In the following sections we review how aging may impact the anatomical structures associated with dry eye including the lacrimal glands, meibomian glands, goblet cells (e.g., lacrimal functional unit, or LFU) as well as the immune system and ocular sensor apparatus (peripheral and central nervous system components).
The impact of aging on the lacrimal glands, meibomian glands, and goblet cells (LFU): Changes in the lacrimal gland (producer of the aqueous component of tears), meibomian glands (producers of the lipid component) and goblet cells (producers of the mucin component) have all been described with aging. The acini, stroma, and ducts of aging rat lacimal glands displayed changes that included acinar degeneration, nuclear abnormalities, lipofuscin-like inclusions; increased collagen, and ductal dilation [37
]. Similarly, mice and human aging meibomian glands demonstrated loss of acne and a reduction in cellular proliferation markers (Ki67) [38
]. Furthermore, decreased goblet cell densities on the ocular surface have been described in older animals, including rats [37
] and mice [42
] (detected using histology), and in humans (detected using in vivo confocal microscopy) [43
] All of the above changes have been shown to alter the properties and function of the aging tear film. These despite changes, clinically measured dry eye signs (as described above) do not mirror patient reported dry eye symptoms, including ocular pain.
Studies in rodents have also found reduced density of parasympathetic and sympathetic nerves surrounding the acini with aging [44
]. These differences were also detected via decreased acetylcholine release from lacrimal gland nerves in 24-month-old mice compared to 3 and 12-month-old animals. Functionally, these changes were also associated with decreased tear secretion. Older lacrimal glands also synthesized less protein and secreted less fluid after stimulation with SP, VIP, histamine or 5-HT [46
The impact of aging on the immune system and ocular inflammation: Inflammation is a well-recognized component of dry eye [47
]. A classic paper that established this concept in animals was published by Niederkorn et al. in 2006 [49
]. In this paper, mice were first subjected to a low humidity environment and were given scopolamine to reduce blinking. These experimental conditions led to the development of T cell-mediated inflammation on the ocular surface with clinical manifestations that resembled dry eye in humans (i.e. corneal staining). The authors were then able to induce a similar disease picture in nude mice by adoptively transferring CD4(+
) T cells . Newer data have since demonstrated that older mice spontaneous develop this phenotype, with CD4+
T cell infiltration into the conjunctiva, expression of inflammatory cytokines (interferon-γ, IL-17, matrix metallopeptidase 9), and increased T and B cells in the lacrimal gland. Interestingly, adoptive transfer of CD4(+
) T cells isolated from these elderly mice transferred the disease into young immunodeficient recipients [42
]. Rats similarly have been found to have increased mast and lymphocytic infiltration in their lacrimal glands with age [37
] Inflammation is a known sensitizer of peripheral nerves [50
] and it is likely that inflammation on the ocular surface alters the function of the corneal nociceptors, and therefore the function of the ocular sensory apparatus.
The impact of aging on the ocular sensory apparatus: Changes in peripheral nerves, including a reduction in the number of specialized peripheral receptors and deterioration of myelinated and unmyelinated axons, are known to occur with age [52
] and have been related to decreased sensitivity within the auditory, gustatory, and visual systems [53
]. Similar changes have been shown within the somatosensory system, including mechanical, thermal, and nociceptive responsiveness. Evidence from this line of research may be useful for understanding changes in mechanical and thermal sensory processing within the ocular sensory apparatus.
In aged animals the proportions of cutaneous mechano- and/or heat-responsive C-nociceptors have been shown to be significantly lower in older compared to younger rats [54
]. Similarly, microneurographic studies in humans have reported decreased ratios of mechano-sensitive to mechano-insensitive C-fibers in healthy older subjects (age range: 41-67 years) compared to younger subjects (21-36 years) when measuring from the peroneal nerve [55
Electrophysiologically, the mechanical stimulus needed to elicit a response in cutaneous mechano-responsive C-fibers was higher in older compared to younger rats, but responses to heat and chemical stimuli were similar irrespective of age [54
]. In contrast, responses recorded from mechano-responsive C-fibers from deep tissue (muscle) in aged rats, showed the opposite effect: lower thresholds and greater responsiveness to mechanical stimuli compared to fibers from young rats [56
]. In humans, decreased thresholds for heat stimuli in cutaneous mechano-insensitive C-fibers, and slightly increased thresholds in mechano-responsive fibers, have been found in aged individuals compared to fibers in their younger counterparts [55
]. Additionally, microneurographic recordings from single C-fibers in humans have demonstrated that a higher frequency of fibers in older adults had atypical discharge characteristics (13%) compared to fibers recorded from younger individuals, similar to, but less pronounced than, what has been described in patients with neuropathic pain [55
Behaviorally, it has generally been found in humans that aging caused increases in threshold for pain report, but decreases in pain tolerance [53
]. Similarly, animal studies have reported enhanced nociceptive behaviors to supra-threshold noxious levels of cold and heat (but similar responses to mechanical stimuli) between aged and young animals [54
]. The interplay of peripheral and central changes due to aging which give rise to these changes in perceptual response involves a complex and minimally understood set of mechanisms.
With regards to the eye, confocal microscopy data studying peripheral corneal nerves have demonstrated mixed results, with one study reporting significant declines in sub-basal nerve fiber density with age (0.9% per year) [57
], and others reporting no alterations in number, density, or beadings of nerves with age [58
Central nerve changes are also known to occur with age, with decreased connectivity and network integrity noted in healthy older adults [60
]. With regards to the eye, changes in peripheral nerves due to age related environmental stress and inflammation may propagate upwards and affect the function of the central aspect of the ocular sensory apparatus. The anatomy of the ocular sensory apparatus lends biologic plausibility to this idea. The first synapse of the ocular sensory apparatus occurs in the Vi/Vc zone, and specific dry-responsive projection neurons have been identified in this area. A subset of these neurons, receive additional converging input from corneal primary afferents sensitive to protons, heat and chemicals [61
]. Therefore, projection neurons in the transition zone integrate innocuous as well as noxious sensory information from the eye, and may constitute the neurophysiological substrate of central sensitization. In fact, the Vi/Vc transition zone is a well-recognized area involved in the generation and maintenance of hyperexcitability and central sensitization [62
With regards to the overall function of the ocular sensory apparatus, both the Cochet-Bonnet and Belmonte aesthesiometry devices have been used to characterize somatosensory function within the eye in animals and humans. Using the Cochet-Bonnet aesthesiometer, corneal sensitivity was found decrease with age in horses [63
] and humans [64
]. The Belmonte aesthesiometer, a more robust instrument, has displayed mixed results with some studies reporting lower detection thresholds (higher sensitivity) [65
] and others reporting higher thresholds (lower sensitivity) with age [66
]. Unfortunately, none of the available instruments can directly measure the function of the peripheral and/or central components of the ocular sensory apparatus in-vivo in humans.