NEUROLOGICAL MANIFESTATIONS OF HYPOTHYROIDISM - A RETROSPECTIVE STUDYOF 29 DOGS

Neuromuscular signs in association with hypothyroidism are described i n 29 dogs. Eleven dogs had lower motor neuron signs, 9 had peripheral vestibular deficits, 4 had megaesophagus, and 5 had laryngeal paralysi s. Primarily older (mean = 9.5 years), large-breed dogs were affected, and there was no sex or breed predisposition. Duration of clinical si gns before presentation ranged from 2 to 8 weeks (mean = 5 weeks). The diagnosis was based on (1) results of neurological examination (29 do gs); (2) electromyographic abnormalities (18 dogs), including fibrilla tion potentials (n = 18), positive sharp waves (n = 15), and complex r epetitive discharges (n = 4); (3) high serum cholesterol concentration (10 dogs; mean = 335 mg/dL); (4) low response to thyroid-stimulating hormone (29 dogs; mean T4 prestimulation concentration = 0.8 mug/dL; m ean T4 poststimulation = 1.2 mug/dL); and (5) good response to thyroxi ne supplementation (26 dogs). Dogs with vestibular deficits had abnorm al brainstem auditory-evoked responses (BAER), including increased lat encies of P1-P6 and decreased amplitude of P4,5 - N5. Seven other dogs had similar BAER abnormalities without manifesting clinical signs of vestibular involvement. Three dogs with vestibular signs had fibrillat ion potentials and positive sharp waves without exhibiting lower motor neuron signs. All dogs were supplemented with levothyroxine (0.02 mg/ kg PO bid). The follow-up period ranged between 6 and 30 months (mean, 14 months). Serum T4 concentrations were measured at least 3 times fo r each dog every 2 months (mean T4 concentration = 2.6 mug/dL). All bu t 1 dog with lower motor neuron signs and 1 dog with vestibular signs recovered after 2 months (mean, 57 days). Signs of megaesophagus becam e progressively less severe over 4 months. Dogs with laryngeal paralys is improved partially after 5 months. We suggest that either vestibula r or lower motor neuron signs, megaesophagus, or laryngeal paralysis m ay be the only clinical signs of an underlying, more generalized polyn europathy associated with hypothyroidism. Electrodiagnostic abnormalit ies may be detected before clinical disease develops.