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Haematology may sometimes provide evidence of systemic infection (e.g. alterations in white blood-cell count), and serum biochemistry may reveal changes consistent with involvement of other organs. Cerebrospinal fluid analysis often reveals increased white blood-cell (WBC) count (pleocytosis) and increased protein concentration. CSF pleocytosis has been classified as mild (6–50 WBC/μl), moderate (51–200 WBC/μl), or marked (>200 WBC/μl), and as mononuclear, neutrophilic, eosinophilic, or mixed based on the predominant cell type on cytological examination (Tipold, 2003).

The type of CSF pleocytosis may be suggestive of a particular aetiology or disease group (Table 5.2).

CSF may be normal when the CNS inflammation does not involve the leptomeninges or the ependymal lining of the ventricular system or if the animal has been treated with anti-inflammatory medications (particularly corticosteroids) prior to CSF collection. Additional tests on CSF (such as polymerase chain reaction (PCR), antibody or antigen titres, immunofluorescence and culture) can help to reach an aetiologic diagnosis. Occasionaly, certain microorganisms (e.g. bacteria, Ehrlichia morulae, fungi, protozoa or parasites) can be visualized on CSF cytology.

Table 5.2. Cerebrospinal fluid characteristics of canine and feline inflammatory CNS disease.

Disease	Type of pleocytosis	Total protein concentration
Viral meningoencephalitis (CDV or other; FIP excluded)	None or mild to moderate mononuclear	Normal to markedly elevated
FIP meningoencephalitis	Moderate to marked neutrophilic or mixed, occasionally eosinophilic	Markedly elevated
Rickettsial meningoencephalitis	Mild to moderate mononuclear or mixed. Can be neutrophilic with granulocytic ehrlichiosis or anaplasmosis	Mildly to markedly elevated
Bacterial meningoencephalitis	Moderate to marked neutrophilic (toxic changes in cell morphology) in acute and subacute infections; mixed in chronic infections; sometimes mononuclear following antibiotic treatment	Mildly to markedly elevated
SRMA	Moderate to marked neutrophilic in acute SRMA, mononuclear or mixed in chronic SRMA	Mildly to markedly elevated
Protozoal meningoencephalitis	Moderate mixed, occasionally eosinophilic, rarely mononuclear	Mildly to markedly elevated
Fungal meningoencephalitis	Moderate to marked mixed, occasionally eosinophilic	Markedly elevated
Algal (Prototheca)	Moderate to marked mixed or eosinophilic	Markedly elevated
Parasitic meningoencephalitis	Mild to moderate mixed, often eosinophilic	Mildly to markedly elevated
GME	None or moderate to marked mononuclear or mixed	Mildly to markedly elevated
Necrotizing meningoencepahlitis/ leukoencephalitis	Mild to marked mononuclear	Mildly elevated
Eosinophilic meningoencephalitis	Mild to marked eosinophilic	Mildly to markedly elevated

Inflammatory CNS disease can cause increased intracranial pressure (ICP) and CSF collection may be contraindicated due to the risk of cerebral herniation and death. Progression from obtundation to stupor, a diminished or absent vestibulo-ocular reflex, the development of unilateral or bilateral midriasis and loss of the pupillary light reflexes are suggestive of increased ICP and transtentorial brain herniation. Any time increased ICP is suspected, MRI of the brain should be performed before considering CSF collection.

MRI of the brain can reveal changes suggestive of inflammatory CNS disease such as multifocal, diffuse or sometimes focal lesions within the brain parenchyma that typically appear hyperintense on T2-weighted and FLAIR images, iso- to hypointense on T1-weighted images and show variable contrast enhancement sometimes with meningeal involvement following administration of contrast medium. The MRI findings sometimes may support the ante-mortem diagnosis of a particular aetiology (e.g. FIP). The MRI features of various inflammatory CNS diseases have been reviewed (Hecht and Adams, 2010b). Sensitivity and specificity of high-field MRI for classifying brain diseases as inflammatory are 86.0 and 93.1% without provision of clinical data and 80.7 and 95.4% with provision of clinical data, respectively (Wolff et al., 2012). Sensitivity and specificity of high-field MRI are lower for detection of specific inflammatory CNS aetiologies (Wolff et al., 2012).