Musculus orbicularis oculi
The orbicularis oculi is a circular sphincter muscle of the eyelids. It originates from the medial palpebral ligament and surrounding orbital bone and inserts into the skin of the eyelids (circular, returning to its origin). Its actions are to close the eyelids and aid in tear drainage. It is better developed in the upper lid. It is innervated by the auriculopalpebral branch of the facial nerve (CN VII). The blink reflex tests both the facial nerve (motor) and the ophthalmic branch of the trigeminal nerve (sensory).
Origin: Medial palpebral ligament and surrounding orbital bone
Insertion: Skin of the eyelids (circular, returning to origin)
Action: Closes the eyelids; aids in tear drainage
Innervation: Auriculopalpebral branch of facial nerve (CN VII)
Dysfunction (inability to blink) leads to corneal desiccation and ulceration, which is a veterinary emergency. Facial nerve paralysis commonly causes incomplete blinking, ptosis, and ear droop. The blink reflex is used to assess facial and trigeminal nerve function.
## Overview When facial nerve paralysis affects the eyelid branch, the horse cannot close the eye or blink normally. This leads to exposure keratitis, corneal drying, excessive tearing, and a high ri
## Overview Blepharospasm is the involuntary, forceful closure or squinting of the eyelids caused by spasm of the orbicularis oculi muscle. In horses it is almost always a sign of ocular pain rather
## Overview Orbital tightening is partial sustained squinting that makes the eye appear smaller, with increased blink rate and wrinkles above and below the eye. It is one of the most reliable indicat
Inability to blink, a drooping eyelid, or a dry or cloudy cornea after trauma or with other neurologic signs needs immediate veterinary attention. Protect the eye until the horse can be seen.
## Overview When facial nerve paralysis affects the eyelid branch, the horse cannot close the eye or blink normally. This leads to exposure keratitis, corneal drying, excessive tearing, and a high risk of corneal ulceration. Eye protection is the immediate priority. ## Causes Same as facial nerve paralysis: trauma, pressure, or central lesions affecting the nerve supply to the orbicularis oculi. ## Diagnosis Observation of incomplete blink and corneal changes. Fluorescein staining checks for ulcers. ## Treatment Frequent lubrication, temporary tarsorrhaphy (partial eyelid closure) if needed, treatment of any ulcer, and management of the underlying nerve injury. ## Recovery and Prognosis Depends on nerve recovery. Corneal scarring can persist even if blink returns. ## Prevention Early recognition and aggressive corneal protection. Signs include: inability to close the eye; dry or dull cornea; excessive tearing; corneal ulceration; eye remains partially open
## Overview Blepharospasm is the involuntary, forceful closure or squinting of the eyelids caused by spasm of the orbicularis oculi muscle. In horses it is almost always a sign of ocular pain rather than a primary disease. The most common underlying causes are corneal ulcers and uveitis; both can threaten vision if not treated promptly. ## Causes Corneal ulceration, equine recurrent uveitis, conjunctivitis, foreign bodies, and eyelid trauma are frequent triggers. Environmental irritants or bright light can produce milder, transient squinting. Primary neurologic blepharospasm is rare. ## Diagnosis Observation of sustained or frequent eyelid closure, often with tearing, swelling, or cloudiness, is the first clue. A complete ophthalmic examination including fluorescein staining, intraocular pressure measurement, and slit-lamp evaluation identifies the cause. ## Treatment Treatment is directed at the underlying disease: topical antibiotics or antifungals for ulcers, anti-inflammatories and mydriatics for uveitis, and removal of foreign material. Pain relief and protection of the eye (fly mask, stall rest) are supportive. ## Recovery and Prognosis Prognosis depends entirely on the primary lesion. Superficial ulcers often heal well; deep ulcers or uncontrolled uveitis can lead to scarring or blindness. ## Prevention Fly control, prompt attention to any eye discharge or squinting, and regular veterinary eye checks in horses with a history of uveitis reduce complications. Signs include: forceful or frequent eyelid closure; excessive tearing; eyelid swelling; light sensitivity; cloudiness or redness of the eye
## Overview Orbital tightening is partial sustained squinting that makes the eye appear smaller, with increased blink rate and wrinkles above and below the eye. It is one of the most reliable indicators of pain and is scored on the Horse Grimace Scale. It reflects discomfort rather than a primary eye disease. ## What It Indicates Pain, which may be ocular, musculoskeletal, or visceral. ## Diagnosis Observation as part of a pain assessment. Examine the eye itself to rule out primary ocular disease. ## Treatment Address the underlying source of pain. ## Recovery and Prognosis Resolves when pain is controlled. ## Prevention Prompt pain management. Signs include: partially closed or squinting eye; increased blink rate; eye appears smaller; wrinkles above and below the eye
Dysfunction (inability to blink) leads to corneal desiccation and ulceration, which is a veterinary emergency. Facial nerve paralysis commonly causes incomplete blinking, ptosis, and ear droop. The blink reflex is used to assess facial and trigeminal nerve function.
Inability to blink, a drooping eyelid, or a dry or cloudy cornea after trauma or with other neurologic signs needs immediate veterinary attention. Protect the eye until the horse can be seen.
See the Orbicularis Oculi on our interactive 3D horse model. Open 3D Model