Skeletal muscle myopathy is a hereditary muscle disorder in which there is a deficiency of type II muscle fibers leading to a notable decrease in skeletal muscle mass. Although several breeds of dogs have been observed to demonstrate similar disorders, this condition is only seen in Labrador retrievers and was first reported and described in 1976. Other names for HMLR include muscular dystrophy, myotonia, generalized muscle weakness, polyneuropathy, and hereditary myopathy. These symptoms become more obvious as the exercise continues and the dog tires or if the dog is exposed to cold weather. Eventually, if not allowed to rest or to keep warm, the dog may temporarily collapse.
Rest improves symptoms but follow-up exercise quickly brings on a relapse.
Additionally, as the disease progresses, atrophy of the muscles in the limbs and head becomes apparent. Diagnosis can be difficult especially in mildly to moderately affected dogs since symptoms of HMLR can often be mistaken as produced by other disorders, particularly hip dysplasia. Often, other than intolerance to exercise, there is no outward evidence of other physiological or neurological abnormalities. Routine blood work is usually normal in affected dogs. However, because other diseases affecting the heart and lungs, muscles and adrenal glands may produce symptoms of exercise-intolerance similar to HMLR, it is important that these disorders be ruled out as well.