Stylohyoid Osteoarthropathy

The Stylohyoid Bone, Could it be Causing a Head Tilt in your Horse?

 Stylohyoid Osteoarthropathy nose droop
Facial paralysis
Eye lid droop
Drooping eyelid
Stylohoid
Radiograph shows thickened stylohoid bone next to blue arrow
Anatomy
Ceratohyoid bone which is removed during surgery

Is your horse ataxic? Do they have a head tilt? Facial paralysis? Ear droop? Horses that present with sudden ataxic with a head tilt might just have an enlarged stylohyoid. This condition has been one of the leading causes of sudden onset of seizures and ataxia in middle age to older horses.  

Although the condition can be classified as a degenerative and chronic in nature, there is often a traumatic episode (often cribbing) that fractures the degenerative bone and produces such acute pain and swelling seizures that facial nerve paralysis occurs.

With less severe forms, the patient presents with a drooping and paralysis of the affected ear and eyelid and lateral deviation of the nostril. There is often linear erosion or ulceration of the cornea from drying of the cornea, as the eyelids do not fully close. They have a poor menace, decreased tone to the eyelid but will have a normal pupillary response to stimulus with a bright light.

There is often pain to deep digital palpation over the TMJ and posterior mandible area.

Diagnosis of Stylohyoid Osteoarthropathy

Usually your horse will be tested for EPM, have a neurological exam to check for wobbler syndrome but most often a radiograph for stylohyoid enlargement is overlooked.

DV radiographs of the skull show a thickened stylohyoid bone and the lateral projection will often show a fracture callous. Endoscopy shows a thickened stylohyoid bone often with an increase in small vessels on the surface of the bone.

A nuclear imaging scan shows increased isotope activity at the petrous temple bone area. A CT scan is very definitive and can increase the clinician’s ability to provide an accurate diagnosis and prognosis.

Treatment

Treatment consists of anti-inflammatory medication and surgical removal of the ceratohyoid bone. The surgery should be done early to afford the best chance to reduce the inflammation to the facial nerve.

If surgery is delayed, there is a good chance vision will be lost and the affected eye may have to be removed.

One Horse's Story

One horse that was overlooked for stylohyoid osteoarthropathy had clinical symptoms and did respond initially to time and anti-inflammatories. A year later he was treated for Borna disease but not getting any better. This time a lateral radiograph of the skull showed an enlarged stylohyoid. The horse had nystagmus for 3 to 4 days with a decreased menace response but normal consensual light response.

A computed tomography scan of his skull was recommended and it showed a much enlarged left stylo and ceratohyoid bones with osteoarthropathy of the stylohyoid/petrous temple articulation.

A ceratohyoidectomy was recommended to reduce chances of more severe facial nerve paralysis and to reduce his discomfort with the arthritic changes.

Surgery was successfully performed and this horse is back to work. He is doing well and almost back to doing everything before the symptom onset. He is expected to make a full recovery.