Chronic idiopathic myopathy in Icelandic horses: A case series

Background: Exertional myopathies are recognised as a cause of poor performance in equines. In Icelandic horses presenting reduced performance and/or multi-limb lameness, no specific myopathy has been identified. Objectives: To characterise the clinical presentation and histopathological findings in muscle biopsy samples from Icelandic horses demonstrating poor performance. Study Design: Case series. Methods: Phenotypic characteristics, clinical examination and diagnoses of 17 Icelandic horses were studied. All horses had a resting serum creatine kinase (CK) and aspartate transaminase activities measured, and CK activities 4–6 and 24 h post-exercise were measured in some horses. The semimembranosus muscle was biopsied in all horses and used to categorise horses into two groups: those with histopathological evidence of chronic idiopathic myopathy and those with normal biopsy findings. Results: Twelve horses displayed non-specific histological features of muscle degeneration and regeneration consistent with a myopathy. The affected horses had significantly increased CK activities after exercise (median 1637 U/L, IQR 861–2480 U/L) compared with the group with histologically normal muscle (median 234 U/L, IQR 211–457 U/L, p = 0.02). Phenotypic traits, such as tachypnoea, fatigue, stumbling and reluctance to tölt or canter, were present in both groups. Main Limitations: The small sample size and absence of a control group with normal performance. Conclusions: This case series suggests the presence of a novel myopathy in Icelandic horses. In horses of this breed, exhibiting decreased performance, multi-limb lameness and/or increased serum CK activity at rest or post-exercise, an underlying myopathy should be considered.