Anaphylaxis in Dogs & Gallbladder Halo Sign
April 2, 2021
Written by Chelsea Liberati, DVM
Diagnosing the Condition
Over the past decade, ultrasound has been slowly integrated into the veterinary industry. As a result, ultrasonography has allowed veterinarians to detect other soft tissue changes and free fluid that would otherwise be missed.
It is recommended that in patients that present to the emergency room for suspected anaphylaxis that an AFAST (focused assessment with sonography for trauma) is performed for abnormalities. The most common abnormalities that are noted within the abdominal cavity secondary to anaphylaxis are gallbladder edema and hemoabdomen. Furthermore, there can be changes with the caudal vena cava.
When intramural edema is present the gallbladder wall is characterized sonography as striated with hyperechoic-anechoic-hyperechoic or hyperechoic-hypoechoic-hyperechoic architecture. Other common names for gallbladder wall edema are halo effect, double rim effect, or halo sign.
Gallbladder wall edema is a sonographic marker for canine anaphylaxis. The canine shock organs that contain the highest concentration of mast cells are the liver and gastrointestinal tract. Interestingly enough, felines do not have the same shock organ as canines. The main shock organ in felines is the lung, therefore gallbladder edema is not a hallmark of anaphylaxis. The cause of the gall bladder edema is due to a massive histamine release within the portal circulation causing hepatic venous sphincter constriction and massive hepatic venous congestion. Anaphylaxis induced gallbladder edema is an immediate occurrence and can last up to 24-48 hours post insult.
Other abnormalities that can be noted secondary to anaphylaxis are an increase in serum alanine transaminase (ALT), hemoabdomen, vomiting, diarrhea as well as cutaneous signs. Although cutaneous signs are absent in nearly 95% of cases. The elevation in ALT typically lags several hours, therefore it cannot be relied upon in the immediate diagnosis of anaphylaxis. The mean value for ALT is 400 IU/L in anaphylactic canines. The hepatic venous congestion along with factors that contribute to acquired coagulopathy leads to a hemoabdomen.
Unfortunately, gallbladder wall edema is not pathognomonic for canine anaphylaxis. In an acutely collapsed or hypotensive canine with gallbladder wall edema, you must rule out other differentials such as pericardial effusion, right-sided heart failure or dilated cardiomyopathy. The caudal vena cava can help determine the cause of the gallbladder wall edema because it is an indirect marker of volume status and central venous pressure. In anaphylaxis the caudal vena cava is flat or small while in pericardial effusion and right sided cardiac cases it is fat or distended due to backflow of blood.
Dogs with anaphylaxis commonly develop abdominal effusion. Abdominal fluid is scored 1 to 4 when using the AFAST (abdominal focused assessment with sonography for trauma) applied fluid scoring system (AFS). The most common abdominal fluid score in anaphylaxis is 1 or 2. The fluid is most commonly located in the diaphragmatic hepatic view. The lower scoring effusions (1 to 2) are often self-resolving and non-coagulopathic. In addition to an AFAST, PCV/TS (packed cell volume/total solids) and coagulation panel (aPTT/PT) is recommended on presentation. A serial AFAST is recommended at least 4 hours post administration or sooner pending patient status. If the fluid score is increasing, then recheck PCV/TS and coagulation panel is recommended. AFAST with AFS should rechecked at least every 4 hours until certain that the acquired coagulopathy was been corrected and the hemoabdomen has been resolved.
The aPTT is more affected by heparin, therefore PT and aPTT times should be discordant with the aPTT being far more prolonged. When the PT is normal or mildly elevated with an out-of-range aPTT, you should have anaphylaxis on your list of differentials for a potential cause of the coagulopathy.
There are cases of canine anaphylaxis that will have a large volume effusion of AFS 3 or 4. Even in large volume effusions the coagulation profile may be within normal limits or slightly above more (<25% above upper reference range). The large volume effusions typically self-resolve in 24 hours if the patient responds favorably to initial resuscitation and therapy. Canine anaphylaxis treatment typically includes fluid resuscitation +/- epinephrine (low dose 0.01 mg/kg IM or IV, repeated every 5-10 minutes, if fails then CRI starting at 0.05 mcg/kg/min), diphenhydramine, famotidine and glucocorticoids (dexamethasone sodium phosphate or prednisone). Abdominocentesis should only be obtained if safely accessible. The PCV of the abdominal fluid is > 50% of the peripheral PCV. When the coagulation profiles are > 25%, clotting factors should be replaced as soon as possible (e.g., fresh frozen plasma).
Treatment
On presentation of a patient that has a hemoabdomen secondary to anaphylaxis it is recommended to administer an initial immunosuppressive dose of dexamethasone sodium phosphate (0.3 mg/kg IV/IM) followed by an anti-inflammatory dose of prednisone (0.25mg/kg q12hr PO for 3 days then 0.25mg/kg PO q24 hours for 3 days). In addition to the administration of steroid, diphenhydramine (2mg/kg IM/PO PRN) and famotidine (0.5 mg/kg IV/IM/PO q12-24 hours for 5-7 days) are recommended.
References:
Gregory R. Lisciandro, G. (2020). Update on Canine Anaphylaxis: Diagnosis, Treatment & Medically-Treated Hemoabdomen & More than Gallbladder Wall Edema. Hill Country Veterinary Specialists & FASTVet.com.