Etiology and Pathogenesis

Despite its recognition as a clinical entity for over 40 years, the underlying cause and mechanism of disease production remains unclear. Genetic, autoimmune, infectious, neoplastic, and even toxic causes have been theorized. Previous work has demonstrated that females are predisposed to GME; this finding is similar to other autoimmune demyelinating diseases including multiple sclerosis and experimental autoimmune encephalitis.79,92,92 The explanation of the female predisposition for autoimmune CNS diseases is unclear; however, a connection between sex steroid–associated alterations in T-helper cytokines, suppression of regulatory cytokines, and X-chromosome susceptibility alleles may be involved.^79,92

GME has been proposed by some to be a delayed-type hypersensitivity reaction with an autoimmune basis, supported by a predominance of major histocompatibility complex class 2 and CD3+ T cells.¹⁰¹ Other investigators have subsequently confirmed a predominance of CD3+ T lymphocytes and a complete absence of the CD79 (B-cell marker) immunoreactivity in four cases of GME.¹⁸² However, these investigators were unable to demonstrate statistical differences in the number of CD3+ cells between GME and necrotizing meningoencephalitis (NME) or GME and central malignant histiocytosis.^181,182 Results of preliminary immunophenotyping studies indicate a consistent pattern among disseminated GME cases.⁸³ Anti-astrocytic antibodies also have been identified in the CSF of dogs with GME.¹²⁹ The complete immunologic profile of GME and whether CNS autoantibodies are the cause or consequence of inflammation remain to be elucidated.

Despite the conventional view that GME is a disorder of immune dysregulation, some veterinary neuropathologists suggest that GME is a lymphoproliferative disorder, with features of both inflammation and neoplasia.¹⁸⁰ Focal GME is particularly similar to neoplasia, because lymphocytes within the perivascular cuffs often have variable degrees of pleomorphism and mitotic indices.⁵⁷ Interestingly, CSF from disseminated GME cases occasionally contains lymphoblasts.¹⁶⁰ It is unclear whether the abnormal lymphocytes within brain lesions or CSF are reactive inflammatory cells or representative of a true neoplastic population.

Potential infectious triggers for GME have also been considered and investigated.159,161 Borna disease virus was reported in several dogs with meningoencephalitis in Japan and Switzerland and has been proposed as a causative agent for GME.^137,211 Borna disease virus, however, is unlikely to be a common etiology for GME, given its predilection for CNS gray matter. Other investigators¹⁶³ have demonstrated positive immunohistochemistry for West Nile, canine parainfluenza, and encephalomyocarditis viruses in severe lesions of dogs with meningoencephalitides of presumed unknown origin. The significance of these observations is unclear, because the positive immunohistochemistry may be caused by antibody cross-reactivity with endogenous proteins, as has been described with measles infections.¹⁶⁸ Alternatively, these results may support the theory that GME is a nonspecific hyperinflammatory response of the host to various antigens, of which pathogens comprise an important subset.

To date, molecular investigations led by one of the authors of this chapter (SJS) have failed to disclose consistent infectious agents associated with GME; however, work is ongoing to investigate an extremely diverse group of potential viral, bacterial, and rickettsial triggers.¹⁵⁹ Further investigation has identified Bartonella spp. and Mycoplasma spp. in sporadic, confirmed cases of disseminated GME.¹¹ GME is most likely a nonspecific immunologic response, and multiple environmental triggers (pathogens, vaccines) and genetic factors are likely to play roles in the etiopathogenesis.

Clinical Findings

The disease occurs most often in young to middle-aged small purebred females, but dogs of different ages, breeds, and sex can develop the disease. The prevalence for smaller breeds, especially poodles, small terriers, Pekingese, and Maltese, appears to be higher.²⁴ The disease rarely occurs before the age of 2 years. Clinical signs usually have an acute onset, and the disease typically is progressive. In acute GME, clinical signs develop rapidly over the course of 1 to 10 days, and without treatment, it is rapidly progressive and fatal. Clinical signs also may develop more slowly over a period of 1 to 4 months. Characteristic neurologic signs with GME cannot be expected because lesions can occur anywhere in the CNS. Signs are associated with the site of most extensive damage, or may reflect a multifocal lesion. Signs consist of disturbances in gait and postural reactions, with ataxia, dysmetria, paresis, and paralysis, and changes in mental status, with confusion, lethargy, and coma. Cranial nerve dysfunction may also be observed.

