, 30). Changes similar to those observed be observed. As a result it becomes rounded and clearly in the type I malformation in humans are also noted widened (40). Similar changes in the spatial arrange- in dogs, thus in veterinary medicine they are defined ment of the cranial bones were described in humans as the Chiari-like malformation. However, the name is with the Chiari syndrome (35). doubtful because the lesions