Corneal Confocal Microscopy Demonstrates Corneal Nerve Loss in Patients With Trigeminal Neuralgia

Nerve-myeloid cell interactions in persistent human pain: a reappraisal using updated cell subset classifications

ABSTRACT

The past 20 years have seen a dramatic shift in our understanding of the role of the immune system in initiating and maintaining pain. Myeloid cells, including macrophages, dendritic cells, Langerhans cells, and mast cells, are increasingly implicated in bidirectional interactions with nerve fibres in rodent pain models. However, our understanding of the human setting is still poor. High-dimensional functional analyses have substantially changed myeloid cell classifications, with recently described subsets such as epidermal dendritic cells and DC3s unveiling new insight into how myeloid cells interact with nerve fibres. However, it is unclear whether this new understanding has informed the study of human chronic pain. In this article, we perform a scoping review investigating neuroimmune interactions between myeloid cells and peripheral nerve fibres in human chronic pain conditions. We found 37 papers from multiple pain states addressing this aim in skin, cornea, peripheral nerve, endometrium, and tumour, with macrophages, Langerhans cells, and mast cells the most investigated. The directionality of results between studies was inconsistent, although the clearest pattern was an increase in macrophage frequency across conditions, phases, and tissues. Myeloid cell definitions were often outdated and lacked correspondence with the stated cell types of interest; overreliance on morphology and traditional structural markers gave limited insight into the functional characteristics of investigated cells. We therefore critically reappraise the existing literature considering contemporary myeloid cell biology and advocate for the application of established and emerging high-dimensional proteomic and transcriptomic single-cell technologies to clarify the role of specific neuroimmune interactions in chronic pain.

Corneal confocal microscopy identifies corneal nerve fiber loss in patients with migraine

BACKGROUND/HYPOTHESIS

Migraine affects >1 billion people but its pathophysiology remains poorly understood. Alterations in the trigeminovascular system play an important role. We have compared corneal nerve morphology in patients with migraine to healthy controls.

METHODS

Sixty patients with episodic (n = 32) or chronic (n = 28) migraine and 20 age-matched healthy control subjects were studied cross-sectionally. Their migraine characteristics and signs and symptoms of dry eyes were assessed. Manual and automated quantification of corneal nerves was undertaken by corneal confocal microscopy.

RESULTS

In patients with migraine compared to controls, manual corneal nerve fiber density (P < 0.001), branch density (P = 0.015) and length (P < 0.001); and automated corneal nerve fiber density (P < 0.001), branch density (P < 0.001), length (P < 0.001), total branch density (P < 0.001), nerve fiber area (P < 0.001), nerve fiber width (P = 0.045) and fractal dimension (P < 0.001) were lower. Automated corneal nerve fiber density was higher in patients with episodic migraine and aura (P = 0.010); and fractal dimension (P = 0.029) was lower in patients with more headache days in the last three months. Automated corneal nerve fiber density predicted a significant amount of the observed variance in pain intensity (adjusted r² = 0.14, partial r = -0.37, P = 0.004) in patients with migraine.

CONCLUSIONS

Corneal confocal microscopy reveals corneal nerve loss in patients with migraine. It may serve as an objective imaging biomarker of neurodegeneration in migraine.

Corneal Confocal Microscopy to Image Small Nerve Fiber Degeneration: Ophthalmology Meets Neurology

Neuropathic pain has multiple etiologies, but a major feature is small fiber dysfunction or damage. Corneal confocal microscopy (CCM) is a rapid non-invasive ophthalmic imaging technique that can image small nerve fibers in the cornea and has been utilized to show small nerve fiber loss in patients with diabetic and other neuropathies. CCM has comparable diagnostic utility to intraepidermal nerve fiber density for diabetic neuropathy, fibromyalgia and amyloid neuropathy and predicts the development of diabetic neuropathy. Moreover, in clinical intervention trials of patients with diabetic and sarcoid neuropathy, corneal nerve regeneration occurs early and precedes an improvement in symptoms and neurophysiology. Corneal nerve fiber loss also occurs and is associated with disease progression in multiple sclerosis, Parkinson's disease and dementia. We conclude that corneal confocal microscopy has good diagnostic and prognostic capability and fulfills the FDA criteria as a surrogate end point for clinical trials in peripheral and central neurodegenerative diseases.