Progressive ethmoidal hematomas (PEHs) in horses are non-cancerous masses that develop from the ethmoidal labyrinth. This a structure with a high concentration of blood vessels within the horse’s nasal sinuses. [1]

PEHs are composed of blood surrounded by a thin layer of connective tissue, making them susceptible to damage and profuse bleeding. [1] The most common presentation of ethmoidal hematomas is bleeding from one or both nostrils.

Veterinarians diagnose ethmoidal hematomas in horses using an endoscope passed into the nasal cavity. Treatment options include surgical removal, laser reduction, cryosurgery, and formalin injection.

The overall prognosis of ethmoidal hematomas is poor, as recurrence is very common despite treatment. Many horses with ethmoidal hematomas can be managed with repeated treatments when symptoms redevelop.

Progressive Ethmoidal Hematomas in Horses

Progressive ethmoidal hematomas are benign masses that can form at the back of the horse’s nasal passages. When PEHs develop in horses, they can grow and spread into the nasal cavity and the surrounding sinuses. [1]

PEHs in horses are usually not life-threatening, but they can limit a horse’s performance and may lead to sinus and airway issues if not addressed. [1][3] This condition is relatively uncommon in horses but may be suspected if a horse experiences recurrent nosebleeds unrelated to exercise. [1]

The name “progressive ethmoidal hematomas” denotes the following:

  • Progressive: Indicates that the condition tends to worsen over time if not treated
  • Ethmoidal: Originates from the ethmoidal turbinates, which are small, scroll-like bony structures covered with mucous membranes, located at the back of the nasal cavity​
  • Hematomas: Blood-filled masses that results from blood pooling or clotting within the tissue​


The underlying cause of PEHs in horses is unknown, however damage to the protective layer of mucus-secreting tissue lining the nasal sinuses is likely a trigger. [1]

Damage to the mucosa lining causes small bleeds underneath the surface, resulting in a pocket of blood. [2] It is theorized that continued irritation and damage enlarges the pocket, eventually forming an ethmoidal hematoma. [2]

Around half of horses that have PEH have a mass in both nasal sinuses (bilateral). [1]

Currently, the only predisposing risk factor for PEH is increasing age, as rates of occurrence are higher in older horses. [1]


The main symptom of PEH is periodic bloody discharge from a nostril (epistaxis). [1] Horses with bilateral PEHs may have bleeding from both nostrils. [1]

Other symptoms of PEH may include: [1][3]

  • Swelling of the face
  • Noisy breathing
  • Bad breath
  • Difficulty breathing
  • Coughing
  • Head shaking


Periodic bloody discharge from the nose usually suggests a diagnosis of ethmoidal hematoma.

Confirming the diagnosis involves endoscopy, a procedure in which a camera is used to examine the inside of the nasal passages and visualize the mass. [1] On endoscopy, ethmoidal hematomas are a purple-red or green-yellow mass that obstructs the nasal cavity. [2]

Often, PEH masses obstruct the entire nasal cavity, which makes it challenging to confirm the exact size and location of the mass on endoscopy. [2] Other diagnostic tests that may help determine the extent and size of the mass include: [1][2]

  • X-rays of the head
  • Computed tomography (CT scan)
  • Magnetic resonance imaging (MRI)

Differential Diagnosis

Other types of masses can also occur within the nasal sinuses. To confirm that the mass is an ethmoidal hematoma, a biopsy (small sample of tissue) is required to analyze what type of cells are present in the mass. [1]

Other masses that can resemble PEHs include: [2]

  • Nasal polyps
  • Fungal infections
  • Traumatic injuries from a foreign body entering the nasal passage
  • Tumors
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The primary treatment options for PEHs in horses include: [1][2]

  • Surgical removal
  • Formalin injection
  • Laser reduction
  • Cryosurgery (liquid nitrogen)

While skull surgery is considered the “gold standard” treatment to resolve PEHs, horse owners may elect for a less invasive option, and surgery may not be appropriate in all cases. Factors including the size of the mass and its location determine which treatment options are suitable for the individual horse. [2]

Surgical Removal

Most veterinarians recommend surgical removal as a first option, because it has the highest chance of removing the entire mass. [2] Surgeons primarily perform this surgery under general anesthesia, however “standing surgery” under sedation is also possible. [2][4][5]


During the surgery, the surgeon makes a bone flap, a hole in the bone that can be removed to access structures within the skull. [4] Surgeons use a bone saw or an osteotome (a tool for cutting bone) to make the bone flap. [4]

Once the ethmoidal hematoma is visible through the bone flap, the surgeon excises (removes) the mass from within the nasal cavity. [2]

After removing the mass, the surgeon packs the nasal cavity with gauze. [2] The gauze puts pressure on the mucosa lining the nasal cavity to staunch the bleeding. [2]

