Chiari-like malformation (CM) is the canine analog of Chiari type I malformation of people. It is typically associated with the development of fluid cavitations within the spinal cord-syringomyelia (SM) - so the combined condition is often abbreviated as CM/SM. This disease is almost exclusive to small breed dogs, with the Cavalier King Charles spaniel (CKCS) being the most over-represented. There is convincing evidence in the CKCS breed that CM/SM is a heritable disease, most likely autosomal recessive with incomplete penetrance. The disorder is a congenital malformation involving the caudal occipital region of the skull, leading to overcrowding of the caudal fossa and compression of the cervicomedullary junction at the level of the foramen magnum. There is recent evidence that CM involves a malformation of the entire skull, with the intracranial volume being too small to accommodate the intracranial contents. In CM/SM, as in Chiari type I of people, the caudal aspect of the cerebellum is often projecting into or through (herniation) the foramen magnum, contributing to obstruction of CSF flow between intracranial and spinal compartments. Progressive alterations in pressure dynamics between the intracranial and spinal compartments are believed to be responsible for the development of clinical signs of CM/SM.

The typical age range at presentation appears to have changed over time, with many dogs developing clinical signs within the first year of life. In general, though the age range at clinical presentation is broad, most dogs present by the time they are 4 years old. Dogs that are presented at less than 2 years of age often have more severe clinical signs than older dogs. In recent years, there have been an increasing number of younger patients (< 1 year of age. Similar to Chiari type I of humans, there is a wide spectrum of possible neurologic presentations for dogs with CM/SM, including cervical myelopathy, cerebellovestibular dysfunction, and forebrain dysfunction (e.g., seizure activity). By far, evidence of cervical dysfunction and cerebellovestibular dysfunction are the most common and are often both present (e.g., multifocal CNS disease). Most of the CM/SM cases that the author encounters are presented for signs referable to the cervical region (e.g., neck pain, scratching activity) and subtle signs of central vestibular dysfunction are apparent on neurologic examination. Some specific clinical findings in dogs with CM/SM include cervical and cranial hyperesthesia, decreased menace responses with normal vision, positional ventrolateral strabismus, thoracic limb weakness, pelvic limb ataxia, persistent scratching (at the head, neck, and shoulder region - often without making skin contact), scoliosis, facial nerve paresis/paralysis (unilateral or bilateral), and hearing abnormalities. The persistent scratching activity and scoliosis are fairly unique clinical signs associated with syringomyelia. Scratching activity and neck discomfort often are exacerbated by abrupt weather changes, stress or excitement, and physical contact with the neck/shoulder region (e.g., collar).

The severity and rate of progression of CM/SM in dogs is variable, ranging from asymptomatic (i.e., finding evidence of CM/SM while imaging for some other reason) to extreme pain and debilitation with rapid worsening. In addition, some dogs with CM/SM have other concurrent disorders (e.g., disk extrusion, inflammatory brain disease) that could explain observed clinical signs. In such situations, it may be difficult to discern if the CM/SM is the main problem, contributory, or an incidental finding. Diagnosis of CM is made by MR imaging. Magnetic resonance imaging is also the preferred imaging modality for diagnosing syringomyelia. The malformation is best visualized on a midsagittal view (preferably T2-weighted), which includes the caudal fossa and cranial cervical cord. Consistent findings on MR imaging indicative of CM are attenuation/obliteration of the dorsal subarachnoid space at the cervicomedullary junction and rostral displacement of the caudal cerebellum by the occipital bone. Other common MRI findings in CM include syringomyelia (usually C2 level caudally), herniation of the caudal cerebellum through the foramen magnum, and a "kinked" appearance of the caudal medulla. Recently, the width of cervical syrinxes as measured on axial MR images was positively correlated with presence of pain in CKCS dogs with CM/SM. Occasionally, dogs with MRI findings consistent with CM/SM will have evidence of other congenital disorders such as intracranial arachnoid (quadrigeminal) cyst, malformation of the C1 and or C2 vertebrae, and hydrocephalus. In the absence of concurrent disease processes, CSF analysis is usually normal; occasionally, however, a mild mononuclear pleocytosis will be apparent. Treatment of CM/SM can be divided into medical and surgical therapy. In people with symptomatic Chiari type I malformation, surgical therapy is considered the treatment of choice, with foramen magnum decompression (FMD) being the preferred surgical procedure. Medical therapy for dogs with CM/SM generally falls into three categories: analgesic drugs (implies relief of dysesthesia / paresthesia also), drugs that decrease CSF production, and corticosteroid therapy. By far the most useful drug available for relief of scratching activity associated with syringomyelia has been gabapentin (10 mg/kg body weight PO, q 8 hrs). The author and colleagues have been using pregabalin more frequently to treat the pain and scratching activity associated with CM/SM. I have also had success using oral tramadol (2–4 mg/kg, q 8–12 hrs). A number of drugs aimed at decreasing CSF production have been used in CM/SM patients, in an effort to diminish the CSF pulse pressure. All information regarding efficacy of these drugs is anecdotal. They include omeprazole, acetazolamide and furosemide. Corticosteroids are often used in medical management of CM/SM. The preferred surgical procedure for treatment of CM/SM in dogs is FMD with cranioplasty. There is little information regarding the prognosis for CM/SM in dogs. Most dogs with CM/SM will respond favorably to medical therapy, although in many cases this response is temporary. In one group of 10 CM/SM dogs treated medically, 5 dogs (50%) were euthanized within 2–3 years due to disease progression and diminished responsiveness to therapy. In another report, 5 of 14 dogs (36%) with the disorder treated medically were eventually euthanized due to disease progression.

