Clinical History
The dog was presented to The Animal Health Trust for episodic exercise-induced or stress-induced collapse with a first episode at 4 months of age, a second episode at 10 months of age and again at 5 years of age. Following these episodes, activity was considered normal until the dog was 8 years of age when it had a four-month-history of exercise or stress associated collapse without loss of consciousness. Cardiac evaluation and Holter monitoring showed no evidence of organic heart disease and all valves were normal.

Physical and Neurological Examination
No abnormalities were detected on general physical examination. Neurological examination revealed normal mental status, posture and gait. Postural reactions were normal on all four limbs. Cranial nerve examination, spinal reflexes, nociception and fundic examinations were normal. Pain could not be exhibited on palpation of the spine.

Diagnostic Tests
Hematology and Biochemistry – only mild elevations of urea and creatinine
Thyroid panel (TT4, FT4,  endogenous TSH) – normal
ACTH stimulation – normal
Insulin/glucose ratio (repeated three times at one-hour intervals after 12 hr fast) – normal at all time-points
Resting and post-exercise lactate, pyruvate and lactate to pyruvate ratios - normal
Muscle biopsy (left cranial tibial muscle) – No abnormalities detected
Abdominal radiography and ultrasound revealed large dilatation of the caudal vena cava suggestive of an aneurysm.

Conclusions
The presence of a caudal vena cava aneurysm could potentially be the underlying cause of exercise-induced weakness by creating abnormal venous return or pressure on the aorta. This could also potentially explain the mild azotemia.  An abdominal MRI with angiography was performed (Figures 1 and 2) and confirmed a dilated caudal vena cava and azygous vein.  Given the lengthy history of episodic weakness, a congenital vascular abnormality is likely. This case demonstrates the importance of considering vascular anomalies in the differential diagnosis of collapse.

Figure1. Maximal intensity projection (MIP) image of a time of flight (TOF)  magnetic resonance angiogram of the abdomen. The MIP image gives a 3-D representation of the abdominal vasculature. The caudal vena cava  (CVC, arrow) caudal to the renal veins (not visible on this image) is normal. Cranial to the renal veins there is a large aneurismal dilation (arrowhead) of the caudal vena cava. At the cranial end of the aneurysm the CVC runs dorsally to join the left hemiazygous vein (*) which is dilated. The CVC appears on this image to have  a segmental aplasia with no visible CVC between the aneurysm and  the  hepatic veins. TOF magnetic resonance angiography is prone  to artefacts particularly with slow venous flow. Slow flow and flow within the image plane may give artefactual loss of signal from the blood vessel. The main portal vein  (PV) is normal.

Figure 2. An MIP image can be rotated to aid visualisation of vascular abnormalities. This is an MIP showing a dorso-ventral  projection of the abdomen. The continuation of  the CVC into the left hemiazygous vein  and  the CVC dilation cranial to the kidney is clearly seen. One disadvantage of MIP images is the superimposition of structures which can be reduced by only constructing a thin MIP of a thinner part of the region of interest. In  this whole abdomen MIP, the PV appears to be continuous with the CVC.