The bone flap slides back into place after surgery and does not need to be re-attached to the surrounding bone. [4] Over time, the lines cut in the bone heal, similar to a broken bone. Stitches or sutures are used to close the skin over the bone flap to reduce healing time and minimize scarring. [4]


After surgery, horses receive antibiotics to reduce the risk of infection and non-steroidal anti-inflammatory drugs (NSAIDs) to reduce swelling and pain. [2] Around 3 weeks of stall rest is required after surgery to allow the nasal tissues to heal, as deep breathing during exercise may cause additional irritation to the surgery site. [2]

The gauze packed within the nasal cavity is removed by the attending veterinary staff at around 12-48 hours after the surgery. [4] If bleeding continues, the nasal cavity may need repacking. [4] Some surgeons recommend rinsing the nasal cavity to remove blood clots and debris once bleeding stops. [2]

Horses are expected to have mild nasal discharge, with or without blood, for around 2-3 weeks after surgery. [2] The nasal discharge should resolve over time, indicating a successful surgery.


Although uncommon, there are several complications associated with bone flap surgery of the skull. [4] Possible complications include: [2][4]

  • Excessive bleeding during surgery
  • Difficulty breathing following treatment
  • Bacterial or fungal infections
  • Suture line periostitis
  • Damage to the infraorbital nerve

Excessive Bleeding:

Excessive bleeding is the most common complication associated with PEH surgery. [2] Since the hematoma has a large blood supply, removal of the mass can result in significant, rapid blood loss. [2] Surgeons typically arrange for a suitable blood donor prior to surgery in case the horse requires a blood transfusion to replace lost blood. [2]

Respiratory Issues

Difficulty breathing can occur due to swelling in the nasal cavity related to inflammation. [2] The swollen tissues block the airway, making it difficult or impossible for the horse to breathe. [2] Affected horses may require a tracheotomy, a temporary hole made in the windpipe, to allow air to enter the lungs. [2]


Bacterial or fungal infections can occur if organisms enter the surgical site. [4] Infection can occur at the skin incision or within the nasal cavity. [4] Treatment typically involves antibiotic or anti-fungal medication. [4]

In some cases, the nasal cavity may need medicated lavage (i.e. flushing) to treat the infected area directly. [4]

Suture Line Periostitis

Suture line periostitis is an inflammatory reaction in the sutures of the skull. [4] Sutures are normal seams between the different pieces of bone that compose the skull. [4] Performing surgery on the skull can destabilize the skull’s structure, causing the sutures to rub against each other and become irritated. [4] This irritation causes bony swelling over the affected suture that typically resolves over 12-18 months. [4]

Damage to Infraorbital Nerve

The infraorbital nerve is the main nerve that provides sensation to the skin on the face. [4] In some cases, skull surgery can damage the infraorbital nerve, resulting in the horse having no pain or touch sensation on the affected side of the face. [4] Affected horses may be more likely to injure their face due to lack of pain sensation. [4]

Formalin Injection

Formalin is a preservative chemical that dries out tissues and breaks down proteins, resulting in reduced tissue size. [2] Veterinarians can inject formalin into PEHs using endoscopy to guide the placement of the injection needle. [2]

Once the injection needle is inserted into the PEH, veterinarians typically inject enough formalin to fill the mass completely and formalin leaks out of the injection site. [2] The veterinarian performs these injections every 2-4 weeks until the mass resolves. [2] Horses may require up to 20 injections to resolve the mass. [6]

Some horses require ongoing formalin injections to prevent recurrence of symptoms. [2] Clinical studies suggest that complete regression of the PEH occurs in around 60% of horses treated with formalin, with the remaining horses having resolution of symptoms. [7] Intervals between symptoms redeveloping range from 9 to 42 months. [2]


The most common complication after formalin injection is abundant nasal discharge for 5-7 days after treatment. [2] The nasal discharge typically resolves without further treatment.

There is one report of a horse developing symptoms of neurologic damage such as circling, head pressing, head shaking, and difficulty breathing after formalin injection. [6] Post-mortem examination revealed that the PEH had damaged the barrier between the nasal cavity and the brain, allowing formalin to interact with the brain. [6]

From this case, many experts now recommend performing a CT or MRI scan of the skull to identify any bone damage prior to using formalin injection as a treatment. [6]

Laser Reduction

Surgical lasers can make cuts in tissue that immediately cauterize (stop bleeding) small vessels. [2] This makes lasers an ideal treatment method for removing PEHs due to their high risk of bleeding. [2] Some surgeons use lasers during traditional PEH surgery through a bone flap to prevent excessive bleeding. [2]

There are also lasers available that can be passed through the nasal cavity and guided using an endoscope, preventing the need for a bone flap. [2] Veterinarians can perform this procedure in a standing, sedated horse. [2]

The laser is used to cut the PEH out of the nasal cavity in pieces. [2] In addition, bursts of heat from the laser trigger tissue death, often resulting in sloughing of any remaining tissue from the mass after treatment. [2]

Several treatments using an endoscopic laser are usually necessary to completely remove an ethmoidal hematoma. [2] Some masses may be too large or too deeply entrenched within the nasal sinuses to allow complete removal using an endoscopic laser. [2] These masses may require surgical intervention.

