Quieting the neuropathic components of ocular pain

Nerve blocks used to treat peripheral neuropathic pain elsewhere in the body can successfully treat chronic ocular pain from a variety of causes.

Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage, according to the International Association for the Study of Pain.

Patients with chronic ocular pain may benefit from the use of nerve blocks that generally are used to treat peripheral neuropathic pain elsewhere in the body.

Pain disorders can be nociceptive, that is, characterized by pain that arises from actual or threatened damage to non-neural tissue, such as with severe ocular surface disease, band keratopathy, and intraocular inflammation, or they can be neuropathic, as with pain resulting from damage to or changes occurring in the nervous system, such as that caused by a previous cataract, LASIK, or RK surgery; neuralgia associated with the herpes virus; and eye drops containing the preservative benzalkonium chloride, noted Ann Quan, MD.

Neuropathic pain is a complex process resulting from various receptors, she continued. The corneal nociceptors are comprised of polymodal nociceptors that sense chemical, thermal, and endogenous inflammatory mediators, mechanoreceptors that sense mechanical stimuli, and cold thermoreceptors that sense evaporation.

The terminal nerve endings of the corneal nociceptors interact with the external environment and by doing so, they are susceptible to damage during inflammation or repetitive environmental injuries.

The nociceptive causes of ocular pain, i.e., inflammation, tear dysfunction, and anatomic abnormalities are commonly treated; however, the pain often persists, suggesting there can be a neuropathic component.

Drs. Quan and colleagues retrospectively reviewed the medical records of patients in the Oculofacial Pain Clinic, University of Miami, from Jan. 1, 2017 to Aug. 11, 2018, to determine if use of nerve blocks can effectively treat chronic ocular pain that likely has neuropathic components.

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How peripheral nerve blocks work

All of the study patients had been treated with a nerve block as part of a treatment regimen. Dr. Quan, who is an Ophthalmology resident, Bascom Palmer Eye Institute, University of Miami Hospital and Clinic, Miami, recounted that nerve blocks were administered using a standardized injectable solution of 4 milliliters  of bupivacaine 0.5% (Marcaine, Pfizer) and 1 milliliter of methylprednisolone acetate (Depo-Medrol, Pfizer) 80 mg/ml that were mixed in a 5-cc syringe and injected using a 25-gauge needle. The terminal branches of the trigeminal nerve that innervate the periocular tissues and the eyelids were targeted. 

Bupivacaine blocks sodium channels, which prevents depolarization, and inhibits N-methyl-D-aspartate receptor-mediated transmission in the spinal cord and trigeminal nucleus, which is important in central sensitization. Methylprednisolone likely further enhances the therapeutic potential of the nerve blocks and decreases ectopic neuronal discharge directly and indirectly through decreased inflammation, she explained.

“One possible explanation for the ability of periorbital nerve blocks to provide pain relief may be that afferent pain signals may originate from the tissues adjacent to the cornea,” Dr. Quan commented.

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Results

Of 11 patients (7 men, 4 women; mean age, 54 years) who received nerve blocks, four developed pain after ocular surgery and one each following trauma, radiation, zoster ophthalmicus, pituitary adenoma resection,  septoplasty, in the setting of neuromyelitis optica, and one with no known pain triggers.

Following administration of a nerve block, seven patients reported immediate pain relief that lasted for varying amounts of time (the longest period, 7 months). Four patients did not have any pain improvement.

The patients had been treated with a variety of topical therapies, such as artificial tears, corticosteroids, diclofenac, cyclosporine, serum tears and oral drugs such as gabapentin and diclofenac prior to the nerve block.

The results of the nerve block were impressive. In one case, a patient complained of pain at the level of 8 out of a possible 10  before the nerve block procedure and wore sunglasses into the clinic. By 20 minutes after the procedure, the patient could go outside without sunglasses.

One week after the nerve block, the patient had significant pain relief and complained of mild pain of 1 to 2 on a scale of 10 . The left infratrochlear and infraorbital blocks were repeated. Four  months later, patients were free of pain and photophobia, and 7 months later, the pain returned partially and the patient underwent repeated peripheral nerve block at the same sites is now pain free and able to return to work full time.

Dr. Quan concluded, “We found that nerve blocks typically used to treat peripheral neuropathic pain elsewhere in the body may be successful for treating chronic ocular pain from a variety of causes. Both the peripheral and central effects of neural blockade may account for the suppression of pain signals.”

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Disclosures:

Ann V. Quan, MD

E: avq7@miami.edu

Dr. Quan has no financial interest in any aspect of this report.