Resistance of the peripheral nervous system to the effects of chronic canine hypothyroidism.
BACKGROUND
Hypothyroidism has been implicated in the development of multiple peripheral mono- and polyneuropathies in dogs. The objectives of this study were to evaluate the clinical and electrophysiologic effects of experimentally induced hypothyroidism on the peripheral nervous system of dogs.


OBJECTIVE
Chronic hypothyroidism will induce peripheral nerve sensorimotor dysfunction.


METHODS
Eighteen purpose-bred, female dogs.


METHODS
Prospective, longitudinal study: Hypothyroidism was induced by radioactive iodine administration in 9 dogs, and the remaining 9 served as untreated controls. Neurological examinations were performed monthly. Electrophysiologic testing consisting of electromyography (EMG); motor nerve conduction studies of the sciatic-tibial, radial, ulnar, and recurrent laryngeal nerves; sciatic-tibial and ulnar F-wave studies; sensory nerve conduction studies of the tibial, ulnar, and radial nerves; and evaluation of blink reflex and facial responses were performed before and 6, 12, and 18 months after induction of hypothyroidism and compared with controls.


RESULTS
Clinical evidence of peripheral nervous dysfunction did not occur in any dog. At 6 month and subsequent evaluations, all hypothyroid dogs had EMG and histologic evidence of hypothyroid myopathy. Hypothyroid dogs had significant (P<or=.04) decreases in ulnar and sciatic-tibial compound muscle action potentials over time, which were attributed to the concurrent myopathy. No significant differences between control and hypothyroid dogs were detected in electrophysiologic tests of motor (P>or=.1) or sensory nerve conduction velocity (P>or=.24) or nerve roots (P>or=.16) throughout the study period, with values remaining within reference ranges in all dogs.


CONCLUSIONS
Chronic hypothyroidism induced by thyroid irradiation does not result in clinical or electrophysiologic evidence of peripheral neuropathy, but does cause subclinical myopathy.
Rossmeisl
J H
JH
Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060, USA. jrossmei@vt.edu
D000818:Animals; D002908:Chronic Disease; D004283:Dog Diseases / Q000209:etiology*; D004285:Dogs; D005260:Female; D007037:Hypothyroidism / Q000150:complications / Q000662:veterinary*; D008137:Longitudinal Studies; D009422:Nervous System Diseases / Q000209:etiology / Q000662:veterinary*; D017933:Peripheral Nervous System / Q000503:physiopathology*