Canthotomy is a surgical procedure often performed in conjunction with cantholysis to correct ocular compartment syndrome (OCS). OCS is an ophthalmic emergency that must be attended to immediately because it can rapidly lead to blindness if not correctly diagnosed and imaging undertaken. OCS occurs in the context of retrobulbar hemorrhage (blood accumulation in the retrobulbar space) with proptosis (displacement of the eye) leading to sudden vision loss. Other manifestations of OCS include afferent pupil defects (Marcus Gunn Pupil), severe eye pain, and intraocular pressure (IOP) greater than 40 mm Hg instead of the normal IOP of 10-21 mm Hg. With OCS, the twelve muscles affecting eye movement can be impaired. 

Causes of OCS include retrobulbar hemorrhage (from recent orbital trauma), eye surgery, eyelid surgery, and anesthetic injections. Though uncommon, other causes include leukemia, intraorbital aneurysm (excessive localized swelling) of the ophthalmic artery, hypertension, von Willebrand disease (a blood clot disease), haemophilia (inability of blood to clot), and atherosclerosis (fat and cholesterol built-up in the artery walls). If left untreated, loss of vision can be permanent within one to two hours of ischemia (short oxygen supply due to restricted blood flow) of the optic nerve and retina.

Canthotomy can also be performed on patients with cherry-red macula, severe pain in the eye, ophthalmoplegia (paralysis of eye muscles), and optic nerve head pallor (whiteness of optic disc and death of optic nerve axons). However, lateral canthotomy should not be performed if the globe is raptured. Hyphema (collection of blood in the anterior chamber), an irregularly shaped pupil, extraocular muscle movement restriction, and exposed uveal tissue, are signs of globe rupture. 

Usually, severing the lateral canthal tendon is insufficient to release the globe from its fixed position, leading to cantholysis (severing of the inferior crus).

Preparation & Expectation Before Surgery

The doctor will explain the risks and benefits of the procedure. 

The patient is placed supine (lying face upwards) with the bed's head slightly elevated 10-15% and the patient’s eyelids and head stabilized. This position helps to prevent iatrogenic injury (injury caused by medical examination or treatment) including globe puncture during the surgical procedure, as the patient may move unexpectedly. In the case of a patient with mental instability, the head may be taped or restrained with the help of an assistant. If the patient is exceptionally uncooperative, the doctor may need to use sedation or general anesthesia (rarely).

Types, Purpose & Procedure

The use of canthotomy is the surgical treatment of choice for OCS and involves exposing the lateral canthal tendon to ease intraocular pressure.

The patient's skin is cleaned with chlorhexidine, and if necessary, the patient is sedated. The doctor will inject local anesthetic or lidocaine 1-2% with epinephrine into the orbital rim through the lateral canthus subcutaneously (under the skin). If necessary, an additional anesthetic may be used during the procedure. The surgeon will inspect the globe and estimate visual acuity (sharpness of vision). S/he will then clean and irrigate (washing using a stream of water or other fluid) the lateral canthus region. The patient is then draped. 

The surgeon injects the site with 1-2 mL of local anesthetic with epinephrine while aiming the tip of the needle away from the globe. S/he will use a hemostat or needle driver to estimate the incision’s path from the lateral canthus to the orbit’s rim. When the hemostat is locked in for less than one minute, it enables the crushing of the tissue (to help prevent or stop bleeding). Once the tissue has been crushed, the needle driver is removed. The surgeon next uses iris scissors and follows the path of crushed tissue to make a full-length incision of about 1-2 cm. S/he lifts the lateral part of the inferior eyelid to expose the lateral canthal tendon. Tissue crushing is essential to minimize bleeding, making it easier for the surgeon to see where to cut in case of extensive traumatic edema (swelling).

Risks, Side Effects & Complications

Bleeding and infection may occur though they are uncommon.

Corneal exposure is a risk that can be addressed by a lubricating ointment and a plastic moisture chamber which is applied without putting pressure on the globe. Cotton or pressure patches over a globe with proptosis are discouraged.

Complications that may arise with canthotomy include iatrogenic globe injury. Ptosis (drooping of upper eyelid) caused by a damaged levator aponeurosis can result. The lacrimal artery and lacrimal gland may also suffer damage. Irreversible loss of vision may occur if ischemia of the retina lasts more than 90 minutes.

After Care, Recovery & Outcome

The surgeon will apply an antibiotic ointment like erythromycin 5% on the site because the patient cannot blink to lubricate the cornea following canthotomy. The eye is covered with a sterile dressing to prevent infection. Pain and inflammation may last several days after the procedure, and the patient will be advised to use ice packs for relief. The patient is also advised to avoid any form of eye strain.

Patients with severe injuries may be admitted to the hospital. Patients with progressive loss of vision often have methylprednisolone prescribed for 3 days. 

Should intraocular pressure remain, systemic and topical therapy can be considered. Topical treatment includes eye drops such as brimonidine 0.2%, timolol 0.5%, and dorzolamide 2%. Systemic therapy can also be prescribed,e.g., mannitol and acetazolamide.