Despite the nonspecific features of the CNS signs, some predictable findings may be observed. In the acute stage of the disease, more than 50% of animals have fever, and many exhibit paraspinal discomfort on palpation¹⁷⁴ or signs of cervical pain.^41,175 In this stage of the disease, central vestibular signs are common (Fig. 82-1); seizures also may be observed. The clinical presentation alternatively may suggest a space-occupying mass referring to a single site of involvement, most frequently the posterior fossa, with brainstem and cerebellar signs. Cerebral or spinal signs sometimes predominate. A particular ocular form of GME, previously called ocular reticulosis, occurs with acute unilateral or bilateral blindness and ophthalmoscopic evidence of optic neuritis.^24,103

Typical hematologic and biochemical findings are not observed. Neutrophilia may be present in the acute phase of disease.174,175 The CSF analysis is abnormal, with a marked pleocytosis; cell counts of 100 to several thousand leukocytes per microliter are usual. Mild pleocytosis or even a normal cell count is observed in animals pretreated with glucocorticoids. Mononuclear cells predominate, but a mixed pleocytosis can also be observed with varying numbers of neutrophils.¹⁰ Cytologic examination of the CSF may reveal large anaplastic mononuclear cells with abundant lacy cytoplasm,²⁴ and up to 20% are macrophages. Protein concentration usually is increased, with a range from reference values up to 1119 mg/dL and a mean of 163.2 ± 25 mg/dL.¹⁰ Electrophoretic examination of the CSF may suggest blood-brain barrier disruption and intrathecal immunoglobulin synthesis.^174,175 Such changes are not specific for GME. Electroencephalographic patterns studied in some dogs with GME were nonspecific.

Pathologic Findings

Lesions are found in the meninges, brain, and spinal cord, especially in the cervical segments. White matter often is more severely affected compared with gray matter. Although the lesions are disseminated, the cerebral hemispheres, the midbrain, and the region around the fourth ventricle are most frequently involved. The lesions, which are strictly associated with the vasculature, consist of perivascular cuffing of monocytes, lymphocytes, plasma cells, and sometimes a few neutrophils and mast cells. The formation of eccentrically situated nodular foci of macrophages within these cuffs is a characteristic finding in GME (Fig. 82-2). The cuffs can be distinguished in three degrees of severity: small disseminated perivascular lesions, confluence of several perivascular cuffs into small granulomas, and large focal coalescing granulomas, leading to the formation of tumorlike lesions. Additionally, necrosis of the neuropil, mitosis in the perivascular cuffs, and edema may be observed in both forms of the disease. Tryptase-positive mast cells can be found in perivascular cuffs of the lesions of GME, in the meninges, and in the CNS parenchyma. Dogs with the acute disease have significantly more mast cells compared to dogs with chronic disease, suggesting that this cell population may contribute to the dynamics of the lesion with resultant CNS injury, and to the rapid clinical deterioration of dogs with GME.⁵² Anti-astrocyte antibodies are increased in the CSF of dogs with GME and NME but not with other inflammatory conditions of the CNS.¹²⁹ Autoantibody was also detected in some dogs with brain tumors, but this may represent only a secondary response to a damaged blood-CNS barrier.

Therapy

No specific therapy for GME exists, although improvement frequently has been observed after administration of glucocorticoids or diverse immunosuppressive drugs (Table 82-1). Because a definitive antemortem diagnosis of GME is difficult to make, treatment studies have been cumbersome. Because glucocorticoid therapy provides only temporary improvement in many cases, other immunosuppressive drugs have been used. The use of procarbazine or cytosine arabinoside, two alternative antineoplastic drugs, may be considered.^45,136 Procarbazine is a prodrug that forms numerous active cytotoxic metabolites after metabolism. This DNA-alkylating agent is an anticancer drug that has also been used in human transplant cases. It can be used alone or in combination to treat presumptive GME.^45,223

