Pituitary Glands in Horses: One Size Fits All

In a recent study of horses’ brains, veterinarians defined normal parameters for equine pituitary glands on MRI. This information will help them diagnose tumors—such as those causing pituitary pars intermedia dysfunction (PPID), or equine Cushing’s disease, and other abnormalities—leading to earlier disease detection and treatment.

“Imaging features of the normal anatomy of the equine pituitary have been described with the use of CT, and abnormal features have been described with the use of CT and MRI,” said Kallie Hobbs, DVM, MBA, MS, during the virtual 2021 ACVIM Forum. But researchers had not yet investigated the normal gland using MRI.

Hobbs, a third-year internal medicine resident at the University of Florida’s (UF) College of Veterinary Medicine, in Gainesville, and her colleagues retrospectively evaluated imaging features of pituitary glands on standard MR images of the horse brain. The patients included 27 horses seen at UF from 2005 to 2020 for intracranial disease diagnostic evaluation.

The horses were 2-16 years old, weighed more than 300 kg (661 pounds), showed no signs of PPID (whether physical or on bloodwork), and had no pituitary abnormalities identified on MRI. Clinicians had captured images of each horse in dorsal recumbency—with the animal on its back—using a high-field 1.5 Tesla magnet and routine imaging protocols.

Hobbs and her colleagues measured maximal width and height of the brain and the pituitary gland on transverse (cross-sectional) plane post-contrast MR images, and maximal length on sagittal (longitudinal) images. In pre-contrast images they also looked at a region of focal increased signal intensity, dubbed the “bright spot,” thought to be a concentration of neurotransmitters in the pituitary. When they saw this feature, they noted the size (height and width on transverse, length on sagittal) and location.

What They Found

In shape 81% of the glands looked like snails, complete with stalks that resembled antennae, whereas 19% were oval, each with a stalk. The mean ± sd (standard deviation) pituitary gland dimensions were:

  • 1±2.4 mm wide;
  • 4±1.9 mm high; and
  • 5±2.7 mm long.

“The size of the pituitary gland was relatively constant between horses in this study,” said Hobbs, “and there was no significant correlation to body weight or brain size. Magnetic resonance imaging appearance of the normal equine pituitary gland is similar to that reported in dogs, cats, and in humans. This can be useful in determining normal and mildly abnormal pituitary gland sizes in horses.”

Further underscoring their findings, the team found that MRI measurements aligned with previous reports of equine pituitary gland measurements on necropsy in clinically normal horses.

Additionally, they found bright spots on pre-contrast images of pituitary glands, with the spot located caudally (toward the rear) in 24 of the horses (88%) and centrally in two horses (<1%). One horse had no bright spot (<1%).

Hobbs said these findings suggest horses might typically have caudal placement of the bright spot, whereas dogs, cats, and humans typically have a central placement. “This may indicate that horses with central placement may have no pathologic abnormalities,” she said. “The information may be indicative that bright spot variations would have little pathologic significance in horses.”

She acknowledged two study limitations: histopathology and metabolic testing results weren’t available for all cases, simply because it wasn’t warranted in these normal cases. In future studies the team wants to conduct dynamic evaluation of contrast enhancement on MRI—to show the timing for contrast to perfuse and wash out of the gland—and investigate abnormal MRI pituitary gland findings in horses.

Hobbs explained to TheHorse.com that veterinarians can use these findings as guidelines as they examine pituitary enlargements suggestive of adenomas (tumors) in horses regardless of horse size or weight.

“Determining normal measurements of the pituitary on MR can help veterinarians to diagnose pituitary disease sooner and may even serve to help in development of more targeted treatments in the future,” she said.

Table 1: Measurements of pituitary gland and brain from MR images in 27 horses

  Pituitary Height (mm) Pituitary Width (mm) Pituitary Length (mm) Brain Height (mm) Brain Width (mm)(
Mean 10.4 21.1 24.5 91.5 102.2
Standard Deviation 1.9 2.4 2.65 5.2 18.48
Range 7.1 – 13.7 16.7 – 24.7 19.5 – 29.6 81.5 – 102.8 90.8 – 110.74