Miosis is the bilateral constriction of the pupils in response to light stimulus or as part of near fixation. When miosis is present without a light stimulus and independent of near fixation, this phenomenon has disease value. Intoxications are as likely a cause as meningitis or lesions of the pons.
What is miosis?
Miosis is the bilateral constriction of the pupils on exposure to light or during near fixation. In miosis, the pupils temporarily constrict up to two millimeters. The constriction can be pronounced either unilaterally or bilaterally and can vary in intensity. The reflex corresponds to an eye reflex to light irradiation and is subject to parasympathetic control. Either the constriction results from a contraction of the autonomically controlled eye ring muscle Musculus sphincter pupillae or from a decreased activity of its antagonist Musculus dilatator pupillae. Both muscles are part of the internal eye musculature. Miosis can be a symptom of various diseases. However, it can also be artificially induced by the administration of parasympathomimetics. The opposite of miosis is mydriasis, in which there is dilatation of the pupils over five millimeters. Lens constriction and lens dilatation both belong to the phenomenon of accommodation. They are physiological in response to certain stimuli. However, without a preceding stimulus, they are pathological phenomena.
Function and task
The third cranial nerve, called the oculomotor nerve, plays a role in miosis. Its nerve fibers originate from the nucleus accessorius, which is also known as the nucleus Edinger Westphal. This is an accessory nucleus of the third cranial nerve, located in the mesencephalon and connected to the eye by preganglionic parasympathetic fibers. The parasympathetic fibers of the third cranial nerve are interconnected in the ciliary ganglion, a ganglion in the orbit responsible for pupillary reflexes. The nerve fibers extend through the ciliary breves nerve to the sphincter pupillae muscle. The reflex arc of pupillary reflexes attaches to the retina. It continues into the area pretectalis via the optic nerve and is bilaterally connected in the mesencephalon. Due to this bilateral circuitry, the pupils always constrict bilaterally during physiological miosis, as is the case with light stimuli. This is true even if only one eye is directly stimulated. For the other eye, this is then referred to as an indirect light reflex. The adaptation to the incidence of light is called adaptation. Narrowing reduces the incidence of light and the eye thus preserves visual acuity. Thus, miosis is both a protective reflex and an adaptive reflex. Physiologically, miosis also occurs during near fixation in the broadest sense. Together with the convergence movement and accommodation, miosis in near fixation constitutes the neurophysiological control loop of the near adjustment triad. Pupil constriction during accommodation helps people see nearby objects with extra sharpness because the reduced lens size generates a greater depth of field. Even in lensless people, miosis improves visual acuity. Therefore, it is specifically and deliberately induced by ophthalmologists to treat various diseases in order to improve patients’ vision.
Diseases and disorders
Pathologic miosis may indicate alcohol abuse or drug use. Opiates, opioids, and morphines in particular cause miosis. The same is true for narcotics or fading anesthesia. Miosis can be specifically induced by drug administration and then usually corresponds to ophthalmologic therapy, such as may be useful for glaucoma. Targeted induction usually occurs with miotics such as pilocarpine. Miosis is also induced by medication during differential diagnosis of certain ocular diseases and during pharmacodynamic studies of pupillotonia. On the other hand, if miosis is to be prevented momentarily for ophthalmologic examinations, the physician gives mydriatics. These include hyoscyamine or atropine, which temporarily paralyze the sphincter pupillae muscle. When parasympatholytics are given, the muscle paralysis is accompanied by a loss of accommodation ability, which comes from paralysis of the parasympathetic nerves in the ciliary muscle.If the miosis is not consciously induced and also does not correspond to a physiological stimulus response, then it can indicate various diseases. For example, the cause may be damage in the sympathetic supply, as in Horner’s syndrome. The so-called Argyll-Robertson syndrome is also a possible cause of pathological miosis. In the context of this disease, there is usually a reflex rigidity of the pupils on both sides, which is triggered by the neurolues. Miosis spastica, on the other hand, is present when there is irritation of the parasympathetic nervous system. As a rule, this special form of pathological miosis changes into a so-called mydriasis paralytica and may result in a complete paralysis of the oculomotor nerve. However, miosis can also be a symptom of meningitis. This potentially life-threatening infection of the pia mater and arachnoid mater primarily affects children and can be either bacterial or caused by fungi, viruses, and parasites. Lesions in the pons may just as well cause pathologic miosis. There are various causes for such lesions. Inflammations as well as hypoxia or strokes can be considered as primary diseases. Not only the presence of miosis, but also the inability to miosis in the presence of light has pathologic value and is indicative of parasympathetic paralysis of the sphincter pupillae muscle.
