What Is Your Diagnosis?

Jackie M. Williams Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Iowa State University, Ames, IA 50010.

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 DVM, MS
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Elizabeth A. Riedesel Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Iowa State University, Ames, IA 50010.

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Nick D. Jeffery Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Iowa State University, Ames, IA 50010.

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 BVSc, PhD
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Cody J. Alcott Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Iowa State University, Ames, IA 50010.

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History

A 3-year-old spayed female German Shepherd Dog was referred for evaluation because of a 2-day history of lethargy, ptyalism, ataxia, anisocoria, and recent, rapidly progressive tetraparesis. On physical examination, the dog was ataxic with dull mentation. Heart rate and rectal temperature were within reference limits, and the patient was panting. The dog was able to stand with assistance and had postural reaction deficits in the pelvic limbs. Neurologic examination revealed bilaterally absent menace and pupillary light responses, anisocoria (left-sided mydriasis), and rotary nystagmus (slow phase to the left). The neurologic deficits were consistent with either a mass lesion in the rostral fossa on the left side causing secondary subtentorial herniation with brainstem compression or multifocal intracranial disease. Magnetic resonance imaging of the brain was performed (Figure 1).

Figure 1—
Figure 1—

T2-weighted transverse (A), T2-weighted dorsal (B), and T1-weighted transverse postcontrast (C) MRI images of the brain of a 3-year-old German Shepherd Dog referred for evaluation because of ataxia, anisocoria, and progressive tetraparesis. The dotted line on the dorsal image corresponds to the plane of the transverse images.

Citation: Journal of the American Veterinary Medical Association 245, 2; 10.2460/javma.245.2.175

Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →

Diagnostic Imaging Findings and Interpretation

On T2-weighted images, a mass effect is evidenced by a midline shift within the brain toward the right (Figure 2). A crescent-shaped high-signal-intensity lesion is identified within the left dorsolateral calvarium along the entire left cerebral hemisphere. After gadolinium contrast administration, focal uniform contrast enhancement of the dura mater and adjacent pia mater and arachnoid highlights the lesion within the subdural space. On postcontrast T1-weighted images, the lesion has low signal intensity, compared with the brain parenchyma. On the basis of the imaging characteristics, differential diagnoses included abscess, granuloma, or hematoma with associated meningitis.

Figure 2—
Figure 2—

Same MRI images as in Figure 1. Notice the high–signal-intensity lesion located along the dorsolateral aspect of the left cerebral hemisphere (white arrows; A and B). A mass effect is indicated by the right-sided deviation of the falx cerebri (black arrow; A). Enhancement of the leptomeninges and dura in the postcontrast image highlights the lesion's location as subdural (arrowheads; C).

Citation: Journal of the American Veterinary Medical Association 245, 2; 10.2460/javma.245.2.175

On additional T2-weighted FLAIR (fluid-attenuated inversion recovery) and T2*GRE (gradient echo) sequences, the lesion had a proteinaceous component and no hemorrhage, respectively. A subdural abscess was considered most likely. The midline shift was considered a consequence of the intracranial space-occupying lesion. Subtentorial herniation of the temporal lobes of the cerebrum and partial obliteration of the fourth ventricle were also evident on MRI images.

Treatment and Outcome

A left-sided rostrotentorial craniotomy was performed. A copious amount of purulent material was identified between the dura mater and the arachnoid and submitted for bacterial culture, confirming a diagnosis of subdural empyema. A strictly anaerobic polymicrobial infection was identified containing Fusobacterium spp, Peptostreptococcus spp, and Actinomyces spp. The dog improved following surgery, and at a 3-month recheck evaluation, most neurologic deficits had resolved. A negative menace response in the right eye continued to be detected 5 months after surgery.

Comments

Empyema of the CNS is uncommon in human and veterinary medicine. Few cases have been reported in the veterinary literature, and most subdural abscesses have occurred in cats.1 Reported locations include the dorsal aspect of the cerebral cortex and adjacent to the pituitary gland and brainstem. Mechanisms of infection are often not definitively identified, although extension of middle ear disease and a retrobulbar abscess have been proposed in previous cases.2,3 Hematogenous spread from bacterial endocarditis and suppurative chronic pneumonia have been described as well as direct inoculation from a bite wound.1,4 In humans, 70% of cases are associated with paranasal sinusitis.1 Obligate anaerobes (Fusobacterium spp, Bacteroides spp, and Actinomyces spp) and facultative anaerobes (Escherichia spp and Staphylococcus spp) are the most commonly cultured organisms in veterinary patients.

On the basis of lesion neurolocalization in the dog of the present report, MRI was selected to evaluate the brain parenchyma. Magnetic resonance imaging has proven superior to CT in detection and evaluation of subdural lesions in human medicine because it provides more accurate localization, characterization, and information on the extension of disease.5 Magnetic resonance imaging affords differentiation of abscesses from most sterile effusions and hematomas. Consistent with the imaging findings in this dog, collections of purulent material have high signal intensity on T2-weighted sequences and low signal intensity on T1-weighted sequences, compared with the cerebral cortex. The purulent material does not enhance after contrast medium administration. A clear distinction between brain parenchyma and the collection of purulent material was made in the dog of the present report, confirming the subdural location.

Subdural empyema is considered a surgical emergency because increased intracranial pressure and consequent brainstem compression are common sequelae. Surgery allows for decompression and lavage as well as collection of material for bacterial culture. Magnetic resonance imaging revealed subtentorial herniation and evidence of increasing pressure within the caudal fossa, accounting for the mydriasis and vestibular signs, respectively, noted on neurologic examination. The mechanism of infection was not identified; no wounds were identified around the head, and there was no evidence of nasal or otic disease or systemic infection to suggest a hematogenous route.

  • 1. Barrs VR, Nicoll RG & Churcher RK, et al. Intracranial empyema: literature review and two novel cases in cats. J Small Anim Pract 2007;48: 449454.

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  • 2. Klopp LS, Hathcock JT, Sorjonen DC. Magnetic resonance imaging features of brain stem abscessation in two cats. Vet Radiol Ultrasound 2000; 41: 300307.

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  • 3. Sturges BK, Dickinson PJ & Kortz GD, et al. Clinical signs, magnetic resonance imaging features, and outcome after surgical and medical treatment of otogenic intracranial infection in 11 cats and 4 dogs. J Vet Intern Med 2006; 20: 648656.

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  • 4. Dow SW, LeCouteur RA & Henik RA, et al. Central nervous system infection associated with anaerobic bacteria in two dogs and two cats. J Vet Intern Med 1988; 2: 171176.

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  • 5. Weingarten K, Zimmerman RD & Becker RD, et al. Subdural and epidural empyemas: MR imaging. AJR Am J Neuroradiol 1989; 152: 615621.

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