Inflammatory Intracranial Diseases: The Meningoencephalitides

4 Inflammatory Intracranial Diseases: The Meningoencephalitides



John H. Rossmeisl, Jr.



1. What is meningoencephalitis?

Meningoencephalitis is the generic term used to describe any inflammatory central nervous system (CNS) disorder involving the meninges (meningitis) and brain parenchyma (encephalitis). Because of the intimate anatomic relationship between the meninges and brain parenchyma, inflammatory intracranial diseases often involve both tissues, prompting the use of the inclusive term meningoencephalitis. Many of the inflammatory intracranial diseases can also originate in or extend to involve the spinal cord parenchyma and meninges, in which case the disease process would be referred to as meningoencephalomyelitis.


2. Are there specific clinical signs that should increase my clinical suspicion that meningoencephalitis is present?

The meningoencephalitides are typically acute in onset and, in the absence of specific therapy, progressive in nature. Ultimately, the clinical manifestations of inflammatory intracranial disease are dependent on the neuroanatomic location of the lesions. Classically, the animal with meningoencephalitis will present with evidence of multifocal or diffuse intracranial disease on the neurologic examination. However, one study reported that approximately 66% of dogs with inflammatory CNS disease presented with clinical signs referable to a focal neuroanatomic area. The results of the neurologic examination are rarely specific for the etiology of the meningoencephalitis.


The antemortem identification of the cause of the meningoencephalitis can be difficult, and has remained elusive in approximately one third of dogs in one study.


3. Do most animals with meningoencephalitis have systemic signs of illness?

Many animals with meningoencephalitis have no extraneural signs of disease. Dogs with viral, bacterial, and granulomatous meningoencephalitis (GME) have been reported to commonly have a fever and funduscopic abnormalities at presentation. Animals with CNS disease from fungal infections can also have extraneural signs of disease, notably in the respiratory, lymphatic, integumentary, and ocular systems. A complete ophthalmic examination is indicated in any animal with suspected meningoencephalitis.


4. What ancillary diagnostics aids are helpful in determining the cause of meningoencephalitis?

Results of the complete blood cell counts, biochemical profiles, and urinalyses rarely provide specific information, but are useful in screening for the presence of concurrent systemic disease. Cerebrospinal fluid (CSF) examination, often performed in conjunction with advanced intracranial imaging, serologic assays, and cultures, is the most commonly used method to establish an antemortem diagnosis of meningoencephalitis. In many cases, definitive diagnosis of meningoencephalitis requires histopathologic examination of brain tissue. Recent studies have shown that both computed tomography (CT)-guided and open surgical brain biopsy techniques are safe and valuable diagnostic tools.


5. Are there any abnormalities present in CSF that are specific for the cause of meningoencephalitis?

Animals with meningoencephalitis often will have a CSF pleocytosis, an increase in the number of leukocytes. The type and number of CSF leukocytes present may provide additional information regarding the specific cause, such as bacterial, viral, or fungal disease, but in most cases, the identification of pleocytosis is a nonspecific finding. Occasionally, analysis of CSF can provide a definitive diagnosis, as is the case with cytologic identification of cryptococcal organisms.


6. Besides routine analysis, should I save CSF for any other ancillary tests?

If CSF collection is performed, a small volume should be saved for the performance of immunologic, serologic, or microbiologic tests. Selection of these adjunctive tests should be based on the animal’s neurologic condition and results of the initial gross, biochemical, and cytologic examination of CSF.



BACTERIAL MENINGOENCEPHALITIS



7. What bacterial species are commonly associated with bacterial meningoencephalitis?

Streptococcus, Escherichia coli, Klebsiella, Nocardia, Actinomyces, and Staphylococcus are frequently identified aerobes in clinical cases. Various anaerobic species have also been incriminated.


