<unk>  is a genetic disorder which occurs in both humans and horses.  It is characterized by muscle hyperexcitability or weakness which, exacerbated by potassium or cold,  can lead to uncontrolled shaking followed by<unk> .  Onset in humans usually occurs in the twenties.
The mutation which causes this disorder is  dominant with linkage to the
sodium channel expressed in muscle (called SCN4A).  In the presence of high potassium levels, sodium channels fail to activate properly.  The mutation causes single amino acid changes in parts of the channel which important for inactivation.
Equine<unk>  occurs in 1 in 50 quarter horses and can be traced to a
single ancestor (A stallion named Impressive).  Episodes in these horses are commonly brought on by alfalfa, which contains high levels of potassium.
Hyperkalaemic periodic paralysis
Myoneurogastrointestinal encephalopathy syndrome
Neurasthenia
Progressive supranuclear palsy
Myotonia congenita
Woodhouse-Sakati syndrome
Lambert-Eaton myasthenic syndrome
Myasthenia gravis