Definition
The inverted Mickey Mouse sign is a transverse abdominal ultrasound pattern in intact female dogs characterized by:
- A large, round anechoic urinary bladder (central structure)
- Two symmetrically enlarged, fluid-filled uterine horns positioned caudolateral or ventral to the bladder
This creates a three-structure configuration resembling an inverted Mickey Mouse head.
Diagnostic Basis
This pattern indicates:
- Bilateral uterine horn distension
- Presence of intraluminal fluid
- Loss of normal uterine diameter and contour
When seen in an intact female with compatible clinical findings, this pattern is highly suggestive of pyometra.
Ultrasound Features Supporting Pyometra
- Tubular, paired structures continuous with uterine body
- Anechoic to hypoechoic luminal contents, often with internal echoes
- Increased uterine diameter (>1 cm typically, varies by size)
- Thickened or irregular uterine wall
- Absence of fetal structures
Diagnostic Interpretation
The inverted Mickey Mouse configuration reflects:
- Simultaneous enlargement of both uterine horns
- Fluid accumulation under pressure
- Anatomical displacement around the bladder
This distinguishes it from:
- Intestinal loops (peristalsis, wall layering)
- Pregnancy (gestational sacs, embryos)
- Mucometra/hydrometra (similar imaging but requires clinical correlation)
Clinical Correlation Required
Diagnosis should be supported by:
- Intact female status
- Recent estrus history
- Systemic signs (lethargy, anorexia, PU/PD, discharge or none)
- Hematologic changes (leukocytosis, left shift)
Diagnostic Conclusion
The inverted Mickey Mouse sign on ultrasound represents:
- Marked bilateral uterine distension with fluid accumulation
In the appropriate clinical setting, this finding is diagnostic for pyometra until proven otherwise.
Sources
- Nyland TG, Mattoon JS. Small Animal Diagnostic Ultrasound, 3rd ed. Elsevier.
- Thrall DE. Textbook of Veterinary Diagnostic Radiology, 7th ed. Elsevier.
- Johnston SD, Root Kustritz MV, Olson PN. Canine and Feline Theriogenology. Saunders.
- European School for Advanced Veterinary Studies (ESAVS). Small Animal Reproduction Imaging Modules.
- American College of Veterinary Radiology (ACVR) imaging references.
Dr. Geoff Carullo is a Fellow and the current President of the Philippine College of Canine Practitioners.
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