8. How do animals typically acquire bacterial meningoencephalitis?

Small animals with bacterial meningoencephalitis usually acquire the causative organism from their own endogenous flora. CNS bacterial infection can occur through several general mechanisms: direct traumatic inoculation, hematogenous spread from a systemic septic focus, opportunistic colonization of mucosal surfaces, progression of an infected local anatomic structure (e.g., orbit, inner ear, sinus), or by congenital or acquired communication of the CSF with body surfaces.


9. Are systemic signs of infectious disease seen in cases of bacterial meningoencephalitis?

Yes. Systemic signs of illness, such as fever, vomiting, diarrhea, pulmonary crackles, and dyspnea can be variably present. The animal should be examined for signs of local infections in anatomic structures in proximity to the head. However, the absence of systemic signs of illness does not exclude a diagnosis of bacterial meningoencephalitis.


10. What are common neurologic manifestations of CNS bacterial infections?

Seizures, cranial nerve deficits, behavioral change, altered mentation, and cervical rigidity are common. Signs of focal intracranial disease may be present in cases of abscessation.


11. What are the most common laboratory abnormalities?

Results of a complete blood count can reveal an inflammatory leukogram, thrombocytopenia, or leukopenia. Serum biochemical abnormalities are present in 70% of dogs with bacterial meningoencephalitis, but are variable and nonspecific.


12. How is a diagnosis of bacterial meningoencephalitis made?

The diagnosis is most often made based on results of CSF analysis, which often reveals a moderate to severe neutrophilic pleocytosis. The further identification of degenerate neutrophils and the finding of intracellular bacterial organisms in CSF confirm the diagnosis. Gram stain preparations of CSF samples may facilitate visualization of bacteria and guide empirical therapy. Culture of CSF with these characteristics is always indicated, but is frequently negative. In one study, a positive antemortem CSF culture was obtained in only 13% of dogs with bacterial meningoencephalitis. CT and magnetic resonance (MR) imaging scans of the brain may also reveal changes consistent with inflammatory disease.


13. How should bacterial meningoencephalitis be treated?

When possible, treatment should be based on culture and sensitivity results. Considering the high frequency of negative CSF cultures despite known infections, treatment is often empiric. Ideally, broad-spectrum, bactericidal antibiotics with the ability to penetrate the blood-brain barrier should be chosen.


Metronidazole, fluoroquinolones, and potentiated sulfonamides are good empirical first choices. Successful treatment of brain abscessation requires both surgical drainage and chronic antimicrobial therapy.


14. What is the prognosis for animals with bacterial meningoencephalitis?

The prognosis associated with bacterial meningoencephalitis is variable. Many animals that survive experience residual neurologic dysfunction or clinical relapses.



THE RICKETTSIOSES



Rocky Mountain Spotted Fever



15. What is the causative agent of Rocky Mountain spotted fever (RMSF)?

RMSF is caused by the obligate intracellular parasite, Rickettsia rickettsii. RMSF is a tick-transmitted disease. Dermacentor tick species are primarily responsible for maintenance of sylvatic disease cycle and transmission to animals and humans in the United States.


16. What neurologic signs are seen in animals with RMSF?

Clinical evidence of neurologic disease occurs in approximately one third of dogs with RMSF. Seizures, alterations in consciousness, vestibular disease, and signs referable to spinal cord dysfunction (e.g., spinal hyperpathia, ataxia, paresis) are commonly noted. Systemic evidence of illness may or may not be present, and is reviewed in Chapter 69.


17. How does RMSF cause neurologic disease?

RMSF is endotheliotropic, with a predilection for small arteries and veins of all body systems. The resulting necrotizing vasculitis is responsible for the clinical and biochemical manifestations.


18. How is RMSF diagnosed?

Because RMSF is an acute clinical disease, initial diagnosis is often made based on tick exposure, physical examination, compatible clinical and laboratory abnormalities, and response to treatment. Serologic diagnosis is dependent on a single elevated serum titer, or demonstration of seroconversion between acute and convalescent serum samples. Polymerase chain reaction genetic testing has also been reportedly used as a diagnostic tool.