An abnormality at the craniocervical junction in small and toy breed dogs has recently been described called atlanto-occipital overlapping (AOO). In this malformation, the atlas (C1) is cranially displaced into the foramen magnum, and there is overlap of the occipital bone and the atlas. This displacement tends to compress the caudal aspect of the cerebellum and elevate and compress the caudal medulla (medullary kinking). AOO is likely a form of basilar invagination, a human craniocervical junction disorder in which the atlas and/or axis (C2) telescope toward the foramen magnum. This disorder can occur as a sole entity and in combination with CM and/or atlantoaxial (AA) instability. Syringomyelia has been associated with AOO, whether as a sole malformation or part of several craniocervical malformations in the same patient. Since bony detail is difficult to distinguish on MR images, it is likely that atlanto-occipital overlapping has been under-diagnosed in dogs, the majority of these patients being incorrectly ascribed a diagnosis of CM. The precise nature of this and other craniocervical malformations is typically apparent on CT images. The author and colleagues routinely follow most craniocervical junction abnormality MR images with CT imaging in order to fully evaluate the malformation or malformations in the region. Clinical signs in dogs with AOO are typically neck pain and varying degrees of ataxia of all four limbs. There have not been enough reported cases of AOO in the literature to make general recommendations concerning therapy. Most of the dogs with this malformation will respond temporarily to medical management, similar to dogs with CM. Other dogs have required surgical stabilization for relief of clinical signs. Both ventral and dorsal methods of stabilization of the atlanto-occipital region have been described in people for basilar invagination, and the author has adapted these methods to several canine patients with favorable results.

Similar to AOO seen in small and toy breed dogs, the author and colleagues have seen a large number of dogs with dorsal compression at the level of C1/C2. This compression varies in severity, with some dogs having a mild divot in the dorsal subarachnoid space, others having severe cervical spinal cord compression. At surgery, the majority of this compressive mass appears to be soft tissue, but some have a bony component. It is thought that this disorder is also a manifestation of instability at the C1/C2 junction, possibly a form of basilar invagination like AOO. This can occur as a sole entity or in combination with CM and/or atlantoaxial (AA) instability. In cases of combined AA instability and C1/C2 dorsal constriction, the author has pursued combined ventral and dorsal approaches in order to surgically address both issues. Similar to AOO in dogs, the optimum treatment strategy for dorsal constriction at C1/C2 has not yet been established. However, due to its compressive nature, this is most likely a surgical disease.

AA instability is usually caused by hypoplasia or aplasia of the dens. Abnormal ligamentous support of the dens may also be involved. This problem is usually seen in miniature and toy dogs, often less than two years of age, but has been reported in older dogs and larger dog breeds. Commonly reported breeds with AA instability include Yorkshire terrier, Pomeranian, Miniature and Toy Poodle, Chihuahua, and Pekingese. It occasionally occurs in cats. The instability may lead to dorsal subluxation or luxation of the axis, with resultant compression of the cranial cervical spinal cord. Associated malformations of the atlas and/or occipital bones may be observed in some patients. Syringomyelia may be evident on MR imaging. Clinical signs can vary from neck pain with no neurologic deficits to tetraplegia with respiratory difficulty. In most cases, AA instability is apparent on lateral radiographs of the neck. Stressed views may be used to demonstrate AA instability, but they must be performed with caution. MR imaging is both safer for this disorder and is also preferable to radiographs in that concurrent craniocervical junction abnormalities will be apparent if present. AA instability is sometimes treated non-surgically, often involving external splinting of the neck, with or without administration of anti-inflammatory drugs (e.g., prednisone). The author believes that surgery is usually recommended for these cases and a ventral approach is typically chosen.