Mechanical corneal sensitivity in people with chronic pain

This article was written by Brandon Baksh, BA, and Anat Galor, MD, MSPH. reviewed

Chronic pain is a common and often debilitating condition. Chronic pain syndromes can be located in the head and face and affect the trigeminal system, for example in migraines and trigeminal neuralgia, explained Brandon Baksh, BA.

He went on to say that chronic pain syndromes can also be prevalent, such as in fibromyalgia, or localized to areas outside the head and face such as back pain, and that nerve abnormalities have been linked to chronic pain syndromes.

“As opticians,” he said, “we can assess this relationship between nerve abnormalities and chronic pain.”

He pointed out that corneal sensitivity can be measured in the office.

Purpose of study
The corneal sensitivity is changed in several eye diseases. However, the relationship to chronic pain conditions outside the eye is not clear.

To learn more about how chronic pain conditions outside the eye affect corneal sensitivity, the authors investigated whether people with chronic trigeminal pain conditions show differences in corneal sensitivity compared to people with chronic pain conditions that are not located in the trigeminal system.

Their hypothesis was that patients with chronic trigeminal pain have differences in the mechanical recognition threshold of the cornea compared to patients with chronic pain conditions that are not localized in the trigeminal system.

Methods
A cross-sectional study was conducted on veterans from South Florida with chronic pain conditions (> 3 months duration) who were treated in an eye clinic.

The most important independent variables were pain diagnoses and localizations, which were recorded by the participants through a survey.

The main dependent variable was the mechanical recognition threshold of the cornea, which was assessed with a modified Belmonte esthesiometer.

To assess confounders, data were collected on demographics, symptoms and signs of dry eye, mental health indices, systemic comorbidities, and quality of life indices.

The people were divided into two groups based on their pain diagnosis:

· Group 1 (trigeminal pain) – Chronic pain conditions with involvement of the trigeminal system

  • migraine
  • Burning Mouth Syndrome
  • Trigeminal neuralgia
  • Trigeminal mandibular disorder (TMD)

· Group 2 (other pains) – chronic pain conditions that are not localized in the trigeminal system

Assessment of the mechanical corneal threshold
The mechanical corneal threshold was determined with a modified Belmonte esthesiometer:

  • Air pulses were used in the right eye at room temperature.
  • Started at 10 ml / min and waited 15 seconds between pulses
  • Increased by 10 ml / min until the patient could feel the pulse
  • This was repeated a second time and the mean value used
  • If the difference between the 2 experiments was> 20, a third measurement was carried out

Baksh found that a lower detection threshold with the Belmonte Aesthesiometer means higher corneal sensitivity.


statistics
Continuous variables were assessed using the independent t-test. Correlations were scored using the Pearson’s r coefficient and categorical variables were scored using the chi-square test.

All reported p-values ​​are bilateral and p <0.05 was considered statistically significant.

Results
This study included 568 people with one or more chronic pain conditions. The mean age was 61 ± 10.5 years. 89% were male, 44% said they were white, and 25% said they were Hispanic.

The mean mechanical detection threshold of the cornea was 81 ± 38 ml / min, with a range of 10-240 ml / min.

The authors found that the trigeminal pain group (n = 122) had a lower mean corneal mechanical detection threshold (i.e. higher sensitivity) (73 +/- 31) compared to the other pain group (n = 446, mean threshold 84 +/- ) exhibited. 40). This was statistically significant.

The authors also rated continuous variables. Variables that significantly correlated with a lower corneal mechanical detection threshold (higher sensitivity) included a high depression score, a high PTSD score, worse dry eye symptoms, worse eye pain, and poorer quality of life, lower age, lower cholesterol, and lower Triglycerides.


Conclusions
A lower mechanical recognition threshold of the cornea (higher sensitivity) may be associated with pain phenotypes such as chronic pain with involvement of the trigeminal system, 5 or more pain conditions, and arthritis.

In addition to pain phenotypes, other factors that may play a role include higher levels of depression, black race, dyslipidemia, and CPAP use.

Brandon Baksh, BA
[email protected]
This article is based on Baksh’s presentation at the ARVO 2021 annual virtual meeting. Baksh has no financial information.