Is it a skull deformity or just a tiny dog

The skull shapes of the tiniest dog breeds can look very different from their larger brethren, posing a challenge for veterinarians attempting to separate problems from normal variation.

A new study from researchers in the College of Veterinary Medicine (CVM) helps deepen the understanding of skull shapes within different sized dogs and draws a link between cranial and facial shapes, body weight and the risk of syringomyelia, a spinal condition common in some dog breeds.

"We wanted to better characterize what happens to skull shape when you shrink dogs down, so we don't misdiagnose normal findings as pathologic processes," said Dr. Peter Scrivani, professor of clinical sciences in CVM and the study's corresponding author.

The study, "Miniaturization in Domestic Dogs: Relationships Among Cranial Shape, Head Indexes, and Body Weight," published May 22 in the journal Veterinary Radiology & Ultrasound.

The researchers identified the medical records of 852 dogs of varying breeds who had received CT scans of their heads at CVM and did not have a known cranial disease.

Study co-author Dr. Ian Porter, former resident at CVM who's now associate clinical professor of diagnostic imaging, performed a quantitative analysis of those images, calculating a set of head indices from the widths and lengths of the skull, cranium and face.

Fellow co-author Dr. Alyssa Froese, imaging resident at CVM, performed a qualitative analysis, grouping dogs into categories: archetypal cranium, intermediate cranium, round cranium and extreme round cranium. Co-author Dr. Patrick Carney, associate professor of clinical sciences at CVM, provided statistical analysis.

"This study provides strong evidence that when dogs get smaller, they don't reduce in size proportionately or isometrically, they actually have a recognizable shape change, which is allometric scaling," Scrivani said. "We're not just looking at a miniature wolf-like skull. We're actually seeing that as the dog shrinks, the cranium undergoes ballooning of the vault that holds the brain, creating a rounder shape."

Similar shape changes are observed in dogs with altered fluid balance in the central nervous system like hydrocephalus and syringomyelia, a painful condition in which fluid-filled cavities form in the spinal cord. The data showed that breeds predisposed to syringomyelia have a high cranial index, regardless of the length of their nose, and they weigh less than 44 pounds. The condition is most common in breeds such as Cavalier King Charles spaniels, Affenpinschers, Brussels Griffons, Chihuahuas and Pomeranians.

"For every 2.2 pounds of body weight that you reduce, there's a 25% increase in the risk of being in a group that's going to get syringomyelia," Scrivani said, "and the shorter and wider the cranium is, the higher risk you have of being in that group."

Further research is needed to understand if bony changes to the cranium and face contribute to syringomyelia or are a consequence of the disease process, Scrivani said. That understanding could inform treatment and breeding strategies.

"This is a study on associations," he said. "We don't prove cause and effect, but we identify things that may be related, or that are related, at least statistically, and then we provided different possibilities for why these associations may have been observed."