Introduction:
In 1976, Kramer and others described a condition of Labrador retriever dogs characterized by a marked deficiency of skeletal muscle mass, abnormal head posture, and a stiff hopping gait. Subsequently, the condition was shown to be inherited as an autosomal recessive trait, and further reports came from the United States, the United Kingdom, and Australia.

Nomenclature:
Hereditary mypoathy of Labrador retrievers (HMLR) was first described as a muscle disorder characterized by a deficiency of type II muscle fibers. It has also been referred to as muscular dystrophy, myotonia, generalized muscle weakness, polyneuropathy, and hereditary myopathy.

Clinical Signs:
This disease is seen only in Labrador retriever dogs. It affects both males and females and has been seen in animals of black and yellow coat color. In typical cases, clinical signs become obvious at 3 to 40 months of age and include muscle weakness, abnormalities of gait and posture, and decreased exercise tolerance. Severely affected pups may have a low head posture, with ventroflexion of the neck. The back is arched, and the gait is characterized by short stilted strides in which the hind legs are often advanced simultaneously (“bunny hopping’). The abnormality becomes more accentuated as the animal tires and, if encouraged to continue, the pup may collapse forwards with the head and neck to one side. There is no loss of consciousness or cyanosis. Exercise tolerance may be reduced to 20 yards in severely affected animals. However, mildly-affected dogs may be presented because they seem to be “slow” pups that are less playful than their littermates and less willing to exercise. These dogs may not collapse unless forcibly exercised at speed for several minutes. Rest results in some improvement, but the clinical signs rapidly recur on resumption of exercise.

Joint posture is often abnormal, with affected dogs having carpal overextension, carpal valgus, splaying of the digits, and a “cow hocked” stance. As the condition progresses, generalized atrophy of skeletal muscles develops. The proximal muscles of the limbs and the muscles of the head are particularly affected, but in milder cases, the atrophy may not be dramatic.

In most cases, the clinical sign stabilize between 6 months and 1 year of age, although signs may be exacerbated by excitement of stress and particularly by exposure to cold weather. After exposure to cold, an affected dog may be unable to stand or to lift the head. Moving the animal to a warm kennel usually results in improvement within a few hours.

A less common sequel, which has been observed in three adult dogs, is the development of megaesophagus. One affected 18-month-old dog, in other respects, appeared to be improving. The other two dogs were both affected bitches in the eighth week of pregnancy. Other sporadic complications that have been observed include the presence of a luxating patella and clinical and radiographic evidence of degenerative joint disease in the hip of one affected dog that was allowed to become obese.

Neurologic Examination:
Affected-dogs are bright and alert, although often poorly muscles when compared with their normal littermates. Temporal muscle atrophy is often a feature, but cranial nerve functions are otherwise normal. Muscle tone may be normal or reduced. There is no muscle pain on palpation nor dimpling on percussion. Severely affected pups are obviously weak and may have difficulty wheelbarrowing or hopping, although in less severely affected pups, postural testing may indicate no abnormalities. Proprioceptive function is normal and no sensory deficits have been observed in affected dogs. Tendon reflexes are generally reduced or absent, even in midly-affected dogs with little muscle atrophy. There is no impairment of bladder function nor other signs of autonomic nervous system dysfunction.

Diagnosis:
A diagnosis of HMLR may be suspected from the signalment data, clinical signs, and results of the neurological examination. Further procedure used in establishing the diagnosis include serology, electrodiagnosis, and muscle biopsy. Serum creatine kinase may be within normal limited or may be moderately elevated. Levels may increase following exacerbation of signs after exposure to cold weather but do not reach the levels reported in other degenerative muscle diseases, such as the inherited muscular dystrophy described in golden retrievers. Other routine hematological and blood biochemical parameters are within normal limits.

Motor nerve conduction velocities are within the normal range in affected dogs, and there is no decremental response to repetitive nerve stimulation. On electromyographic examination, there frequently is spontaneous activity, particularly in the proximal limb muscles, musculature of the head, and the thoracolumbar paraspinal muscles. The most commonly recorded abnormalities are fibrillation potentials, positive sharp waves, and bizarre high frequency discharges. Electromyographic changes may be less pronounced in midly-affected dogs and may be difficult to detect in very young dogs. Results of electrocardiographic examination of affected adults and pups have indicated no cardiac involvement.

Despite the abnormal joint posture seen in many affected dogs, on radiography of hocks, carpi, and the vertebral column, there have been no abnormalities. However, in some cases, changes consistent with hip dyslpasia have been present.

A wide range of morphological features may be observed in muscle biopsies from affected dogs. The changes reported include small and large group atrophy, small angular fibers of both fiber types, and occasional fiber type grouping. All of these changes are generally considered characteristic of neurogenic disease. In other biopsies, there may be large numbers of internal nuclei, disturbances in myofiber architecture, necrosis, regeneration, and replacement of muscle fibers with fat and fibrous tissue. These changes are more commonly associated with destructive myopathies or dystrophies. Alterations in fiber type percentages are a common finding. In most muscles, there is a reduction in the proportion of type II fibers, but in others, such as the cranial tibial, an increase in the percentage of type II fibers may occur. These changes in fiber type proportions appear to become more accentuated as the disease progresses.

Preliminary biochemical data indicate significantly elevated concentrations of sodium, calcium, zinc, copper, chloride, fat, and intracellular water and reduced levels of potassium and magnesium in muscles from affected adult Labrador retrievers. In addition, a significant decrease in muscle specific proteins has been identified in the biceps femois muscle of affected dogs.

Despite the presence of some apparently “neurogenic” features, examination of the various parts of the lower motor neuron has so far failed to identify morphological abnormalities. The underlying pathophysiologial mechanisms involved in this disease are, therefore, still unclear.

Prognosis:
In most cases, the clinical signs stabilize between 6 months and one year of age, and affected dogs may become acceptable house pets, although they are not suitable for work. Owners of affected dogs should be warned that stress, including exposure to low temperatures, can result in a dramatic worsening of clinical signs, even in clinically stable adults. The life span of affected dogs does not appear to be directly affected by the condition, although the prognosis for dogs with megaesophagus should be more guarded, due to the risk of developing inhalation pneumonia.

Treatment/Control:
There is no definitive treatment for this condition. As there is as yet no way of detecting heterozygous carriers, breeders should be advised against breeding parents or siblings of affected pups.

Reprinted by permission of the author.