19. How is RMSF treated?

Tetracycline, doxycycline, chloramphenicol, and enrofloxacin are all effective antimicrobials for the treatment of RMSF. Antibiotic therapy should be continued for 2 to 3 weeks in animals with neurologic signs. In cases with severe multisystemic involvement, intensive supportive care and monitoring is also necessary.


20. What is the prognosis for animals with RMSF?

Animals with RMSF that have mild clinical signs or that are treated early in the course of the disease can often make a complete recovery. Most cases will demonstrate clinical improvement within 48 hours of initiation of treatment. In some cases of RMSF, continual neurologic deterioration is seen despite appropriate therapy. Rapidly progressive necrotizing meningoencephalitis, intracranial hemorrhage, or thrombosis of CNS vasculature may be responsible for these refractory neurologic signs. Some animals that survive may have residual neurologic deficits or experience a prolonged neurologic recovery.



Ehrlichiosis



21. What causes ehrlichiosis?

Several species of ehrlichial organisms can infect small animals. Most commonly, Ehrlichia canis has been incriminated in canine cases. Ehrilichial infections can occur throughout the year in endemic temperate areas, and clinical disease can be acute, subacute, or chronic in nature.


22. What percentage of dogs with ehrlichiosis will have neurologic signs?

Approximately 33% of dogs with ehrlichiosis will have neurologic signs, which are similar to those described for RMSF. Neurologic disease is primarily caused by inflammation and bleeding of meningeal vessels. Meningitis can be found on postmortem examination of many animals with ehrlichiosis, even in the absence of clinical signs.


23. What are the diagnostic tests available for ehrlichiosis?

Serologic testing, immunoblotting procedures, and genetic (polymerase chain reaction) testing are all used for diagnosis and to distinguish between the ehrlichial species capable of causing clinical disease. CSF analysis of animals with ehrlichiosis typically reveals mononuclear pleocytosis. Occasionally, ehrlichial morulae have been identified in CSF leukocytes. Serology can also be performed on CSF to confirm the diagnosis.


24. How is ehrlichiosis treated?

Tetracycline, doxycycline, oxytetracycline, imidocarb diproprionate, minocycline, and chloramphenicol can be used to treat ehrlichiosis. In some cases, concurrent usage of corticosteroids is necessary to control signs of polysystemic inflammation, and in particular meningitis, which likely has an immune-mediated component.


25. What is the prognosis for animals with neurologic disease attributable to ehrlichiosis?

The overall prognosis is good to fair with early recognition and appropriate treatment. Animals that have systemic evidence of overt bleeding diatheses may develop refractory neurologic signs.



INTRACRANIAL MYCOTIC INFECTIONS



Cryptococcosis



26. What organism causes cryptococcosis?

The dimorphic fungus, Cryptococcus neoformans var neoformans most often causes clinical disease. Several antigenic variants of cryptococcal species have also been reported to cause disease with lesser frequency.


27. How do animals acquire cryptococcosis?

In the warm climates of the southern and southeastern United States, cryptococcosis is ubiquitous in the environment, especially in areas where pigeon excrement is found. The presumed method of infection is via inhalation and hemolymphatic dissemination, or possibly through direct extension of the organism through the cribiform plate.


28. What are the common clinical signs associated with cryptococcosis?

Dogs with cryptococcosis are often young adult, are large breeds, and have presenting complaints referable to the eye or CNS disease. In dogs, neurologic signs are often multifocal and commonly involve the brain stem (vestibular dysfunction, facial paralysis, ataxia), forebrain (seizures, circling), cervical spinal cord (tetraparesis, spinal hyperpathia, ataxia), or optic nerves (optic neuritis).

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Jul 31, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Inflammatory Intracranial Diseases: The Meningoencephalitides

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