What Is Your Neurologic Diagnosis?

Stephanie Chang BluePearl Pet Hospital North Dallas, Lewisville, TX

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 DVM
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Rebecca Sandler Avets Specialty and Emergency Trauma Center, Monroeville, PA

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 DVM, DACVIM (Neurology)
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Emma Davies Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY

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 BVSc, MSc, DECVN

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Introduction

A 9-month-old sexually intact female Havanese was evaluated at the Cornell University Hospital for Animals because of an acute onset of abnormal behavior and seizures. A few hours prior to presentation, the patient had suddenly started to exhibit abnormal behavior in the form of frantic sniffing, whining, excessive drooling, and hyperactivity. The patient then developed whole-body shaking and altered mentation with no loss of consciousness. The dog was fully vaccinated and had been a normal puppy with normal appetite, thirst, and energy prior to the onset of clinical signs, but house training had been challenging. Historically, the patient had had a few episodes of stridor that resolved following antimicrobial treatment for presumed rhinitis.

On presentation to the emergency clinic, the patient was exhibiting focal facial seizures. Point-of-care bloodwork performed during initial stabilization revealed no abnormalities. Seizure activity was not responsive to levetiracetam (20 mg/kg, IV, once) but was responsive to midazolam (0.3 mg/kg, IV, once).

Assessment

Anatomic diagnosis

The rule-out location for the seizures in this dog was the forebrain (cerebral cortex and thalamus). Rule-out locations for the decreased to absent postural reactions in all 4 limbs and tetraparesis were the forebrain, brainstem (midbrain, pons, or medulla oblongata), and C1-C5 spinal cord segments. The rule-out locations for bilateral miotic pupils and diminished pupillary light reflexes were the forebrain and rostral midbrain.

Likely location of a single lesion

A lesion in the forebrain could fully explain the seizures, proprioceptive deficits, tetraparesis, and bilateral miosis with diminished pupillary light reflexes. Because the proprioceptive deficits were more severe in the left thoracic and pelvic limbs, it was likely that the right forebrain was more severely affected.

Etiologic diagnosis

Most likely differential diagnoses for a bilateral forebrain lesion lateralizing subtly to the right included congenital anomalies (eg, meningoencephalocele or portosystemic shunt), infectious meningoencephalitis (eg, toxoplasmosis or neosporosis), and degenerative diseases (eg, lysosomal storage disorders). Inflammatory causes such as meningoencephalitis of unknown etiology and neoplasia were considered less likely.

Diagnostic Plan

The initial diagnostic plan included bloodwork (a CBC and serum biochemical profile) to evaluate for metabolic disorders and MRI of the brain, followed by CSF analysis and infectious disease testing.

Diagnostic Test Findings

Results of a CBC and serum biochemical profile were unremarkable. Magnetic resonance imaging of the brain was performed with a 1.5-T MRI unit (Vantage Atlas 1.5-T MR Scanner; Toshiba America Medical Systems Inc). The following sequences were obtained: sagittal T2-weighted images and T1-weighted (T1W) images before and after contrast (gadodiamide) administration; transverse T2-weighted, FLAIR, T2*-weighted, pre- and postcontrast T1W, diffusion-weighted (with calculation of apparent diffusion coefficient), postcontrast fat saturation T1W, and postcontrast subtraction T1W images; and dorsal postcontrast T1W images.

Magnetic resonance imaging revealed that both olfactory bulbs were displaced rostrally into the caudal half of the right nasal cavity owing to absence of the cribriform plate on the right (Figure 1). The nasal septum was moderately deviated toward the left, resulting in a moderately stenotic left nasal passage; the right nasal cavity was relatively void of nasal conchae and had evidence of chronic rhinitis. Within the right nasal passage, the displaced olfactory bulb parenchyma was heterogeneous with faint, patchy contrast enhancement, and there was mild, locally extensive enhancement of the meninges. The left cerebrum and thalamus were mildly larger than the right cerebrum with deviation of the falx cerebri.

Figure 1
Figure 1

Axial T2-weighted MRI images (A and B) and a right parasagittal postcontrast T1-weighted MRI image (C) in a 9-month-old Havanese with an acute onset of abnormal behavior and seizures. A—Both olfactory bulbs are displaced rostrally into the caudal half of the right nasal cavity owing to absence of the cribriform plate on the right. Note the relative absence of conchae in the right nasal cavity. B—The left cerebrum and thalamus are mildly larger than the right cerebrum with deviation of the falx cerebri. There is also mild narrowing of the left lateral ventricle. C—There is faint, patchy contrast enhancement of the displaced olfactory bulb parenchyma with mild, locally extensive enhancement of the meninges.

Citation: Journal of the American Veterinary Medical Association 260, 11; 10.2460/javma.21.01.0030

Cerebrospinal fluid was collected from the cerebellomedullary cistern. Cerebrospinal fluid protein content was 8 mg/mL (reference range,1 < 30 mg/dL), and the nucleated cell count was 2 cells/µL (reference range,1 < 5 cells/µL). A differential cell count on 5 nucleated cells yielded 4 small lymphocytes and 1 small, quiescent macrophage. No infectious organisms were identified in the examined sample.

Congenital right-sided nasal meningoencephalocele secondary to a cribriform plate malformation was diagnosed and presumed to be the cause of the dog’s seizures. To rule out secondary infectious meningoencephalitis caused by extension of bacteria or other infectious agents related to the patient’s historical chronic and recurrent rhinitis, additional samples were submitted for diagnostic testing against common infectious agents. Serum testing for antibodies against both Toxoplasma gondii and Neospora spp yielded negative results. Fungal serologic testing revealed no detectable serum Cryptococcus antigen and no detectable antibodies against Aspergillus spp, Blastomyces spp, Coccidioides spp, or Histoplasma spp.

