Occipital dysplasia

From Dog

Occipital dysplasia is a rare neurological disease of dogs[1].

Occipital dysplasia refers to an abnormally large foramen magnum, resulting from a defect in development of the occipital bone (incomplete ossification of the ventromedial part of the supraoccipital bone[2], has been described in small/medium and toy breed dogs that are often brachycephalic, including Yorkshire Terrier, Pomeranian, Maltese Terrier, Chihuahuas, Pekingese, as well as in Miniature and Toy Poodle, Miniature Keeshond, and Beagle[3][4][5].

Pathology

The abnormality, which is readily revealed by frontal radiographs of the skull, consists of a key shaped dorsal midline extension of the foramen magnum into the occipital bone. In one morphometric radiographic study of skulls from 80 Pekingese dogs (75 adult and 5 juvenile), the shape of the foramen varied from ovoid to rectangular and the dorsal notch was observed in all but 2 skulls[6]. Variability in the area of the foramen was mainly correlated with total height of the foramen, including the dorsal notch. The foramen magnum index (the ratio between the maximum width and the total height of the foramen) was not significantly correlated with age, but was significantly larger in female dogs. It was concluded that the large variability in the shape and size of the foramen magnum and the absence of any neurological problems in dogs of this study indicated that the dorsal notch of the foramen magnum in brachycephalic dogs is a normal morphological variation, rather than a pathological condition. This has been confirmed by other studies[7]. In another morphometric study involving German Shepherd puppies, occipital dysplasia was not found[8].

Clinical signs

The absence of neurological deficits in animals of the above-mentioned studies is consistent with earlier reports of occipital dysplasia being a subclinical (or nonclinical) condition[9], and that presence of neurological signs such as ataxia, cervico-occipital pain, personality changes, convulsions, pawing at the side of the face, ear or neck, protrusion of the tongue, and dysphagia in dogs with this malformation reflects some other underlying condition, such as hydrocephalus.

A more recent report suggests that intramedullary CNS abnormalities, such as hydrosyringomyelia, may be present concurrently with occipital dysplasia and should be considered as a possible cause of clinical signs such as cervical hyperesthesia and paresis/tetraparesis[10]. Occipital dysplasia and hydrosyringomyelia are sometimes seen in people with Chiari malformations. Interestingly, some of the signs originally described by Bardens[11], especially the cervical pain and frequent scratching, have been reported in dogs with Chiari I malformation, usually with accompanying hydrosyringomyelia.

It is has also been suggested that there may be an increased potential for herniation of the cerebellum or brainstem through the enlarged foramen magnum[12], although such prolapse is normally prevented by a fibrous membrane (dura mater and connective tissue) covering the dorsal notch. In the report by Bagley and colleagues, this membrane appeared to compress the underlying spinal cord and brainstem in one dog[13].

References

  1. Vite, Ch (2004) Developmental disorders. In: Braund's Clinical Neurology in Small Animals: Localization, Diagnosis and Treatment. IVIS, Ithaca, New York, USA
  2. Watson AG, de Lahunta A, Evans HE. (1989) Dorsal notch of foramen magnum due to incomplete ossification of supraoccipital bone in dogs. J Small Anim Pract 30:666-673
  3. Parker AJ, Park RD. (1974) Occipital dysplasia in the dog. J Am Anim Hosp Assoc 10:520-525
  4. Parker AJ, Park RD. (1974) Unusual deformity of the occipital bone in a dog (a case report). Vet Med Small Anim Clin 69:438-441
  5. Kelly JH. (1975) Occipital dysplasia and hydrocephalus in a Toy Poodle. Vet Med Small Anim Clin 70:940-941
  6. Simoens P, Poels P, Lauwers H. (1994) Morphometric analysis of the foramen magnum in Pekingese dogs. Am J Vet Res 55:34-39
  7. Watson AG, de Lahunta A, Evans HE. (1989) Dorsal notch of foramen magnum due to incomplete ossification of supraoccipital bone in dogs. J Small Anim Pract 30:666-673
  8. Onar V, Mutus R, Kahvecioglu KO. (1997) Morphometric analysis of the foramen magnum in German Shepherd dogs (Alsatians). Ann Anat 179:563-568
  9. de Lahunta A. (1983) Veterinary Neuroanatomy and Clinical Neurology. 2nd ed. Philadelphia: WB Saunders Co, pp:6-29
  10. Bagley RS, Harrington ML, Tucker RL, et al (1996) Occipital dysplasia and associated cranial spinal cord abnormalities in two dogs. Vet Radiol Ultrasound 37:359-362
  11. Bardens JW. (1965) Congenital malformation of the foramen magnum in dogs. Southwest Vet Summer:295-298
  12. Colter SB. (1993) Congenital anomalies of the spine. In: Bojrab MJ, ed. Disease Mechanisms in Small Animal Surgery. 2nd ed. Philadelphia: Lea & Febiger, pp:950-959
  13. Bagley RS, Harrington ML, Tucker RL, et al (1996) Occipital dysplasia and associated cranial spinal cord abnormalities in two dogs. Vet Radiol Ultrasound 37:359-362