Phenylephrine Eyedrops as a Diagnostic Test in Grass Sickness

Research at the Royal (Dick) School of Veterinary Studies, funded in part by the Equine Grass Sickness Fund, has enabled the development of a non-invasive, simple test which is proving to be useful in the diagnosis of grass sickness.

Grass sickness has been causing high mortality of horses at grass since the beginning of last century but despite a great deal of work by veterinary surgeons over the years, and the existence of very similar diseases in dogs, cats and hares, the cause is still unknown.

Diagnosis is difficult and relies on the interpretation of clinical signs associated with the loss of nerve cells (neurons) in the involuntary portion of the nervous system causing a loss of gastrointestinal movement and colic. Signs can be subtle and are shared by numerous other causes of colic, and in the absence of a simple diagnostic test the only definitive way to diagnose grass sickness pre-mortem is by taking a biopsy of small intestine at surgery.

One of the consistent clinical signs of grass sickness is ptosis, drooping of the upper eyelids, that give cases a characteristic sleepy appearance. Three different nerves supply the muscles which raise the eyelid of the horse and work at the ‘Dick’ on the effects of grass sickness on the brain of horses showed that all three could be affected in grass sickness. A detailed investigation however suggested that the cause of the ptosis was principally due to the loss of neurons supplying the smooth (involuntary) muscles in the eyelid, a condition generally known as ‘Horner’s syndrome’. These neurons supply the muscle with a neurotransmitter related to adrenaline, causing it to contract, and it was speculated that applying eyedrops with similar characteristic could reverse the ptosis.

The administration of commercial phenylephrine eyedrops to horses with grass sickness was found to result in a dramatic reduction of ptosis, lasting for several hours. The use of phenylephrine at the concentration used in the diagnosis of Horner’s syndrome in small animals and humans however, also resulted in marked effects on the eyelids of control horses. In addition it was noticed that the easiest way of assessing the change in ptosis was to measure the angle of the eyelashes to the head. The eyelashes in horses also have a smooth muscle inserted on them and have a decreased tone in grass sickness cases.

Further work determined that administering phenylephrine eyedrops diluted to 0.5% to one eye of unsedated grass sickness cases resulted in a marked difference in the angle of the eyelashes on the treated eye of grass sickness cases within 30 minutes (see figure 1) while having a minimal effect on control horses.

The use of eyedrops is routinely used at the ‘Dick’ Large Animal Hospital to diagnose cases of grass sickness. A paper has been published in the veterinary press and it is hoped that this advance will be useful to practitioners in the field presented with this devastating disease.

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Figure 1. Frontal photograph of a grass sickness case 30 minutes after administering 0.5% phenylephrine into the right eye.

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Figure 2. Normal (bottom left) and chromatolytic neurons, oculomotor nucleus, subacute grass sickness