Corneal mechanical sensitivity in individuals with chronic pain

Chronic pain is a common and often debilitating condition. According to Brandon Baksh, from the University of Miami, Florida, United States, chronic pain syndromes can be localised to the head and face and involve the trigeminal system: for example, in migraine and trigeminal neuralgia. Mr Baksh noted that chronic pain syndromes can also be widespread, such as in fibromyalgia, or localised to areas outside the head and face, such as back pain.

Nerve abnormalities have been linked to chronic pain syndromes. “As eye care providers,” he said, “we can assess this link between nerve abnormalities and chronic pain.” Corneal sensitivity—which, according to Mr Baksh, can be measured in a clinic—is altered in several eye conditions. However, its relationship with chronic pain conditions outside the eye is not clear.

Study purpose

To learn more about how chronic pain conditions outside the eye impact corneal sensitivity, the authors investigated whether individuals with chronic trigeminal pain conditions have differences in corneal sensitivity compared with individuals who have chronic pain conditions not localised to the trigeminal system.

The investigators’ hypothesis was that patients with chronic trigeminal pain have differences in corneal mechanical detection thresholds compared with those who have chronic pain conditions that are not localised to the trigeminal system.

Design and methods

A cross-sectional study that included south Florida veterans with chronic pain conditions (> 3 months’ duration) who had been seen at an eye clinic was performed. The study included 568 individuals with one or more chronic pain conditions. The mean age was 61+/-10.5 years. Of the group, 89% were male, 44% self-identified as White and 25% self-identified as Hispanic.

The main independent variables were pain diagnoses and locations, as recorded by the participants in a survey. The main dependent variable was corneal mechanical detection threshold, which was assessed using a modified Belmonte aesthesiometer.

To assess confounding variables, data were collected on demographics, dry eye symptoms and signs, mental health indices, systemic comorbidities and quality-of-life indices. Individuals were split into two groups based on their pain diagnosis:

• Group 1 (trigeminal pain) — chronic pain conditions that involved the trigeminal system
  • Migraine
  • Burning mouth syndrome
  • Trigeminal neuralgia
  • Trigeminal mandibular disorder
• Group 2 (other pain) — chronic pain conditions that were not localised to the trigeminal system
  • Back pain
  • Knee pain

Assessing corneal mechanical threshold

Corneal mechanical threshold was assessed with a modified Belmonte aesthesiometer:

1. Pulses of air at room temperature were used on the right eye;
2. They started at 10 mL/min with 15-second waits between pulses;
3. Pulses increased by 10 mL/min until the patient could feel them;
4. This was repeated a second time and the mean was used; and
5. If the difference between the two trials was greater than 20, a third reading was done.

Mr Baksh noted that a lower detection threshold using the Belmonte aesthesiometer meant a higher corneal sensitivity.

Results

Table 1. Other categorical variables that are associated with a lower corneal mechanical detection threshold. Note that a lower result = higher sensitivity.

In terms of the statistics used, continuous variables were assessed via the independent t test. Correlations were assessed via Pearson r coefficient and categorical variables were assessed via chi-square test. All reported P values were 2-tailed and P < 0.05 was considered statistically significant.

In the study, the mean corneal mechanical detection threshold was 81+/-38 mL/min, with a range of 10 to 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) of 73+/-31, compared with the other pain group (n = 446, mean threshold 84+/-40). This was statistically significant. The groups shown in Table 1 were also associated with a higher corneal sensitivity.

The authors also assessed continuous variables. Variables that came out significantly correlated with lower corneal mechanical detection threshold (higher sensitivity) included a high depression score, a greater post-traumatic stress disorder score, worse dry eye symptoms and more severe eye pain. Other significantly correlated factors were a lesser quality of life, younger age and lower cholesterol and triglycerides levels.

Conclusions

Lower corneal mechanical detection threshold (higher sensitivity) may be associated with pain phenotypes, such as the presence of chronic pain involving the trigeminal system, five or more pain conditions and the presence of arthritis. Beyond pain phenotypes, additional factors that may play a role include a higher depression score, race, dyslipidaemia and continuous positive airway pressure use.

Brandon Baksh, BA

E: bxb619@med.miami.edu

This article is based on Brandon Baksh’s presentation at the 2021 Association for Research in Vision and Ophthalmology virtual annual meeting. Mr Baksh has no financial disclosures.