Nervous System, Eyelid Blink and Pain
The ocular surface is controlled by the nervous system consisting of the peripheral nerves and the central nervous system. Irritations from the ocular surface influence the control of the glands and the eyelid muscles. Tearing and blinking are controlled in the brainstem and perceived in the cortex. If the stimulus from the ocular surface is very strong, a 'paradoxical' wet dry eye can develop, often with tears dripping over the edge of the eyelid.
The eyelid blink is reflectory, but it is influenced by external stimuli from the surface and by internal signals from the cortex. Increased attention through concentrated visual tasks leads to rare blinking and can contribute to dry eyes. Frequent distracted 'nervous' eyelid blinking is often incomplete and also causes a part of the ocular surface to dry out.
Pain is more than a simple perceived stimulus. Pain is a complex negative feeling that is 'evaluated' and influenced by various brain centers. This can lead to a disturbance of the pain system in chronically irritated dry eyes. There is typically a difference/disparity between the visible clinical findings and the subjectively perceived intensity of the pain.
In dry eye, complex disturbances of the pain system can occur, which can lead to the development of a chronic pain syndrome. This may require an equally complex treatment strategy that goes beyond effective local ophthalmological therapy.
The nervous system regulates the function of the ocular surface
The ocular surface is regulated via the nervous system of the peripheral nerves and the central nervous system … in addition to the important influences of the endocrine hormonal system that provides more of a trophic influence and the immune system for protection.
The nervous system serves for
the regulation of function (e.g. gland secretion and eyelid blink) and also for
the sensory transmission of external stimulaly and generation of pain.
Apart from the nervous system also other regulatory systems such als the endocrine hormonal system and the immune system contribute to keep the ocular surface healthy.
Important functions such as the secretion of tear fluid and sufficient eyelid blinks are regulated automatically by reflex.
At least the regulation of eyelid blinks offers a certain voluntary input:
voluntary vigorous blinks (as blink exercise) allow to improve the release of more aqueous tears and more oil in order to reform a new tear film and improve wetting and refraction.
in concentrated visual tasks it is possible to suppress blinks to a certain extent so that a period of rare blinks occurs, that is often not sufficient to preserve sufficient moisture as well as refraction.
on the other hand sometimes too high a frequency of blinks can occur, due to a certain visual task of simply due mental stress (nervousness) which is often combined with incomplete blinks. Since the blink is not full in such cases, a certain lower part of the interpalpebral surface is not sufficiently covered with a new tear film and gets dry.
The sendory irritation of the ocular surface in dry eye conditions, e.g. by drying, friction, hyperosmolarity or wounding, travels as a signal via the sensory facial nerve (cranial nerve V, N. trigeminus) to the central nervous system (CNS).
In the CNS the signals are processed and the function of the eye is adjusted accordingly by automatic/ reflectoy regulation in the brain stem.
Tear fluid production and eyelid blinks are regulated in the brain stem and sensed in the cerebral cortex
Efferent nerve fibers run from the brain and brain stem via the secretory-motor facial nerve (crainal nerve VII, N. facialis) to the periphery. They regulate functions such as tear fluid production by the ocular glands and blinks by the eyelids.
At the same time signals also travel to the cerebral cortex where they can be consciously realized. This also offers the chance for a deliberate response.
Disturbances of the neural regulation may lead to alterations of tear secretion and eyelid blinks, such as e.g.:
rare eyelid blinks
incomplete eyelid blinks
This explains, why chronic excessive irritation from a dry eye can often lead to episodes of an excessive neural response in the form of excessive tearing and a wet eye
Excessive lacrimation is a protective reflex of the eye. It is in fact a useful try to simply flush away an irritant ocular surface impact as is often successful with a foreign body. If, however, the tissue itself is chronically irritated this approach does unluckily not work.
This leads to with tears dripping over the lid margin (medically termed epiphora) (see animated figure).
Other reasons for a wet eye may be:
Changes of the eyelids in shape of position, e.g. inward or outward twisting of the eyelid margin.
Obstruction of the normal outflow of tears from the eye into the nose
Regulation of the eyelid blink - normal, rare or incomplete
The break-up of the thin tear film becomes visible when the tears are stained with the fluorescein dye that is observed in blue light. A break-up occurs as a dark area (seen here in the middle of the faint “C” of the OSCB overlay). The break-up of the tear film is the stimulus that triggers an eyelid blink in order to distribute a new tear film.
The procession of sensory nerve signals from the ocular surface is complex and is regulated by different centers in the central nervous system.
The eyelid blink is governed in principle by the blink reflex in the brain stem. When the tear film on the ocular surface breaks up (see animated image) this leads to immediate drying of the surface tissue with activation of receptors of the local nerve fibers.