Treatment

While infectious disease test results were pending, the patient was started empirically on ampicillin-sulbactam (30 mg/kg, IV, q 8 h) and dexamethasone (0.1 mg/kg, IV, q 24 h) and was given a small loading dose of phenobarbital (8 mg/kg, IV, given as 4 separate 2-mg/kg doses). During hospitalization, the patient had a generalized seizure that spontaneously resolved and the next portion of the phenobarbital loading dose was given immediately afterward. The patient was seizure free for a 24-hour period before being discharged. Amoxicillin–clavulanic acid (12.5 mg/kg, PO, q 12 h), prednisolone (1 mg/kg, PO, q 24 h), and phenobarbital (2.5 mg/kg, PO, q 12 h) were continued at home. A telephone call 1 week after discharge revealed that the patient had normal mentation and behavior and had not had any additional seizures. However, mild ataxia persisted. The prednisolone dosage was slowly tapered over the course of 6 weeks, and the amoxicillin–clavulanic acid was discontinued after 5 months. The patient was reported to be doing well at home and to have remained seizure free at a recheck examination 6 months after initial presentation. Recheck neurologic examination revealed mild proprioceptive ataxia involving all 4 limbs and mildly delayed hopping and paw placement in the left thoracic and pelvic limbs. The remainder of the exam was otherwise unremarkable. Results of biochemical analyses were unremarkable, and serum phenobarbital concentration was within the expected range (17 µg/mL; reference range, 15 to 45 µg/mL).

Comments

Meningoencephalocele is a herniation of the cerebral parenchyma with its meninges through a defect in the cranium, whereas meningocele is a herniation of only the meninges.2 Both can result from a congenital defect arising from a disturbance in the final phase of neural tube formation, whereby the surface ectoderm (epithelial layer) improperly separates from the neuroectoderm (nervous tissue).35 In humans, this defect is thought to occur during the fourth gestational week.4 In veterinary medicine, a hereditary predisposition has been shown in Burmese cats6; however, environmental influences during gestation such as hyperthermia, teratogenic drugs, and nutritional deficiencies may also contribute to the pathogenesis.79 Other causes of meningoencephaloceles such as head trauma and cranial surgery have also been described in veterinary patients.3,10,11

In a retrospective study10 involving 22 dogs with cranial meningoencephaloceles, meningoceles, or both, epileptic seizures were the main presenting neurologic sign. Generalized seizures were the most common type of seizure, followed by a mixture of generalized and focal seizures, with focal seizures being the least common. Abnormal behavior was reported as the second most common clinical sign and included aggression, compulsive behavior, hyperactivity, intermittent yelping, and fly catching or stargazing.10 The patient in the present report had the same common clinical signs; there was hyperactivity and frantic behavior, focal seizures on initial presentation, and an episode of generalized seizures during hospitalization. In agreement with the same retrospective study,10 this patient had asymmetrical neurologic deficits and showed clinically consistent asymmetry of the intracalvarial part of the cerebrum on MRI images.

The protruded meninges of the meningoencephalocele described in our patient had patchy contrast enhancement, which could be consistent with focal meningitis. However, in the aforementioned retrospective study,10 results of cytologic examination of CSF samples did not support inflammation or correlate with meningeal enhancement in those dogs that had contrast enhancement of the protruding meninges, as was the case for the dog in the present report. In human medicine, nasal meningoencephaloceles have been reported to be an unusual cause of recurrent bacterial meningitis.12,13 Suppurative meningoencephalitis secondary to suppurative rhinitis and infection of a unilateral meningoencephalocele of the olfactory bulb has also been previously reported in an Australian Shepherd.3 Because our patient had evidence of chronic rhinitis as well as meningeal enhancement on MRI image, we elected treatment with a broad-spectrum antimicrobial for presumptive infectious meningoencephalitis, even though there was no cytologic evidence of inflammation or infection in the CSF sample collected. Additional studies examining the long-term outcome of meningoencephaloceles and the risk of ascending meningitis in dogs are required.

In animals with mild meningoencephaloceles and neurologic signs, medical treatment with antiepileptic drugs appears to be a valid option associated with a fair prognosis.10 In human medicine, surgical intervention is the treatment of choice, especially when seizures become refractory, because meningoencephaloceles are considered to be epileptic foci.14 Irritative traction, herniation of involved cortex, hemorrhage, malacia, white matter degeneration, and inflammatory infiltrates contributing to abnormal excitability of cortical neurons in meningoencephaloceles have all been proposed as possible causes of seizures.5 Surgical correction of meningoencephaloceles is rarely done in veterinary medicine owing to the invasive nature of the procedure, and there are only 2 published case reports15,16 describing surgical management of meningoencephaloceles. In a 5-month-old Border Collie, surgical removal of a meningoencephalocele and closure of the congenital defect of the cribriform plate resulted in complete resolution of epileptic seizures and other clinical signs 28 months after surgery, even without the use of antiepileptic drugs.15 Surgical correction of a parietal meningoencephalocele in a 4-month-old domestic shorthair cat resulted in complete resolution of clinical signs of aggression and restlessness.16 Other indications for surgical intervention of meningoencephaloceles other than refractory seizures may include recurrent meningitis, rhinorrhea, and the potential for brain damage resulting from herniation.10 Spontaneous nontraumatic tension pneumocephalus as a rare complication of a nasal meningoencephalocele has been reported once in the veterinary literature,17 and prevention of such a complication may be another indication for surgical correction of a meningoencephalocele.

The patient described in the present report remained seizure free for 6 months with medical management after initial diagnosis. Continued long-term follow-up of this patient is needed to determine whether surgical intervention might be indicated in the future.

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