8-year-old neutered male Boxer

A roughly fusiform shaped, irregularly marginated mass measuring approximately 8 x 3 x 3 cm, is present in the right side of the pelvic canal, medial to the right ilial shaft. This mass is T1, STIR and T2 hypointense with stippled hyperintense foci. There is increased signal intensity of this mass on GRE images and contrast enhancement on postcontrast T1 spin echo images. The mass displaces the right sciatic nerve laterally at the level of the ilial shaft and displaces the rectum to the left. The right internal iliac, internal pudendal and caudal gluteal arteries are moderately dilated compared to the left. These vessels converge and join with the aforementioned mass. The right internal iliac, internal pudendal and caudal gluteal veins emerge from this mass and are moderately dilated. The median sacral artery courses cranially, becoming tortuous and dilated, to anastomose with the dorsal aspect of the right internal iliac artery. The right gluteal musculature is smaller than that on the left. The lumbar vertebral column is unremarkable. Incidentally, a T2 hyperintense, T1 hypointense, 0.9 cm, spherical nodule is present within the dermis of the dorsum at the level of L5. No other significant findings are seen.

Summary

• Vascular plexus of the right pelvic canal with secondary sciatic neuritis or neurogenic atrophy of the right sciatic nerve and neurogenic or disuse muscle atrophy of the right gluteal musculature. Differential diagnoses include vascular anomaly, such as an arteriovenous malformation, or, less likely, neoplasia, such as hemangiosarcoma.
• Probable dermal cyst.

Diagnosis: Arteriovenous malformation

The patient was referred for vascular occlusion. An approximately 95% occlusion was achieved using vascular coils and plugs. The preferred method of vascular occlusion, glue embolization, was not pursued in this patient due to the risk of inadvertent occlusion of an artery supplying the colon or rectum.

I would like to acknowledge Dr. Chick Weiss for providing information regarding management of this patient.

Discussion

Arteriovenous malformations (AVMs) are rare in dogs. An AVM is a shunt involving multiple arteriovenous connections between arterial and venous circulation. More common but still rare in dogs, are arteriovenous fistulas (AVF). An AVF is a arteriovenous communication involving a single shunt vessel. AVFs occur most commonly in dogs between the hepatic artery and the portal vein and are typically congenital, likely related to the failure of arterial and venous differentiation in the embryo. Similarly, AVMs have been described between the hepatic artery and portal vein in dogs. Acquired AVFs are also described in the human and veterinary literature, secondary to trauma, sometimes iatrogenic, or neoplasia. On physical exam, pulsatile blood flow may be palpated beneath the skin in a peripheral AVF or AVM. Upon auscultation, a bruit may be heard. Nicoladoni-Branham sign, a reflexive decrease in heart rate associated with temporary occlusion of an arteriovenous fistula, may be seen with digital compression on physical exam. Complications associated with AVFs and AVMs include compression of adjacent anatomy, and right side heart failure due to increased preload from the loss of capacitance afforded by peripheral vascular beds. Imaging is an important diagnostic test and aids in prognosis and surgical planning. In human medicine, MR angiography is the gold standard. However, fluoroscopic or CT angiography are also commonly used modalities. Typical MR features with AVFs and AVMs have been described. Multiple vascular flow voids (the faster the blood flow, the darker the vessels and plexus) are seen on spin echo sequences. These voids will increase in intensity with IV contrast. On gradient echo imaging, a signal will be present within the vascular plexus. This appearance allows distinction of flow from other causes of signal void (air or calcified tissue). Additionally, although not seen in this patient, motion artifact characteristic of flowing blood or “ghosting” may be seen on multiple sequences and can help in attaining a diagnosis.

References

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