The disorder usually affects the left side of the larynx (voice box) and occurs most commonly in larger horses.
The term recurrent laryngeal neuropathy (RLN) is more accurate than other names that have been used for this condition such as 'laryngeal hemiplegia' or 'laryngeal paralysis'.
The abnormal noise is heard during inspiration (i.e. breathing in) and may be anything from a high-pitched soft whistle to a harsh 'roar'.
With each breath, air is taken in through the nostrils and passes via the nasal passages to the throat. From here it passes through a cartilaginous valve, the larynx (voice box), before entering the windpipe (trachea) and lungs.
During exercise, the nostrils dilate and the horse extends its head and neck, further opening the throat and larynx to take in more air.
Anything which interferes with the smooth passage of this increased air flow may result in the horse making an audible noise.
RLN is the most common cause of horses making abnormal noises during inspiration at fast exercise. The noise is caused by partial or total paralysis of one (virtually always the left) side of the larynx.
It occurs mainly in larger horses, because it is thought that this conformation can predispose to injury to the long nerve (recurrent laryngeal nerve) which innervates the left side of their larynx.
In mild cases the noise may only be heard during strenuous exercise but in severe cases the noise might be evident during trotting.
Exercise intolerance (ie, getting tired quickly) can be a problem as the horse has difficulty getting enough air through the incompetent larynx. The disease is progressive and gets worse with time.
In a severely paralysed larynx, the airway actually gets smaller rather than bigger during strenuous exercise as the paralysed side collapses inwards.
RLN is usually diagnosed firstly by listening to your horse exercising to detect an abnormal inspiratory noise.
Next, the horse should undergo an endoscopic examination, ie, a flexible periscope-type instrument is passed through the nose to look at the larynx as the horse breaths. Both sides of the larynx should open and close in synchrony, almost symmetrically and completely.
In the horse with RLN, usually the left side of the larynx moves sluggishly and incompletely, 'hanging' into the larynx and obstructing airflow during inspiration. In some difficult cases, it may be necessary to have an endoscopic examination performed while the horse is exercising on a high-speed treadmill, in order to make the diagnosis. The treadmill has now been superseded by "overground endoscopy".
This consists of a camera inserted up the horse's nose and secured to the bridle. Using these dynamic methods we can now assess more accurately the extent to which the paresis affects the horse when stressed.
What treatment options are available?
Horses used for hacking or less strenuous jobs can cope without treatment. It is important to keep their respiratory tract healthy from infections and allergies with good management (i.e. low dust, good ventilation, proper vaccination regime etc.).
In moderately affected cases an affected horse may undergo a 'Hobday' operation in which a piece of laryngeal tissue (laryngeal saccule) and the adjacent vocal fold is surgically removed to encourage a scar to form to 'tighten' the larynx in a more open position.
This procedure used to be always carried out with the horse under general anaesthesia and an incision, under the throat, made into the larynx to gain access to the vocal folds. In the last 10 years the procedure is now often carried out by laser via an endoscope placed via the nose.
The advantage of this procedure is potentially less lay off time post-surgery and does not require a general anaesthesia.
In more severe cases a 'tie-back' operation is often recommended. This more elaborate operation places sutures in the paralysed side to pull and hold it open.
The 'Hobday' operation is usually also performed at the same time. The tieback procedure should not be undertaken lightly as we now recognise there are considerably more complications follow this procedure than originally realised, these include chronic coughing; difficulty swallowing and low grade lung infections; failure to open up the larynx wide enough and rupture of the implant.
Traditionally in horses where a tie back has failed or is not an option for other reasons, a brass or plastic tracheotomy tube may be inserted into the windpipe to allow air to bypass the larynx completely.
Very few people nowadays undertake this procedure due the daily intensive treatment involved in maintaining the tube and many people understandably find it unacceptable for aesthetic reasons.
Instead of this occasionally vets will undertake an arytenoidectomy whereby the large arytenoid cartilage is removed from the affected left side of the larynx to allow adequate airflow.
If you horse is making a noise whilst exercising we would advise you ask your vet to investigate the problem to determine if your horse is a roarer or not.