Procarbazine is well absorbed orally and may cause neurotoxicity, vomiting, or hepatic dysfunction. The patient should be monitored closely for cytotoxicity, and after 1 month of daily therapy, every-other-day dosing should be used. Cytosine arabinoside, a parenteral drug, is also effective, has few side effects, and is reasonably priced.⁴⁵ Because of its parenteral route, it is used primarily for induction therapy in the hospital. Both drugs cause myelosuppression, and animals must be monitored for signs of toxicity. Leflunomide is a pyrimidine-synthesis inhibitor that suppresses activated T cells. It has been used as an alternative treatment because of side effects or lack of responsiveness when other drugs are used. Leflunomide can affect cell replication, causing leukopenia, diarrhea, or alopecia in humans. Mycophenolate mofetil has been used to treat dogs with various immune-mediated diseases such as pemphigus foliaceus and myasthenia gravis. It inhibits an enzyme in purine synthesis, which in turn suppresses lymphocyte function. It causes minimal myelosuppression. Mycophenolate mofetil is expensive; however, it has a low degree of toxicity, which is usually associated with vomiting or diarrhea. Ciclosporin has shown to be of benefit in treating dogs with localized presumptive GME.¹ The response to ciclosporin alone or initially in combination with glucocorticoids achieved remission and halted progression of neurologic signs for up to 1 year of monitoring with continued daily administration of a maintenance dosage. Use of ciclosporin without glucocorticoids avoided many of the undesirable side effects. Nephrotoxicity of ciclosporin was not observed, and blood monitoring showed safe therapeutic blood levels of the drug. Excess shedding of hair, alopecia, and potential worsening of an existing urinary tract infection were the observed side effects. In addition, the use of lomustine has been reported.⁶¹ In a study using prednisolone alone, or in combination with lomustine, the response to both regimens was equivocal.^60a Therefore, before using immunosuppressive drugs, infectious disease causes of meningoencephalitis should be ruled out; otherwise, an exacerbation of the infection might occur.

Radiation therapy with cobalt or linear accelerator has been used to slow the progression of the disease.¹³⁴ Dogs with focal lesions survive longer than do those with multifocal neurologic signs.¹³⁴ Radiation injury of cranial tissues is an adverse effect; however, this may be an alternative to systemic immunosuppression caused by chemotherapy. Anticonvulsants may be needed to control seizures associated with inflammatory forebrain lesions in dogs with GME. Oral phenobarbital and potassium bromide are indicated for general use, and intravenous or rectal diazepam should be used in cases of status epilepticus.

The prognosis of GME is generally guarded given that most dogs develop progressive neurologic signs despite treatment. Dogs with multifocal GME may have a rapid and fulminant course of illness over several weeks that may be unresponsive to anti-inflammatory or immunosuppressive therapy. Patients with more focal signs can have progressive illness over several months¹⁵³; those with focal forebrain localization have the longest life spans.¹³⁴

Etiology and Pathogenesis

The suspected genetic basis and subsequent pathogenic mechanisms resulting in NME and NLE are poorly understood. Pedigree information on a large cohort of pug dogs is suggestive of a strong familial inheritance pattern for NME.⁶⁹ While this genetic transmission data is not surprising, a simple mendelian inheritance pattern could not be demonstrated. The latter suggests that NME is a multifactorial disorder. A complex genetic and infectious pathogenesis has been demonstrated for acute necrotizing encephalopathy in children, which occurs in susceptible individuals in association with influenza and parainfluenza infections.¹³⁵ Missense mutations in the nuclear pore gene RANBP2 have been identified as susceptibility alleles for familial and recurrent cases of acute necrotizing encephalopathy. A similar combination of genetics and infectious factors may be responsible for canine NE. Molecular studies on affected dogs are ongoing to assess for genetic susceptibility loci and screen for a diverse group of potential infectious triggers that may lead to immune dysregulation in NME and NLE.

Although it is tempting to consider NME and NLE as distinct entities, these disorders may represent a spectrum of CNS injury with similar pathogeneses. Interestingly, neuropathologists have evaluated the brains from pugs, several Maltese terriers, and Chihuahuas with lesions that have been typical of NLE, rather than NME, which has been reported in these breeds.49,83 Conversely, NME has been histopathologically confirmed in a Yorkshire terrier.^160,180 Experimental autoimmune encephalomyelitis models in rats may provide insights into such variations in lesion topographies within and among different dog breeds. Minor modifications of major histocompatibility complex haplotypes in rats with experimental autoimmune encephalitis results in different but reproducible patterns of brain inflammation after exposure to a myelin-oligodendrocyte-glycoprotein.¹⁷⁹ Similarly, the histopathologic differences among the NEs may result from minor genetic differences among dog breeds, modifying genes, and/or variations in antigenic exposures.

An autoimmune pathogenesis has been suggested for NME based on the presence of anti-astrocytic and glial fibrillary acid protein autoantibodies in CSF of affected dogs.129,170,170 However, similar antibody levels occur in the CSF of dogs with GME or brain tumors, and even some clinically healthy dogs.^170,198 Whether glial fibrillary acid protein autoantibodies are the inciting cause of NME, representative of a breed-specific fragility of astrocytes, or the consequence of prolonged tissue destruction secondary to infectious disease, requires further investigation. Results of genomic analysis of pug dogs with NME showed the presence of specific alleles at a locus near the dog leukocyte antigen complex on CFA12.⁷⁰ This strong dog leukocyte antigen class II association with NME is similar to the pathogenic mechanisms and gene associations found in human multiple sclerosis.

