Don't rush to carry out MRI evaluations after performing intrasynovial anaesthesia of the digital flexor tendon sheath, warn researchers at the University of Guelph, in Canada. Their work shows that local anaesthetic injected into the digital sheath results in changes that are visible on the magnetic resonance image for at least 72 hours.
The very act of injecting local anaesthetic into tissues can produce physical changes that need to be taken in to account when interpreting bone scans and ultrasound images. Until recently, knowledge of how local anaesthetic techniques influenced the appearance of magnetic resonance images was limited.
Researchers, led by Dr Belinda Black, at the University of Guelph 's Ontario Veterinary College conducted a study into whether anaesthesia of structures in the equine foot caused changes that were detectable on subsequent MRI evaluation.
The study involved fifteen healthy adult horses. None of them was lame, neither had they received non-steroidal anti-inflammatory drugs or undergone perineural or intrasynovial anaesthesia in the previous two weeks.
A baseline MRI examination of both front feet of each horse was carried out before performing the local anaesthetic techniques on one forelimb, selected at random. Mepivicaine was injected around the lateral and medial palmar digital nerves; into the podotrochlear (navicular) bursa; the digital flexor tendon sheath and the distal interphalangeal joint.
The MRI examination was repeated 24 and 72 hours after injection and the images compared.
The research team found that in the case of palmar digital nerve blocks, and injections in the podotrochlear bursa and distal interphalangeal joint, the MRI findings were not significantly different from baseline 24 and 72 hours after injection.. So, they suggest that, when these structures appear distended in MR images after diagnostic anaesthesia, it probably is a true reflection of disease and not simply due to the anaesthetic injection.
This information is particularly important, they point out, since distal interphalangeal joint effusion had been reported as the most common finding on MR images of the equine foot.
In contrast, injection of mepivicaine into the digital flexor synovial sheath resulted in distension of the sheath which was still present 24 and 72 hours later.
Dr Black comments that, although a definite time frame for resolution of digital flexor tendon sheath distension was not determined, she recommends waiting more than 3 days between intrasynovial anaesthesia of the digital flexor sheath and MRI assessment.
She concludes: “our study showed that perineural anaesthesia of the palmar digital nerves and intrasynovial anaesthesia of the podotrochlear bursa or distal interphalangeal joint in sound horses did not have any significant effect on the interpretation of subsequent MR images. However, intrasynovial anaesthesia of the digital flexor tendon sheath caused a significant iatrogenic increase in synovial fluid, which was detectable on subsequent MR images for at least 72 hours.”
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