Inherited neurologic disorders
Epileptic partial seizures. Idiopathic epilepsy is a common, presumably inherited condition in Labrador retrievers and golden retrievers.68,69 Exercise, excitement, and hyperventilation can all serve as triggers for seizures in affected dogs. Most often the provoked
event is a typical generalized seizure, with sudden loss of consciousness and tonic-clonic convulsions.68 Partial seizures can be more variable in their presentation, with a sudden onset of clinical signs that might include mydriasis,
staring, ataxia, crawling, swaying, loss of balance, salivation, and disorientation.68-72 When these signs develop suddenly during or after exercise, the dog may be presented to a veterinarian for evaluation of
exercise intolerance or, in a Labrador retriever, for a suspected diagnosis of exercise-induced collapse (EIC).73
The acute onset and rapid spontaneous recovery help to identify these episodic events as probable seizures. Also, many Labradors
with exercise-provoked partial seizures will progress over months to years to having a more classic form of epilepsy with
generalized seizures and seizures not necessarily associated with an exercise or excitement trigger.72 Affected dogs can be managed with standard anticonvulsant protocols using phenobarbital or potassium bromide.72
Exercise-induced collapse in Labrador retrievers. An inherited syndrome of EIC is the most common cause of exercise-induced weakness or collapse in otherwise healthy Labrador
retrievers.73 Affected dogs are normal at rest and after mild or moderate exercise, but strenuous exercise in conjunction with excitement
results in ataxia and rear limb weakness that sometimes progresses to collapse.73,74 Most affected dogs experience their first episode of weakness or collapse before the age of 3 years.73,75 During collapse, affected dogs are hyperthermic and they hyperventilate, but physiologic and clinicopathologic parameters
are not dramatically different from normal exercise-tolerant Labrador retrievers taking part in the same exercise.74 Physical examination and neurologic examination findings are normal at rest, but patellar reflexes are absent during collapse.74 Affected dogs may experience a profound loss of balance (disequilibrium) during collapse and recovery.73,74 Most dogs gradually recover within 10 to 20 minutes after exercise is halted, with no residual clinical or clinicopathologic
abnormalities, but rarely a dog will die during an episode of collapse.73,74 Muscle biopsy results are normal.74 Diagnosis is by eliminating other causes of exercise intolerance and demonstrating that the affected dog is homozygous for
the causative mutation in the gene for dynamin 1 (Veterinary Diagnostic Laboratory, University of Minnesota, St. Paul MN:
http://www.cvm.umn.edu/vdl/ourservices/canineneuromuscular/home.html.76
Dynamin 1 (DNM1) plays a key role in forming synaptic vesicles containing neurotransmitters and is essential for sustained
synaptic transmission in the brain and spinal cord during periods of intense exercise, excitement, and perhaps hyperthermia.76 EIC is inherited as an autosomal recessive trait, and dogs with two copies of the EIC mutation are susceptible to collapse.76
Testing has revealed that the EIC mutation is common in field, show, and pet Labrador retrievers, with current data showing
that 30% to 40% of Labradors are carriers and 3% to 5% of dogs in the breed are affected.75 So far, the DNM1 mutation causing EIC has only been found in Labrador retrievers, curly coated retrievers, and Chesapeake
Bay retrievers.75,76 The best treatment is to avoid intensive exercise in conjunction with extreme excitement and to terminate exercise at the
first sign of weakness or wobbliness.
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