Because of the neuropathologic similarities of NME to viral meningoencephalitides in other species, viral etiologies have been considered.¹⁶¹ The histopathologic lesions associated with canine NME are especially similar to those present in human herpesvirus meningoencephalitis.^42,214 Because canine herpesvirus-1 may cause encephalitis in neonates,^2,143 it is conceivable that NE is triggered by a recrudescence of a latent herpesvirus infection. Interestingly, a “herpes-like virus” was isolated from one pug dog with NME; however, the viral isolate was not retained.⁴²

Further attempts at viral isolation from dogs with NME have been unsuccessful180,192,192; however, Mx proteins, interferon-induced GTPases associated with viral and inflammatory diseases, have been demonstrated in brain tissues from pug dogs with NME.¹⁴⁸ Results of broadly reactive PCR assays for herpesviruses have been negative on a large number of paraffinized and freshly frozen NME-affected brains.^69,161 Moreover, PCR screening of NME-affected brains for an extremely diverse group of DNA and RNA viruses has failed to identify viral nucleic acids.¹⁶⁰ Although the lack of viral nucleic acids in NME-affected brains argues against a directly acting, neurotropic virus, these PCR data do not exclude the possibility of a viral trigger for NE via molecular mimicry.^56,138,138 Another possibility is that a pathogen is present in the CNS (but at undetectable levels) in the presence of a self-perpetuating immune response, a phenomenon that has been described for flaviviral infections.¹⁰⁸ Investigations are ongoing to pursue direct acting neuropathogens as well as para- and postinfectious pathogeneses for autoimmune inflammation in canine NE.

Clinical Findings

The onset of neurologic signs associated with NME varies from 6 months to 7 years of age and most commonly occurs in young dogs, with a mean age of 29 months.42,113 In comparison, NLE typically manifests between 4 months and 10 years of age, with a mean age of onset of 4.5 years.¹¹¹ Dogs with both NME and NLE commonly manifest cerebrothalamic signs due to the predominance of lesions in the prosencephalon; NLE also may cause mid to caudal brainstem signs.⁵⁰ However, because of the multifocal nature of inflammatory disease, variations may occur with either disorder, and clinical signs are primarily reflective of the lesion locations. In general, the signs associated with NE typically are rapidly progressive and most commonly include seizures, depression, circling, vestibulocerebellar signs, visual deficits, and ultimately death.

Diagnosis

No uniquely characteristic hematologic or biochemical findings have been observed with NE. CSF analysis results are abnormal in most dogs, with a predominantly mononuclear pleocytosis (usually 90 to 600 cells/µL) and an increased protein concentration (usually 50 to 200 mg/dL)42,94; however, these are not specific, because other encephalitides are associated with similar CSF changes.

The massive necrotizing lesions in the cerebrum can be detected with cross-sectional medical imaging methods such as CT or MRI.110,177 Typical MRI lesions associated with NME include asymmetric, multifocal prosencephalic lesions affecting the gray and white matter, with variable contrast enhancement on T1-weighted imaging.²²⁰ Loss of gray and white matter demarcation also may be discernible.^60,220 Lesions appear hyperintense on T2-weighted images and isointense to slightly hypointense on T1-weighted images, with slight contrast enhancement. In NLE, multiple, asymmetric bilateral prosencephalic lesions mainly affecting the subcortical white matter have been described.²⁰⁸ The NLE lesions are hyperintense on T2-weighted and fluid-attenuated inversion recovery images and often include multiple cystic areas of necrosis. These lesions are hypointense or isointense on T1-weighted images, and contrast enhancement is variable. MRI findings may increase the clinician’s confidence in a presumptive diagnosis of NE. Positron emission tomography has been used on a limited basis in an attempt to distinguish NME from other inflammatory brain diseases.^55b,99a

Pathologic Findings

Necrotizing lesions in the cerebrum may be visible on macroscopic examination. The histologic findings are unusual and highly typical for this disease: disseminated meningitis, choroiditis, and encephalitis, with greatest involvement in the cerebrum. The perivascular and meningeal inflammatory infiltrates consisting of lymphocytes, plasma cells, histiocytes, and macrophages have a strong tendency to invade the adjacent parenchyma. Extensive compact subpial and subventricular lesions are present and may extend deep into the underlying brain tissue, with unusually intense microglial proliferation and total destruction of the original tissue elements. In some areas, frank cerebrocortical necrosis occurs. The cerebral white matter also is severely affected, with intense perivascular cuffing and gliosis (Fig. 82-4). In some areas, leukomalacia with liquefaction and cavitation of the tissue is noted.

