In contemporary veterinary medicine, magnetic resonance imaging (MRI) is an important tool in the diagnosis of neurological
problems.
However, MRI does not simply display the diagnosis. Other diagnostic tests, such as survey radiographs, myelogram, serologies,
cerebrospinal fluid analysis, electro-encephalography and histopathology still remain crucial in the thought process toward
the definitive diagnosis.
In a recent article, 76 percent of the dogs with inflammatory cerebrospinal fluid (CSF) had detectable MR brain lesions, while
24 percent did not (C.R. Lamb et al.: Magnetic Resonance Imaging Findings in 25 Dogs with Inflammatory Cerebrospinal Fluid,
Vet Radiol Ultrasound, Vol. 46, No. 1, 2005, pp 17-22). Two out of the 25 dogs with inflammatory CSF had a focal lesion, and
one of these patients had neoplasia (choroid plexus papilloma). Seventeen had multifocal or diffuse lesions throughout the
brain. The definitive and the tentative diagnoses for these patients included granulomatous meningoencephalitis, toxoplasmosis,
neosporosis, distemper, non-specific meningitis, necrotizing fasciitis and pug encephalitis. Six dogs out of 25 had no lesions
visible on MRI.
Photo 1 A (left), B: T2 (A) and post-gadolinium T1 (B) images of a 9-year-old Poodle with cervical pain. Hydrocephalus found
on magnetic resonance (MR) imaging, but no other detectable MR lesions. Interestingly, the CSF was inflammatory. The patient's
signs resolved on prednisolone/cyclosporine therapy.
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Some structural brain abnormalities are common features of different disease processes, and 28 percent of the dogs with inflammatory
CSF in the article showed a mass effect on MRI. Mass effects are not exclusively associated with brain tumors. They can be
a feature of several other conditions, such as vascular disease (G.B. Cherubini et al: Utility of Magnetic Resonance Imaging
for Distinguishing Neoplastic from Non-neoplastic Brain Lesions in Dogs and Cats, Vet Radiol Ultrasound, Vol. 46, No. 5, 2005,
pp 384-387) or some of the previously mentioned inflammatory diseases. Hydrocephalus is another non-specific finding of different
intracranial lesions. In addition to the congenital form found primarily in brachiocephalic breeds, hydrocephalus was found
in some dogs with inflammatory diseases in the first article and in 39 percent of the neoplastic and 31 percent of the non-neoplastic
conditions in the second article (Photo 1).
In general, the inflammatory lesions appeared hypointense on T1 and hyperintense on T2 sequences. The use of gadolinium contrast
material increased the sensitivity of MRI with inflammatory lesions in the first article and had a predictive value of 74
percent for neoplasia in the second article.
Follow-up MRI studies are not often performed in veterinary medicine because of the expense and the owners' concerns regarding
the anesthesia. It would be of diagnostic value to perform follow-up MRI examinations after certain treatments, tracking the
size, intensity and enhancement of the lesion(s).
As a summary, MRI is important but not the only diagnostic procedure that should be performed in a patient with intracranial
disease. It is important for the clinician to explain to the owner the significance of other neurological diagnostic tests
and the limitations of the MRI.
Dr. Nanai is a resident of the European College of Veterinary Neurology/Neurosurgery at the Animal Emergency and Referral
Center in Fort Pierce, Fla.
Dr. Lyman is a graduate of The Ohio State University College of Veterinary Medicine. He completed a formal internship at the
Animal Medical Center in New York City. Lyman is a co-author of chapters in the 2000 editions of Kirk's Current Veterinary
Therapy XIII and Quick Reference to Veterinary Medicine.
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