Long-Term Follow Up of Refractory Myotonia Associated with Hyperadrenocorticism in a Maltese Dog

A 12-year-old, female Maltese was referred with uncontrolled hyperadrenocorticism (HAC). Despite well management of HAC through treatment with trilostane and serial monitoring with ACTH stimulation tests for over three years, stiffness of the neck and limbs progressively worsened over time. Neurological test was performed, which showed no abnormalities of cranial nerves. Proprioception was delayed but the cause appeared to be due to stiffness of limb muscles. Muscle tone had increased over time and stiffness had worsened to the extent where it made walking difficult. MRI scans showed no orthopedic or spinal diseases, and pituitary microadenoma was confirmed with pituitary gland measurement of 6 × 6.4 × 4.5 mm (H × W × L). Electromyography presented random discharges with fluctuating amplitude and frequency, which were consistent with myotonic discharges. There were no improvements of myotonic signs despite treatment for HAC with trilostane. Supplementation of L-carnitine and coenzyme Q-10 to mitigate muscle stiffness, following diazepam and methocarbamol to help with muscle rigidity, failed to show any positive effect and the dog died a sudden death, 1,182 days after the initial visit.