Roaring is a condition that causes some horses to make a whistling or roaring sound during exercise. It can affect a horse’s performance during high-intensity exercises, such as racing.
This condition is also referred to as recurrent laryngeal neuropathy (RLN) or laryngeal hemiplegia. It is estimated that 2-8% of Thoroughbreds and up to 35% of draft breeds are affected by RLN. 
The larynx is part of the trachea (windpipe) and includes the vocal cords. RLN involves the partial collapse of cartilage in the larynx caused by progressive weakening of nerves.
This narrows the space that air passes through while the horse is breathing, causing a characteristic roaring noise, especially under heavy work. 
The condition is usually first noticed when the horse starts training around 2-3 years of age. In hunting and sport horses, laryngeal hemiplegia may not be noticed until around the age of seven. 
Early and frequent monitoring of laryngeal function in affected horses is important to determine whether surgical intervention is necessary or whether the horse should be retired from exercise.
If your horse is roaring, you can support their respiratory health through good nutrition and management to improve oxygen delivery and exercise tolerance.
The Equine Larynx
The larynx is the narrowest portion of the horse’s upper airway. It starts at the back of the mandible (jaw bone) and extends down into the neck.
The larynx serves as a channel between the pharynx and the trachea (windpipe). It transports inhaled air to the lungs for gas exchange. 
The intrinsic laryngeal muscles are involved in respiration, phonation (vocalization) and protecting the airway.
In horses with laryngeal hemiplegia, the left recurrent laryngeal nerve that controls the laryngeal muscles is damaged. In rare cases, the right laryngeal nerve is also affected.
The left recurrent laryngeal nerve loops around the horse’s aorta while the right takes a shorter route around the right subclavian artery. The left nerve can be up to 250 cm long – twice as long as other motor nerves in the horse’s body.
It is this length and the complexity of the left recurrent laryngeal nerve that likely puts it at risk of pathological changes that cause roaring. 
Recurrent Laryngeal Neuropathy
RLN is usually caused by damage to the left recurrent laryngeal nerve, leading to a loss of nerve supply in the laryngeal muscle. This results in muscle atrophy within the larynx.
In healthy horses, the laryngeal muscles pull on (abduct) the aryteoid cartilage of the larynx to open the airway for inhalation. Following exhalation, the muscles relax and the airway closes slightly.
In horses with RLN, there is incomplete abduction of the arytenoid cartilage and vocal cord. Instead, these structures collapse into the horse’s airway during exercise.
This results in abnormal and often loud breathing noises, best described as a “roar”.
RLN also leads to increased respiratory resistance, a reduction in airflow, and a low concentration of oxygen in the blood (hypoxemia). 
Damage to the left recurrent laryngeal nerve (and sometimes the right) can also lead to total or partial paralysis of the larynx. 
Causes of Roaring
For the vast majority of RLN cases, the trigger leading to the development of the condition is unknown. 
However, there have been documented cases of laryngeal paralysis caused by:
- Injury to the right or left recurrent laryngeal nerve, possibly by intravenous injection that misses the vein
- Trauma to the neck
- Thymic lymphosarcoma (a type of cancer)
- Guttural pouch infection
- Thyroid carcinoma
- Abscessation of the head and neck
In addition, poisoning from organophosphates, toxic plants or lead have been associated with laryngeal paralysis. Liver disease or complications from general anaethesia may also result in RLN.
Researchers hypothesize that RLN could also be caused by mechanical strain, such as tension and stretch of the recurrent laryngeal nerve during neck movement or growth. 
Risk Factors Associated with Roaring
Roaring mostly occurs in tall horses, such as Thoroughbreds and draft breeds.
Because the left recurrent laryngeal nerve is the longest in the horse, researchers think that conformational traits may be associated with RLN risk. These include:
- Wither height
- Neck length
- Jaw width
- Body weight
Age and sex might also affect RLN risk. Male and older horses are more commonly affected than female or young horses. 
Genetics may also play a role. Selective breeding for a smaller size might help to reduce the prevalence of RLN. 
Symptoms and Diagnosis of Roaring
The main noticeable sign of RLN is the whistling or roaring sound made by horses during exercise.
Horses affected by roaring will often have reduced tolerance to strenuous exercise. This is due to less oxygen reaching the pulmonary capillaries, which leads to premature fatigue. 
In rare cases of RLN that affect both the left and right recurrent laryngeal nerve, the horse can experience severe respiratory distress even while resting. 
Endoscopy and Scoring
Diagnosing severe cases of RLN is fairly easy, but detecting milder cases can be more difficult. Diagnosis is usually made via endoscopic examination of the larynx.
In this procedure, a thin, flexible tube equipped with a light and camera (endoscope) is used to look at the airway.
Endoscopy should be performed at rest and during exercise. Airway obstruction and vocal cord collapse may not be present on a resting exam and are usually only detected on an exercising exam. 
Specialized treadmills can be used to evaluate the horse during exercise. With improvements in technology, exercise examinations can now be done overground as well. 
Laryngeal Hemiplegia Scoring System
A four-point system is commonly used to score movement and abduction of the arytenoid cartilage. 
Grade 1: No sign of RLN. There is synchronous and symmetrical movement, and full abduction occurs.
Grade 2: Asynchronous and/or asymetrical movement occurs sometimes but full abduction can still occur.
Grade 3: Movements are asynchronous and/or asymmetrical. Full abduction cannot occur.
Grade 4: No movement of the cartilage or vocal cords.
Grades 1 and 2 of laryngeal function do not result in laryngeal collapse. However, most grade 3 and all grade 4 horses have airway obstruction during strenuous exercise.
In racehorses with grade 3 or 4 laryngeal function, surgical intervention is necessary to enable the horse to continue to perform. 
The degree of laryngeal dysfunction in RLN horses changes over time and can be worsened by exercise. For example, cases that initially present with normal laryngeal function may later be found to have mild to moderate laryngeal paralysis. 
A complete examination of the upper airway, including both resting and exercising endoscopy, is important for any horse with poor performance and respiratory noise. Ongoing assessment may be needed to monitor the progression of the condition.
Additional Diagnostic Tests
Muscle atrophy along the larynx is often palpable on the left side, though this requires an experienced veterinarian or technician to identify.
Your veterinarian or veterinary technician can palpate for atrophy, cartilage asymmetries and scars along the outside of the larynx. 
Ultrasound of the larynx can also be a helpful diagnostic tool. 
Surgical Treatment for Roaring
Roaring has been recognized as a medical condition in horses for hundreds of years. Though it may seem like a simple problem, the condition is quite complex and difficult to resolve.
RLN is typically treated in one of three ways: surgical intervention, retirement, or euthanasia.