Abstract
This review discusses the role of opioids in the corneal surface and the different pathways and therapeutic methods of management. A literature review was performed using PubMed database. For the database search, the main searching words “opioid” and “topical opioid treatment” were used with the descriptors “cornea”, “ocular surface”, “neuropathic corneal pain”, “corneal sensitivity” and “naltrexone”; original scientific articles and reviews were included to achieve the purpose of the review. The endogenous opioid system has relevant functions in the organism, and in daily use, opioids are used as painkillers. However, these drugs may be employed for other indications as opioid pathways have a wide spectrum. The corneal surface for topical treatment is easily accessible, hence sparing the side effects of systemic opioids. Instillation of opioid antagonist substances, such as naltrexone, increases corneal healing rates and stimulates the division of corneal epithelium cells without deleterious effects. The natural modulation of endogenous opioids controls different forms of pain, including inflammatory and neuropathic pain, both in the ocular surface and in the central nervous system. There are diverse methods in controlling pain using opioids, especially in refractory forms. This review attempts to collect the literature about corneal surface and opioid pathways to provide an overview image and a possible direction of the news treatments.
Highlights
The endogenous opioid system is formed by a series of neuropeptides, namely: enkephalins, divided into leucine-enkephalins and methionine-enkephalins, endorphins and dynorphins
For achieving the aims associated with the research question, a narrative literature search was performed in PubMed using the keywords: “opioid” AND “cornea” OR “ocular surface”, “topical opioid treatment” AND “cornea”, “neuropathic corneal pain” OR
With the aim to collect the different forms to modulate the opioid system in the cornea, this article has been divided according to the main functions of the opioid system in the ocular surface
Summary
The endogenous opioid system is formed by a series of neuropeptides, namely: enkephalins, divided into leucine-enkephalins and methionine-enkephalins, endorphins and dynorphins These neuropeptides exert their functions through the widely expressed opioid receptors in the central and peripheral neurons, and in the neuroendocrine, immune, and ectodermal cells [1]. As with other peripheral nerve systems, corneal nerve fibers release endogenous met- and leu-enkephalins peptides in the basal form [13] and play a role in modulating acute and chronic pain [1]. Part of the normal immune system present in the ocular surface, such as lymphocytes, dendritic cells, and monocytes, release enkephalins and dynorphins [16] These cells and native opioids, recruited especially in inflammatory situations, have demonstrated a function in analgesia and neuropathic pain [17–19]. The aim of our review is to portray the evidence of the opioid therapeutic targets in the ocular surface, their clinical approach, and future treatments
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