Aftercare following laser treatment is similar to surgery, involving gauze packing of the nasal cavity, antibiotics, and NSAID therapy. [2] Horses also require a period of stall rest to prevent irritation of the surgical site. [2]


The most common complication of laser reduction is difficulty breathing following treatment, with some horses requiring a temporary tracheostomy. [2][8]


Veterinarians can use liquid nitrogen to freeze sections of an ethmoidal hematoma, making the tissue easier to remove. [2] Cryosurgery is only effective when there is limited bleeding, as bleeding can prevent adequate freezing of the tissue. [2]


Sub-freezing temperatures from the liquid nitrogen may cross the barrier between the nasal cavity and the brain, causing damage to brain tissue. [2] Veterinarians can reduce this risk by placing sensors that measure temperature within the nasal cavity, to alert them of excessively cold temperatures. [2]


The long-term prognosis for PEHs is poor, even with appropriate treatment. [2] Ethmoidal hematomas commonly recur after treatment, resulting in return of symptoms. [3] Most recurrences of PEHs occur within the first 6-12 months after treatment. [3]

For surgical removal, recurrence rates range from 0 – 44% depending on the study. [3] Reports suggest similar recurrence rates for both cryosurgery and laser treatment. [2]

Long-term studies on formalin injection recurrence are limited, as most studies examine only a small number of horses. One study showed that 2 out of 3 horses treated with formalin injection had no recurrence of symptoms for two years after treatment. [9] Another similar study reported no recurrence in 5 out of 6 horses 2.5 years after treatment. [10]

Horses that have recurring symptoms often require additional treatment procedures. Studies show additional treatments are safe to perform, and horses often return to athletic performance even with multiple treatments. [2]


Progressive ethmoidal hematomas are non-cancerous masses of the nasal sinuses that contain a high proportion of blood vessels, resulting in recurring bloody nasal discharge in horses.

  • Treatment involves surgical removal, laser reduction, cryosurgery, or formalin injection
  • Complications of PEH treatment can be mitigated with imaging prior to intervention
  • Many PEHs recur after treatment, causing further symptoms
  • Horses with recurring symptoms benefit from repeated treatments

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  1. Reed, S. M. et al. Equine internal medicine, 3rd ed. St. Louis, Mo: Saunders Elsevier, 2010.
  2. Stich, K. L. et al. Progressive Ethmoid Hematoma in Horses. Veterinary Compendium. 2001.
  3. Freeman, D. E. Sinus Disease. Veterinary Clinics of North America: Equine Practice. 2003. doi: 10.1016/S0749-0739(02)00062-7. View Summary
  4. Schumacher, J. et al. Surgery of the Paranasal Sinuses of Horses. Equine Veterinary Education. 2024. doi: 10.1111/eve.13916.
  5. Smith, L. J. and Perkins, J. Standing Surgical Removal of a Progressive Ethmoidal Haematoma Invading the Sphenopalatine Sinuses in a Horse. Equine Veterinary Education. 2009. doi: 10.2746/095777309X468186.
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  7. Schumacher, J. et al. Transendoscopic Chemical Ablation of Progressive Ethmoidal Hematomas in Standing Horses. Veterinary Surgery. 1998. doi: 10.1111/j.1532-950X.1998.tb00116.x. View Summary
  8. Rothaug, P. G. and Tulleners, E. P. Neodymium:Yttrium-Aluminum-Garnet Laser-Assisted Excision of Progressive Ethmoid Hematomas in Horses: 20 Cases (1986–1996). javma. 1999. doi: 10.2460/javma.1999.214.07.1037. View Summary
  9. Tremaine, W. H. and Dixon, P. M. A Long-Term Study of 277 Cases of Equine Sinonasal Disease. Part 2: Treatments and Results of Treatments. Equine Veterinary Journal. 2001. doi: 10.2746/042516401776249787. View Summary
  10. Dixon, P. M. et al. Equine Paranasal Sinus Disease: A Long-Term Study of 200 Cases (1997–2009): Treatments and Long-Term Results of Treatments. Equine Veterinary Journal. 2012. doi: 10.1111/j.2042-3306.2011.00427.x. View Summary