Portal Vein Hypoplasia (Microvascular Dysplasia)

Chapter 146

Portal Vein Hypoplasia (Microvascular Dysplasia)

In normal dogs approximately 70% of the blood flow to the liver is delivered by the portal circulation, with the remaining 30% derived from the hepatic artery. Hepatopetal blood flow from the portal system enters the portal triad, traverses the sinusoids, and returns to the systemic circulation via the hepatic veins. A spectrum of congenital hepatic vascular anomalies involving the hepatic portal vasculature exists in the dog. Macroscopic portosystemic vascular anomalies (PSVA) occur as single vessels within (intrahepatic) or outside (extrahepatic) the liver that shunt blood from the portal to the systemic circulation (see Chapter 145). Hypoplasia of the portal vein (also known as microvascular dysplasia, or MVD) is a microscopic intrahepatic vascular abnormality in which portal venous blood is diverted into the hepatic veins within the intrahepatic microcirculation (Allen et al, 1999; Center, 2008; Christiansen et al, 2000; Rothuzien et al, 2006; Schermerhorn et al, 1996). Originally called MVD, in 2004 the World Small Animal Veterinary Association (WSAVA) Liver Standardization Group renamed the syndrome primary hypoplasia of the portal vein (PHPV) (also referred to as portal vein hypoplasia [PVH]) because the association felt MVD was part of a group of congenital disorders (excluding PSVA) associated with the PVH resulting in hepatic parenchymal hypoperfusion. Although it is clear that some dogs with congenital portovascular disease have true hypoplasia (atresia) of the extrahepatic portal vein, it is not clear whether all dogs with the syndrome recognized as MVD have hypoplastic vessels.

PVH/MVD may occur independently or concurrently with PSVA. Multiple small-breed dogs are at increased risk for PVH/MVD, including many of the same breeds predisposed to PSVA. These breeds include Cairn terriers, Tibetan spaniels, Maltese, Havanese, Yorkshire terriers, Norfolk terriers, and miniature schnauzers. A small-breed genotyping initiative underway at Cornell University in these breeds has established that macroscopic or microscopic portovascular anomalies occur in 30% to 80% of those breeds (Center S, personal communication). They found the incidence of PVH/MVD exceeds PSVA 30 : 1 in all breeds that were studied. Genotyping data suggest that the PSVA/PVH/MVD trait is allelic and may represent an ancestral founder mutation. An autosomal incomplete penetrant or polygenic mode of inheritance is most consistent with trait transmission.

Clinical Features

Dogs with PVH/MVD are typically asymptomatic. They have none of the clinical features seen in dogs with PVSA, such as small body stature, neurobehavioral signs consistent with hepatic encephalopathy (amaurosis, head pressing, staring, vocalizing, seizures, lethargy, and coma), intermittent gastrointestinal signs (anorexia, vomiting, diarrhea), or urinary signs (polyuria/polydipsia, stranguria, hematuria, pollakiuria). Rarely dogs may exhibit drug intolerance to substances metabolized or extracted by the liver. These drug sensitivities often are noted initially at the time of neutering.

Reports of PVH/MVD dogs with neurobehavioral and gastrointestinal signs can be found in the veterinary literature (Allen et al, 1999; Christiansen et al, 2000). Although initial clinical impressions were that dogs with PVH/MVD could be subdivided into asymptomatic and symptomatic groups, some symptomatic PVH/MVD dogs actually may represent PSVA dogs in which the vascular anomaly escaped detection or dogs with other disorders such as noncirrhotic portal hypertension (NCPH) (see later) or ductal plate abnormalities, which share some clinical and histologic features with PSVA/PVH/MVD. The latter is differentiated from primary vascular disease by the presence of intense cytokeratin positive bile duct profiles on hepatic biopsy.

Clinical Pathology

The hallmark of PVH/MVD is the presence of increased total serum bile acids in a dog that is otherwise clinically normal. Diagnosis requires ruling out the presence of a PSVA. Although increases in total serum bile acids are relatively lower in dogs with PVH/MVD as compared with PSVA, the degree of elevation cannot be used to differentiate the two disorders because of the potential for overlap in bile acid values. Complete blood count, serum biochemistry, and urinalysis typically are normal in dogs with PVH/MVD, with occasional mild increases in serum aminotransferases. In contrast, dogs with PSVA may have an RBC microcytosis, hypocholesterolemia, low BUN and creatinine, hypoglycemia, and ammonium biurate crystalluria. If readily available, plasma protein C level may aid in the differentiation of PSVA and PVH/MVD because it typically is subnormal with PSVA (protein C < 70% in 88% of dogs) and normal with MVD (protein C ≥ 70% in 95% of dogs) (Toulza et al, 2006). The low protein C levels in PSVA likely reflect hepatic hypoperfusion and not synthetic failure; attenuation of the anomalous vessel results in normalization of these values.

Diagnostic Imaging

Abdominal ultrasound is an easily accessible imaging modality that can identify features consistent with PSVA. Dogs with PVH/MVD typically have normal abdominal ultrasound examinations. With an experienced operator, the use of color-flow Doppler, and adequate restraint of the patient (which often requires sedation), the sensitivity of ultrasound to identify a macroscopic extra- or intra-PSVA can be as high as 98% (d’Anjou et al, 2004). In addition to visualization of the anomalous vessel, other signs suggestive of a PSVA on ultrasound include the presence of a small hypovascular liver, turbulence in the portal vascular, renomegaly, urolithiasis, and a portal vein-to-aorta ratio less than 0.65 in a dog without portal hypertension. Unfortunately, a normal ultrasound examination does not definitively exclude PSVA, so additional imaging may be necessary to make a diagnosis of PVH/MVD.

Transplenic portal or per rectal portal scintigraphy can be used to confirm the presence of portosystemic shunting (either congenital or acquired) with a sensitivity and specificity approaching 100% (Sura et al, 2007). In these procedures, small volumes of technetium 99m pertechnetate are administered to the sedated patient per rectum or transcutaneously into the spleen, and distribution of the radioactivity within the portal and systemic circulation is monitored with a computer-linked gamma camera. Dogs with PVH/MVD have a normal or mildly increased shunt fraction with these techniques, whereas dogs with PSVA usually have a shunt fraction greater than 60% (Center, 2008). Transplenic scintigraphy is a minimally invasive and relatively cost-effective secondary imaging modality often used by the authors as a second-line modality after ultrasound has failed to find a PSVA. In the authors’ (CRLW) hospital a clinically normal dog with increased total serum bile acids but normal CBC, biochemical profile, urinalysis, abdominal ultrasound, and transplenic portal scintigraphy routinely is given a diagnosis of PVH/MVD without pursuing additional diagnostics (Box 146-1).

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Portal Vein Hypoplasia (Microvascular Dysplasia)
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