Resistance of the Peripheral Nervous System to the Effects of Chronic Canine Hypothyroidism

• Published in: Journal of veterinary internal medicine 2010-07, Vol.24 (4), p.875-881
• Main Author: Rossmeisl, J.H. Jr
• Format: Article
• Language: English
• Subjects: action potentials; Animals; Chronic Disease; Dog; Dog Diseases - etiology; Dogs; electromyography; Electrophysiology; Female; hypothyroidism; Hypothyroidism - complications; Hypothyroidism - veterinary; iodine; irradiation; larynx; long term effects; Longitudinal Studies; medicine; muscles; muscular diseases; nerve tissue; Nervous System Diseases - etiology; Nervous System Diseases - veterinary; peripheral nervous system; Peripheral Nervous System - physiopathology; peripheral nervous system diseases; Peripheral neuropathy; Thyroid; tibia
• ISSN: 0891-6640; 1939-1676
• DOI: 10.1111/j.1939-1676.2010.0515.x

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. Hypothesis: Chronic hypothyroidism will induce peripheral nerve sensorimotor dysfunction. Animals: 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≤ .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≥ .1) or sensory nerve conduction velocity (P≥ .24) or nerve roots (P≥ .16) throughout the study period, with values remaining within reference ranges in all dogs. Conclusion: Chronic hypothyroidism induced by thyroid irradiation does not result in clinical or electrophysiologic evidence of peripheral neuropathy, but does cause subclinical myopathy.