Does Your Dog Have Hepatic Microvascular Dysplasia?
Often found by accident on bloodwork. Here's what HMD is, how it differs from a liver shunt, and what management looks like.
If your dog’s bloodwork just came back with elevated liver enzymes or high bile acids, and your vet mentioned something called hepatic microvascular dysplasia, it’s okay to take a breath. This isn’t necessarily the emergency it might sound like.
Hepatic microvascular dysplasia (HMD) in dogs is a congenital condition, meaning dogs are born with it, where the tiny blood vessels inside the liver are abnormally formed. Instead of routing blood through the liver’s filtering system the way they should, some of those vessels let blood slip past without being properly processed. Think of it as a microscopic version of a liver shunt, but one that often causes far fewer problems. Many dogs with HMD live full, normal lives and are only diagnosed because routine bloodwork flagged something worth investigating.
TL;DR
- HMD is a congenital liver condition where tiny abnormal vessels bypass normal filtering. It’s often found as an incidental finding on bloodwork, not because your dog is visibly sick.
- Small breeds like Yorkshire Terriers, Cairn Terriers, and Maltese are most commonly affected.
- HMD is not the same as a portosystemic shunt (PSS). A shunt is one large, often correctable vessel. HMD involves many microscopic vessels that can’t be surgically fixed, but also rarely need to be.
- Many HMD dogs are managed with a liver-supportive diet and routine monitoring. Anesthesia and certain medications require extra caution, so always flag the diagnosis with your vet before any procedure.
Which dogs are most commonly affected?
HMD is most common in small and toy breeds, particularly:
- Yorkshire Terriers
- Cairn Terriers
- Maltese
- Other small breeds
If you have one of these breeds and their bloodwork has flagged elevated bile acids or liver enzymes, HMD is one of the first things your vet will want to investigate.
What are the symptoms of HMD in dogs?
Here’s the thing: a lot of dogs with HMD show no symptoms at all. The diagnosis often comes as a surprise during routine bloodwork or a pre-surgical panel. When symptoms do appear, they tend to be subtle:
- Poor or slower-than-expected growth in puppies
- Occasional vomiting or diarrhea, especially after high-protein meals
- Mild lethargy or low energy
- Increased thirst and urination in some dogs
- Sensitivity to anesthesia or certain medications
Neurological signs like disorientation, circling, or seizures are much more typical of a large portosystemic shunt than HMD. If your dog is showing those symptoms, your vet will want to rule out a shunt first.
How do vets diagnose HMD?
The most common trigger for an HMD workup is elevated bile acids on bloodwork. Bile acids are substances the liver produces to help digest fat, and they get recycled through the liver after each meal. When the liver isn’t filtering blood properly, bile acid levels stay higher than they should, especially after eating.
Your vet will likely run a bile acid stimulation test, which compares a fasting bile acid level with one taken two hours after a meal. High bile acids in a dog with an otherwise normal-looking liver on imaging are a strong signal for HMD.
From there, diagnosis typically involves:
| Test | What it’s looking for |
|---|---|
| Liver enzyme panel | Checks for elevated ALT, ALP, and other markers of liver stress |
| Ultrasound | Looks at liver size and checks for a visible shunt vessel. In HMD, imaging often looks normal or near-normal. |
| Liver biopsy | The only definitive way to confirm HMD. A small tissue sample is examined under a microscope, where the microscopic vessel abnormalities can actually be seen. |
The biopsy matters because HMD can only be confirmed at the microscopic level. That’s also why ruling out a portosystemic shunt through imaging is an important step in the process.
HMD vs. liver shunt: what’s the difference?
This is the part that trips a lot of owners up, and it’s worth getting clear on because the two conditions are managed very differently.
A portosystemic shunt (PSS), sometimes called a liver shunt, is a single abnormal blood vessel that routes blood around the liver entirely. It’s usually visible on imaging, often causes more significant symptoms, and in many cases can be corrected with surgery.
