Effects of phenytoin in two myotonic horses with hyperkalemic periodic paralysis.

This study explores the effects of phenytoin treatment on two myotonic, or muscle spasming, horses suffering from hyperkalemic periodic paralysis (HPP) – a genetic disease causing irregular heartbeat, muscle weakness or paralysis. The study found that phenytoin alleviated the horses’ symptoms, suggesting it may be a viable medication for managing this condition in horses.

Effect of Phenytoin on Horses with Myotonia and Hyperkalemic Periodic Paralysis

• The research focuses on assessing the effects of phenytoin, a common anti-seizure medication, on horses showing signs of myotonia and hyperkalemic periodic paralysis (HPP). HPP is a potassium-aggravated disorder characterized by episodes of muscle weakness or paralysis, usually triggered by a high intake of potassium or a sudden surge in potassium levels in the bloodstream.
• Two horses with clinical signs of myotonia and HPP were subjected to phenytoin treatment. Results showed an evident decline or complete abolition of muscle twitching or fasciculations following oral potassium ingestion.

Phenytoin’s Effect on Electromyographic Examination Results

• An electromyographic (EMG) test is a diagnostic procedure to assess the health of muscles and the nerve cells that control them. Following the phenytoin treatment, the EMG tests of the horses showed a decrease or total abolition of repetitive firing and myotonic discharges — findings indicative of myotonia.

Impact of Phenytoin on Calcium Release in Muscles

• In both horses, heavy sarcoplasmic reticulum fractions from the skeletal muscle were used to assess calcium-induced calcium release (CICR). CICR is essential for the contraction of muscles. Both horses had an abnormally low threshold for CICR, which increased with phenytoin treatment.

Phenytoin’s Role in Regulating Disordered Ion System in Skeletal Muscles

• The study concludes by suggesting that phenytoin may be effective in modifying the disordered ion regulation in the sarcolemma and the sarcoplasmic reticulum of the skeletal muscle of horses with HPP.
• The sarcolemma is a thin membranous sheath, while the sarcoplasmic reticulum is a form of endoplasmic reticulum found in muscle cells that plays an integral role in muscle contraction and relaxation. Disordered ion regulation in these structures has been associated with muscle-related diseases, including HPP.