Hereditary Myopathy of Labrador Retrievers (HMLR)

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 Autosomal Recessive Muscular Dystrophy (ARMD), Myotonia, Generalized Muscle Weakness, Polyneuropathy, and Hereditary Myopathy. The disease is inherited as an autosomal recessive trait, meaning that both parents of an affected dog must carry at least one copy of the a disease gene, and that affected dogs have two copies of the disease gene.

Symptoms include muscle weakness, abnormal gait and posture, and a decrease in tolerance to exercise. Onset of symptoms usually occur at three to four months of age; however, some dogs demonstrate symptoms as early as six to eight weeks or as late as six to seven months. The symptoms of abnormal gait and posture become more obvious as exercise continues and the dog tires or if the dog is exposed to cold weather. Rest improves symptoms but a relapse will occur when the activity level is increased again. Other signs of HMLR include abnormalities of the joints and, as the disease progresses, atrophy of the muscles in the limbs and head becomes apparent.

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