Local Anesthesia's Effect on MRIs of Horse Feet (AAEP 2011)

Certain things just don't mix: oil and water, or wearing metal during X rays, for instance. But what about diagnostic anesthesia (nerve blocks) and magnetic resonance imaging (MRI), a combination that sometimes occurs because a horse undergoes an MRI study soon after nerve blocks in a lameness exam? A team of researchers recently examined whether diagnostic anesthesia could skew the results of equine foot MRIs.

"The effect of diagnostic anesthesia on the interpretation of MRI is unknown," explained Belinda Black, BVMS, a resident at the University of Guelph’s Ontario Veterinary College. "Our objective was to determine if mepivacaine (a local anesthetic) injection in the distal (lower) limb would cause variation (that’s) detectable with MRI."

During her presentation at the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas, Black explained that she and colleagues performed a baseline MRI (with horses under general anesthesia) up to six days prior to administering mepivacaine injections in 15 sound adult horses. They injected the local anesthetic into each horse's navicular bursa, digital flexor tendon sheath, or the coffin joint--all textbook areas for nerve blocks. They also performed a palmar digital nerve block in each horse.

The team then repeated the MRI procedure at 24 hours and 72 hours post-injection. The horses were also under general anesthesia for these MRI procedures.

Key findings from the study included:

  • Images taken at 24 and 72 hours post-injection in the coffin joint and the navicular bursa did not differ significantly from pre-injection images;
  • Images taken at 24 and 72 hours post-injection in the digital flexor tendon sheath showed a significantly increased amount of synovial fluid than the pre-injection images (this could be interpreted as joint swelling); and
  • No changes were noted in the palmar digital nerve block pre- and post-injection images, although needle tracts were often evident from the navicular bursa injection.

Black concluded that it remains unclear how long the mepivacaine in the digital flexor tendon sheath will yield the skewed results; however, she stressed, it's important for veterinarians to take these findings into consideration during MRI evaluation. 

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.