Narcolepsy – sick while sleeping

In narcolepsy, the cerebrum and brain stem no longer produce enough neurotransmitters (chemical messengers) that are responsible for controlling the sleep-wake cycle. There is a congenital, hereditary form of narcolepsy that is already apparent in foals and an acquired form that only develops later in a horse's life. The exact triggers for this malfunction are not yet known.

It is suspected that it involves a widespread biochemical malfunction of various neurotransmitters throughout the brain. In humans, it is believed that an autoimmune disease leads to the destruction of the nerve cells that release the affected neurotransmitter.

In the hereditary form of narcolepsy, which occurs mainly in Shetland ponies and miniature ponies, a gene mutation is believed to cause the receptor for a certain neurotransmitter to be missing, so that although the concentration of the messenger substance is not reduced, it cannot work because of the missing receptors.

The signs of the disease can vary from mild muscle weakness to complete collapse. Since the disease is extremely rare in horses ridden by riders, it is not always immediately noticeable to owners.

Affected horses often stand with their eyes half open, dozing and appearing absent-minded until they suddenly lose their balance and stumble, stagger, and sway. Some are able to pull themselves up again at the last moment, while others actually fall to the ground. This sequence of dozing, staggering, falling, getting up, and collapsing again can repeat itself several times in a row. In many cases, it is very specific situations or events that can trigger an attack. This can be a certain movement when grooming the horse, leading it out of the stable, feeding it, or putting on the saddle girth, but also simply playing in the pasture. In sick horses, injuries to the carpal joint are often observed, as the horses usually collapse at the front first and land on their carpal joints.

These attacks can last from a few seconds to up to 10 minutes. As the disease progresses and nerve cells become increasingly damaged, the seizures occur at ever shorter intervals. In the time between seizures, the animal shows no neurological abnormalities and behaves completely normally.

It is not always easy for a veterinarian to diagnose narcolepsy. An initial suspected diagnosis is based on a detailed preliminary report and a thorough general and specific neurological examination by the veterinarian. Video monitoring and recordings of the sleep behavior of the horse suspected of having narcolepsy have also proven helpful. The next step should be to rule out other diseases that can cause similar symptoms. These include, in particular, cardiovascular diseases and tumorous growths that can press on the brain stem.

In this context, it is also very important that the veterinarian observes the horse in its environment on site in order to distinguish true narcolepsy from another sleep disorder. This so-called sleep suppression is much more common than narcolepsy. Caused by stress or pain, affected horses no longer lie down to sleep until they eventually collapse from exhaustion. Particular attention should be paid to horses kept in a running or open stable. If there are not enough places to retreat to, lower-ranking animals often cannot rest.

Furthermore, a blood test can provide information about other underlying diseases. With the help of various pharmacological test procedures, the veterinarian can confirm the diagnosis by stimulating or reducing the seizures. The horse is given medication that either triggers or prevents the seizures. As both procedures can cause certain side effects, they should only be carried out under veterinary supervision in a clinic.

As the triggers for the disease are still not precisely known, causal treatment is not yet possible. Symptomatic treatment depends on the extent and severity of the narcolepsy. The disease can be treated with certain psychotropic drugs used in human medicine, but cannot be cured, and the success of treatment varies greatly.

Since unexpected, violent falls often occur, especially in horses with cataplexy, there is a high risk of injury to both the horse and the owner. In horses, the tarsal joints, fetlocks, and head are often affected. Special padded bandages can be used to temporarily protect the horses. In advanced stages of the disease and in cases of severe attacks with increasing loss of control, the horse should be euthanized to prevent further accidents involving the animal or humans.

The disease is currently considered incurable. However, in the rarer familial or hereditary form, there are reports of Shetland ponies and miniature ponies whose narcolepsy attacks have decreased with age or disappeared altogether.

Prevention of the disease is not possible.