Therapy

Immunosuppressive drugs may be used, as described previously, for dogs with GME (see Table 82-1).^67,184 Controlled studies, however, are lacking, and the prognosis with immunosuppression remains guarded. Glucocorticoid monotherapy does not alter the course of this NE in confirmed cases, and antiepileptic drugs may be ineffective in controlling seizures.^48,76 However, results of one study indicated that use of antiepileptic drugs was associated with a prolonged survival time.¹¹³ Affected animals usually develop persistent seizures and progressive neurologic signs and do not usually live longer than 6 months to 1 year after the onset of clinical illness.

Etiology

Some investigators proposed that such inflammatory changes could be the result of ischemia induced by rapidly rising intracranial pressure.^219a Because of the severity of the inflammatory changes seen with hydrocephalus, an infectious cause has been suspected. On bacteriologic examination of brain tissue and CSF in two dogs with this disease, three different bacteria were isolated; however, these were thought to be contaminants.^84,217 Staphylococci were isolated in another case.⁵⁴ Possibly, the underlying disease might be caused by a virus, although no infectious agent was found on electron microscopy study of the periventricular tissues of two dogs.²¹⁷ Canine parainfluenza virus has caused hydrocephalus in dogs after experimental inoculation (see Clinical Findings, Chapter 7).^15,16 However, the clinical and pathologic findings in experimental parainfluenza-induced hydrocephalus are totally different from those of the condition described herein. Serum-neutralizing antibodies against canine parvovirus (CPV) were absent.²¹⁷

Clinical Findings

The disease occurs in puppies between 2 and 6 months of age. No predilection for miniature breeds exists, as is the case with congenital hydrocephalus. The animals are clinically healthy at birth and exhibit normal development for the first 2 months of life. At 2 or 3 months of age, acute neurologic signs occur together with rapidly progressing skull enlargement. Neurologic signs include behavioral changes such as depression, dullness, and hyperactivity. Progressive incoordination occurs in all limbs, and most animals develop central blindness. Less consistent findings are cranial nerve deficits, which may include deafness, and abnormal eye motility, such as strabismus, head tilt, and dysphagia. The course of the disease is usually progressive over several days to a few weeks. In some animals, the clinical condition may stabilize.

Diagnosis

No consistent hematologic or biochemical findings are associated with this form of hydrocephalus. CSF abnormalities consist of xanthochromia, increased protein concentration, and pleocytosis, including erythrocytes, mononuclear cells, and macrophages containing erythrocytes. Radiographic examination of the skull reveals abnormalities such as thinning of the cranial vault and homogeneous appearance of the intracranial contents. CT and MRI confirm the enlargement of the ventricles. However, these findings are indicative of hydrocephalus, regardless of its cause.⁸⁸ The same can be said for the electroencephalographic findings indicative for hydrocephalus.⁸⁸

Pathologic Findings

Massive enlargement of the cerebral ventricles occurs, which are filled with cloudy, hemorrhagic CSF. The internal surface of the ventricles is focally roughened, with a brownish discoloration. A typical finding is the presence of large dissecting cavities, also called false diverticula, in the cerebral mantle (Fig. 82-5, A). These dissecting cavities communicate with the lateral ventricles. Histologically, severe inflammation with necrosis of the ependymal and subependymal tissues can be found.^32,203 The lesions are always hemorrhagic. Hemosiderin-laden macrophages are found in old lesions. Perivascular cuffing with inflammatory cells occurs and, in acute lesions, diffuse infiltration with neutrophils and macrophages (Fig. 82-5, B, and C). Glial mesenchymal repair tissue is formed later in the course of the disease.

Therapy

By the time the condition becomes evident to the owner, severe damage has already taken place. In most cases, the disease is rapidly progressive, leading to severe neurologic impairment. However, in cases in which the condition stabilizes, conservative therapy with glucocorticoids or surgical treatment may be considered.⁸⁸ Antibacterial therapy and a ventriculoperitoneal shunt was successful in treating a puppy with external hydrocephalus associated with staphylococcal infection.⁵⁴ Prognosis remains cautious given that placed catheters may become obstructed by the inflammatory reaction.