How pain gates affect dry eye and chronic pain

If an OD sees a lot of dry eye patients or has relatives with chronic pain, she may be familiar with how people feel pain differently. I am fortunate enough to be in both situations.

I see a large number of dry eye patients, most with significant chronic pain complaints. These patients often suffer from pain that is out of proportion to the clinical signs. Common treatments often to do not relieve their symptoms, leaving us both frustrated.

My view on these patients is certainly colored by experiences with chronic pain in my family. I have a child with hypermobility joint syndrome (HJS) (think Ehlers-Danlos syndrome [EHS]) who has been dealing with chronic pain since 2013. It is extremely difficult to undergo tests, scans, scopes, surgeries, and countless doctor’s visits, only to be told nothing can be found.

It took five years for a proper diagnosis-primarily because our specialists narrowed their wheelhouse so much that they could not see the forest through their own set of trees.

Previously by Dr. Swartz: Blog: A parent's perspective on genetic testing Pain theory  
I was educated about pain gate by a rheumatologist. The Gate Control Theory of Pain is credited to psychologist Ronald Melzack and biologist Patrick Wall in 1965.¹ Since that time, the Gate Control Theory was modified in 1978 and 1996.^2,3 Even as knowledge of the nervous system developed, the theory has remained consistent.^4,5 

According to this theory, pain messages travel from peripheral nerves to nerve “gates” located in the spinal cord and continuing to the brain. These central nervous system “gates” describe how some pain messages are allowed to continue to the brain while others are blocked. 

In addition to controlling pain message “traffic,” the gates can also amplify or diminish pain signals. According to this theory, patients who suffer more pain have more open gates. Those who suffer less pain have fewer open gates.

Related: 3 mental health conditions to watch for in patients

Acute and chronic
In order to understand this theory, it is necessary to understand the difference between acute and chronic pain.

Fast pain signals use A-delta fibers. These are crucial to protecting the body from injury, such as removing one’s hand from a hot plate. This may be referred to as “warning pain.” A-delta signals do not last long and are delivered to the brain’s sensory cortex.

Chronic pain messages move more slowly along C-fibers, and these signals last longer. This pain may be described as nagging, aching, dull, or burning-this may be referred to as “reminder pain.”

These signals may be responsible for reminding the brain that it has suffered an injury. 

Unfortunately, chronic pain may continue after the injury heals. The signals are delivered to the hypothalamus and the limbic system to release stress hormones and handle emotions. Thus, stress, depression, and anxiety are associated with chronic pain.

Related: How to help patients who are depressed Pain gates
Factors that open the pain gates and increase suffering may be sensory, cognitive, and emotional.

Examples of sensory factors include musculoskeletal injury, physical inactivity, problematic body mechanics, and long-term narcotic use.

Cognitive factors include a focus on the pain, lack of pleasurable interests, anxiety about pain, and perseverating on things associated with it.
Emotional factors include stress, anger, depression, anxiety, frustration, and feelings of hopelessness.

Factors that close the pain gates reduce suffering. Sensory factors including increasing physical activity and aerobic exercise, short-term use of pain medication, relaxation training, and meditation. 

Related: Diet, exercise can diminish eye diseases 

Emotional factors include a positive attitude, reducing depression, and understanding that the pain is not harmful.

Learning to take control of the pain, taking control of non-pain aspects of life, and learning stress management techniques are also beneficial.
According to the pain theory, open gates allow pain signals to reach the brain while closed gates blocks the signals. If a fast stimulus can close the spinal gates, the slower messages are blocked-reducing the pain. 

Management 
Pain control options that close the gates include peripheral stimulation, acupuncture, and auditory interventions. Peripheral stimulation includes transcutaneous electrical stimulation (TENS) and peripheral nerve field or spinal cord stimulation.

TENS units are widely available without prescription and are typically battery-powered. Pads are placed on the skin over the pain area and connected to the TENS unit to supply a current. These produce a less painful buzzing or tingling feeling, which competes with chronic pain signals.

For example, both the threshold and tolerance of tooth pain have been effectively controlled using electrical activation of dental nociceptors for 7 minutes.⁶Related: ODs must examine more than just eyes 
Peripheral nerve field stimulation involves surgical implantation of electrodes directly on nerves or under the skin in the region of pain. Spinal cord stimulation involves surgical placement of the stimulating device at the spinal cord, where the nerve originates.⁷

Acupuncture or dry needling uses thin needles to activate small pain fibers to close pain gates. A meta-analysis of studies using acupuncture for treatment of chronic pain found that acupuncture treatment was better than no acupuncture treatment, and the effect lasted up to 12 months after treatment.⁸Related: Process key to pain management 

Melatonin, an endogenous neurohormone that contributes to circadian rhythms, may also be used to address chronic pain. When circadian rhythms are disturbed, the body produces hormones, chemicals, and neurotransmitters in aberrant amounts or at the wrong time of day. Exogenous melatonin analgesic and neuroprotective effects may be useful for chronic pain.⁹

Because chronic pain is closely tied to depression, its treatment may include various antidepressants, including tricyclics, selective serotonin reuptake inhibitors, serotonin norepinephrine reuptake inhibitors, other miscellaneous antidepressants, and atypical antipsychotics in the treatment of chronic pain.¹⁰Related: Spectacles, lenses offer relief for headaches, ocular symptoms  Wrapping up
An increasing number of my patients take amitriptyline nightly for pain. Unfortunately for those patients suffering from ocular surface disease (OSD), the use of antidepressants may exacerbate dry eye symptoms.

Patients with chronic dry eye and other chronic pain syndromes will be more difficult to treat-they may continue to report symptoms of pain and discomfort when the clinical signs have improved with treatment.

I often use topography, aberrometry, and slit-lamp photos to illustrate OSD pre- and post-treatment.

If the clinical picture improves significantly but the pain fails to improve, I often instill proparacaine. If the pain fails to improve with topical anesthesia, I will educate the patient that the pain is not ocular, and begin to investigate other methods of pain control.

As primary-care providers, optometrists need to bear this in mind when diagnosing and treating these patients.

Read more about patient care 

References:

1. Melzack R, Wall PD. Pain mechanisms: a new theory. Science. 1965 Nov 19;150(3699):971-9.
2. Wall PD. The gate control theory of pain mechanisms. A re-examination and re-statement. Brain. 1978 Mar;101(1):1-18.
3. Melzack R. Gate control theory: On the evolution of pain concepts. Pain Forum. 1996 Summer; 5, 128-138.
4. Dickenson AH. Gate control theory of pain stands the test of time. Br J Anaesth. 2002 Jun;88(6):755-7.
5. Mendell LM. Constructing and deconstructing the gate theory of pain. Pain. 2014 Feb;155(2):210-6.
6. Zampino C, Ficacci R. Pain Control by Proprioceptive and Exteroceptive Stimulation at the Trigeminal Level. Front Physiol. 2018 Aug 7;9:1037.
7. Deogaonkar M, Slavin KV. Peripheral nerve/field stimulation for neuropathic pain. Neurosurg Clin N Am. 2014 Jan;25(1):1-10.
8. Vickers AJ, Vertosick EA.  Acupuncture for chronic Pain: Update of an individual patient data meta-analysis. J Pain. 2018 May;19(5):455-474.
9. Kaur T, Shyu BC. Melatonin: A New-Generation Therapy for Reducing Chronic Pain and Improving Sleep Disorder-Related Pain. Adv Exp Med Biol. 2018;1099:229-251.
10. Khouzam HR. Psychopharmacology of chronic pain: a focus on antidepressants and atypical antipsychotics. Postgrad Med. 2016;128(3):323-30.