NEUROPATHIC OCULAR PAIN AND ALLODYNIA
Most pain sensations result in direct response to an injury, but neuropathic pain is somewhat different. Neuropathic pain results when nerve fibers send incorrect signals to pain centers in the body, sometimes without any obvious injury. Two common symptoms of neuropathic pain are allodynia and hyperalgesia.1 Allodynia is pain resulting from stimulus that does not usually provoke pain; hyperalgesia is a heightened sensitivity to pain. Whether it manifests as allodynia or hyperalgesia, neuropathic pain that affects the eyes is referred to as neuropathic ocular pain and, in most cases, the condition is chronic.
WHAT CAUSES NEUROPATHIC OCULAR PAIN?
Much like an alarm system, pain is the body’s way of alerting you that something is wrong. In the eye, for example, a principal role of the corneal pain system is to protect, sustain and restore the optical tear layer, without which vision would be non-functional.2 With neuropathic pain, the alarm system doesn’t work as it would if you were suffering from normal physiological or nociceptive pain. For example, if you break your arm, it will cause physiological pain—meaning it will hurt, which will likely require you to rest so that the arm can heal. Neuropathic ocular pain, on the other hand, is caused by damage to eye-nerve terminals, causing them to malfunction, which in turn evokes pain as a response.3 Inflammation is a common feature of neuropathic pain and, when this occurs, the matching between the stimulus and the sensation of pain disappears. 4 There are many possible causes for the eye’s alarm to sound and for neuropathic pain to occur. These include trauma, infections, metabolic processes and central nervous system damage. 5 Specific examples include eye diseases and ocular surface surgery that can cause inflammation and/or nerve injury that activate sensory fibers of the eye.6 In fact, the dry eye alarm, in particular, has evolved into the most powerful, sensitive and complex pain system in the body. 7
HOW IS NEUROPATHIC OCULAR PAIN DIAGNOSED?
Identifying neuropathic ocular pain is important because it can influence treatment.8 Patient histories and detailed questionnaires are both key elements in making a diagnosis, as are specific tests such as esthesiometry.9 Patients who have neuropathic ocular pain often use descriptors such as burning, sharp, needle- or foreign body-like to explain symptoms.10 In addition, their pain usually has been present for an extended period of time and has become chronic. In addition to the standard elements of a comprehensive eye exam, esthesiometry using a cotton-tipped applicator or handheld instrument may be performed. This test helps measure and quantify corneal sensation. Also, since neuropathic pain is often caused by dry eye, specific tests to evaluate the ocular surface may be performed. These may include tear film osmolarity, corneal and conjunctival staining, tear break-up time, Shirmer’s testing, and testing for certain biomarkers in the blood.
MEDICAL THERAPY FOR NEUROPATHIC OCULAR PAIN
Neuropathic pain is notoriously difficult to treat,11 and people who have it are more likely recalcitrant to conventional ocular surface therapies.12 When appropriate, anti-inflammatory agents and anti-neuropathic pain drugs may be prescribed. However, more research is needed to understand the role of neuropathic pain treatments in treating dysfunctions of the ocular sensory apparatus associated with dry eye.
SCLERAL LENSES FOR NEUROPATHIC OCULAR PAIN
Since dry eye and corneal evaporative hyperalgesia are common causes for neuropathic pain, doctors are increasingly fitting these patients in scleral lenses. These uniquely designed lenses vault across the entire corneal surface and rest on the white part of the eye. This helps block corneal surface evaporation, which may be beneficial in reducing the symptoms of corneal evaporative hyperalgesia. 14 Even better, when scleral lenses are worn, a liquid reservoir is formed between the lens and the cornea. Scleral lenses also protect the eye from the ongoing mechanical shearing effect of the eyelids that occurs during blinking. Multiple studies have indicated improvement in ocular symptoms including a reduction in eye pain, improvement in photophobia and an improvement in quality of life.1516 If you have any form of eye pain, it is important to visit your eye doctor who can help identify the cause and determine the most appropriate management strategy
REFERENCES
Jebsen TS. Allodynia and hyperalgesia in neuropathic pain: clinical manifestations and mechanisms. Lancet Neurol. 2014 Sep;13(9):924-35.
Rosenthal P, Borsook D. Ocular neuropathic pain. Br J Ophthalmol 2016;100:128-134.
Belmonte C, et al. What Causes Eye Pain? Curr Ophthalmol Rep. 2015; 3(2): 111–121.
Belmonte C, et al. What Causes Eye Pain? Curr Ophthalmol Rep. 2015; 3(2): 111–121.
Belmonte C, et al. What Causes Eye Pain? Curr Ophthalmol Rep. 2015; 3(2): 111–121.
Belmonte C, et al. What Causes Eye Pain? Curr Ophthalmol Rep. 2015; 3(2): 111–121.
Rosenthal P, Borsook D. Ocular neuropathic pain. Br J Ophthalmol 2016;100:128-134.
Abelson M, et al. Neuropathic Pain: The Artifice of Dry Eye. Review of Ophthalmology. January 2016.
Galor A, Levitt RC, Levitt ER, et al. Neuropathic Ocular Pain. Eye 2015;29(3):301-12.
Abelson M, et al. Neuropathic Pain: The Artifice of Dry Eye. Review of Ophthalmology. January 2016.
Abelson M, et al. Neuropathic Pain: The Artifice of Dry Eye. Review of Ophthalmology. January 2016.
Galor A, Levitt RC, Levitt ER, et al. Neuropathic Ocular Pain. Eye 2015;29(3):301-12.
Abelson M, et al. Neuropathic Pain: The Artifice of Dry Eye. Review of Ophthalmology. January 2016.
Gemoules G. A Novel Method of Fitting Scleral Lenses Using High Resolution Optical Coherence Tomography. Eye & Contact Lens 34(2): 80-83, 2008.
Takahide K, Parker PM, Wu M, et al. Use of fluid-ventilated, gas-permeable scleral lens for management of severe keratoconjunctivitis sicca secondary to chronic graft-versus-host disease. Biol Blood Marrow Transplant. 2007;13:1016-1021.
Jacobs DS, Rosenthal P. Boston scleral lens prosthetic device for treatment of severe dry eye in chronic graftversus-host disease. Cornea. 2007;26:1195-9.