The respective centripetal sensory stimulus results in the triggering of a new eyelid blink that reforms a new tear film and thereby removes the irritating stimulus to the brain.
This reflectory blink occurs mainly unconscious, although we can certainly become aware of a blink and can even deliberately influence blinks to a certain extent.
During centrated visual tasks the frequency of blinks can be deliberately slowed down.
This applies to many daily occupations such as reading, particularly if this is done using any kind of computer screen. It also applies to watching television or driving a car, particularly at night etc..
The eye is then opened for a longer time in order not to interrupt the ´important´ visual impression. And this continues even when some initial neural irritation by surface dryness arrives in the brain.
the mental attention in concentrated visual tasks overrides the incoming stimulus that is intended to trigger a blink
as seen in the animated figure the eye even starts to develop a certain redness before eventually a blink occurs that reforms the tear film and refreshes the eye.
This individual will likely have sensations ot dry eye due to the drying of the surface.
Thereby the eyelid blink can occur more rarely that the usual normal value of about 5-10 blinks per minute. Since the tear film is normally stable for only about 10-20 seconds, the ocular surface can easily become dry in concentrated visual tasks.
This explains why rare blinking can induce or worsen a dry eye condition.
In addition, not only the frequency but also the fullness of the eyelid blink can be influenced by the nervous system.
In ´nervous blinks´ that occur due to nervousness or habitual, the eyelid blink can occur more frequently than necessary and is often incomplete without being noticed.
Incomplete blink means, that the upper eyelid is not completely closed during the downstroke phase. Therefore the tear film can not be completely reformed on the interpalpebral ocular surface. The inferior part of the tear film remains continuously ´old ´ (see animated image) and thus the tissue surface dries out and is increasingly damaged.
Incomplete eyelid blinks occur more frequently than assumed and can therefore be an important factor in dry eyes. However, the patient does not realize them and for the doctor it is not easy to recognize them, except with the help of specific diagnostic machines.
Stimulus, neural signal and the generation of pain feeling
There is in principle a certain difference between the pure 'perception' of a stimulus from the ocular surface, which takes place in the cortex, and the emotional evaluation of this stimulus in the 'subordinate' limbic system.
´Pain´ is no simple perception but it is a feeling/emotion based by on a construction of the brain.
This construction results from the emotional evaluation of an incoming neural signal. The evaluation takes place primarily in the so-called limbic system and through other emotional instances. Numerous other qualities, such as experiences and expectations contribute to the subjective evaluation.
However, for the individual human being, the two instances of 'perception' and what appears as 'pain' cannot always be clearly separated. So in the end it is not always clear which of the different instances in the brain is 'cook or waiter'., i.e. which instance reflects the reality at the ocular surface best.
At the ocular surface this means that there can be a certain disparity between the subjectively experienced intensity of pain and the objective degree of stimulation or wounding. Clinically this translates into a disparity between ´signs and symptoms´.
Disparity between signs and symptoms of the patient
It is typical for dry eye disease the the severity of subjective symptoms in the sense of pain does not always meet the degree of objective clinical signs.
Pain is based on subjective evaluation
The subjective evaluation of a stimulus from the ocular surface may, therefore, well dominate over objective perception. This may depend on the 'form of the day' of the individual person, but can also be very different for different people interindividual.
This can be expressed in such a way that the subjective symptoms cause the patient to suffer a great deal while clinically little or sometimes no objective findings are detectable - in this case, one might think of a pain syndrome.
On the other hand, a patient with significant clinical findings, e.g. a stainable surface damage (vital staining), may have little or no subjective symptoms in the sense of irritation or pain. Such a patient will then probably have little inclination to undergo any medically indicated therapy.
Pain syndrome
Disturbance of pain procession can lead to a pain syndrome.
Pain has the biological sense to indicate a damage of the body and thus to give a signal for danger. If chronic pain stimuli persist, disturbances in the pain system can occur. Transmission paths of the signals in the pain system can then be channelled and become increasingly solidified. In this case, the pain impression can persist even if the pain stimuli on the body decrease or the tissue damage has already healed.
Pain then loses its actual meaning as a warning signal and becomes part of the disease. This pain is described as "neuropathic" pain, i.e. pathological pain.
This means that a subjective impression of pain may persist for the patient, even though a successful dry eye therapy has already led to an improvement in the condition of the ocular surface.
Psychosomatic influences
Psychosomatic influences on the course of an illness and on the perception of the patient are an established and serious part of medicine for many years. According to many studies, According to many studies, psychosomatic influences can also play a role in functional disorders such as unspecific back pain, functional cardiovascular complaints or digestive disorders and may also play a role in dry eye disease.
If the occupation with the disease becomes the most important part in the life of a patient, this could be a reason to seek the help of psychosomatics.