HMD involves many tiny abnormal vessels deep inside the liver tissue. You can’t see them on an ultrasound, you can’t remove them surgically, and in most dogs, you don’t need to.
| Liver shunt (PSS) | HMD | |
|---|---|---|
| Vessel type | One large abnormal vessel | Many tiny microscopic vessels |
| Visible on imaging | Usually yes | No |
| Symptoms | Often more significant, including neurological signs | Often mild or none |
| Surgical fix | Possible in many cases | Not applicable |
| Management | Surgery or medical management | Diet and monitoring |
If your dog was diagnosed with HMD after imaging ruled out a shunt, that’s actually reassuring. It means the more surgically complex condition has already been crossed off the list.
How is HMD managed day to day?
For many dogs with HMD, management is straightforward. Your vet may recommend a liver-supportive diet, typically one that’s moderate in high-quality protein and low in ingredients that stress the liver. Some dogs also benefit from supplements like SAMe or milk thistle, though always check with your vet before adding anything.
Routine monitoring matters too, usually periodic bloodwork to track liver enzymes and bile acid levels, and watching for any new symptoms at home. Many HMD dogs have stable numbers for years.
The anesthesia and medication caution
This is something every owner of an HMD dog should know. Because the liver processes many anesthetic drugs, a liver that isn’t filtering efficiently can cause those drugs to stay in the system longer than expected. Before any procedure requiring sedation or general anesthesia, make sure your vet and any specialists know about your dog’s HMD diagnosis. Your vet may adjust the drugs used, the doses, or the monitoring protocol.
The same applies to certain medications. Drugs that are heavily metabolized by the liver, including some antibiotics, anti-inflammatory medications, and dewormers, may need dose adjustments. When in doubt, flag it with your vet first.
Does pet insurance cover HMD in dogs?
Yes, HMD is eligible for coverage under a Lemonade Pet accident and illness policy, when the condition isn’t pre-existing. That means your dog shouldn’t be showing any signs or have a documented diagnosis before your policy starts or during your waiting period.
Bile acid testing, liver enzyme panels, ultrasound, and potentially a biopsy can add up quickly. And that’s before factoring in follow-up monitoring, prescription diets, or supplements over time. For Yorkshire Terriers, Cairn Terriers, Maltese, and other higher-risk breeds, getting coverage in place early, before bloodwork flags something, is what keeps those future costs eligible for coverage.
Before we go
A diagnosis of hepatic microvascular dysplasia can feel alarming at first, especially when it comes with words like congenital and liver dysfunction. But for most dogs with HMD, life looks pretty normal. A good diet, regular check-ins with your vet, and a heads-up before any anesthesia or new medication are usually enough to keep things on track. Your dog is lucky to have someone paying this close attention.
Frequently Asked Questions
Is hepatic microvascular dysplasia fatal in dogs?
Not typically. Many dogs with HMD live normal lifespans with minimal symptoms. It’s a manageable condition for most affected dogs, especially when monitored regularly and supported with an appropriate diet.
How is HMD different from a portosystemic shunt?
A portosystemic shunt (PSS) is a single large abnormal vessel that’s often visible on imaging and may be surgically correctable. HMD involves many tiny microscopic vessels inside the liver that can’t be seen on imaging or surgically removed. HMD is generally less severe and is managed rather than corrected.
What does elevated bile acids mean in a dog with HMD?
High bile acids indicate the liver isn’t filtering blood as efficiently as it should. In HMD, some blood bypasses normal liver filtering through abnormal microscopic vessels, so bile acid levels, especially after a meal, tend to run higher than normal.
Do dogs with microvascular dysplasia need surgery?
No. Unlike a portosystemic shunt, HMD can’t be corrected surgically because the abnormal vessels are microscopic and spread throughout the liver. Management focuses on diet, supplements if recommended by your vet, and routine monitoring.
Which dog breeds are most at risk for HMD?
Yorkshire Terriers, Cairn Terriers, and Maltese are among the breeds most commonly diagnosed with hepatic microvascular dysplasia. Other small and toy breeds can be affected as well.
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