Mydriasis is the dilation or widening of the pupil. This reduces the total iris area, increases intraocular pressure, and decreases aqueous humor leakage. During the Renaissance, pupil dilation was even fashionable and looked attractive at the time, which is why people dripped various substances into their eyes for cosmetic reasons, such as the juice of belladonna. Today, mydriasis is used more to examine the eye and to check a person’s state of consciousness; if necessary, also to determine his or her death. The opposite of mydriasis is miosis. This refers to the condition of the eye when the pupil narrows.
What is mydriasis?
Mydriasis refers to the dilation or widening of the pupil. Mydriasis occurs when there is parasympathetic inhibition of the sphincter muscle or sympathetic stimulation of the dilator muscle. Both are internal eye muscles that act on each other. The causes are various, e.g. it occurs due to the intake of pupil dilating drugs, which are necessary, among other things, during an examination of the retina, as this examination can only be performed in the presence of mydriasis. More precisely, there are three phases in which mydriasis occurs. One is due to dark adaptation, e.g. when a person steps out of bright light into a dark room, the other is due to strong excitation, including excitement, fear, pain, fright, joy, or due to pathological irritation and because of paralysis or inhibition of the sphincter pupillae muscle. Other drugs or intoxicants, such as cocaine or amphetamines, in turn cause stimulation of the sympathetic nervous system, which irritates the musculus dilatator pupillare and causes the pupil to enlarge. The pupil is always round when maximal mydriasis occurs. Pupil dilation is perfectly normal in the dark. Likewise when looking into the distance. The eye responds in these circumstances with an unconscious pupillary reflex involving the optic nerve and the third cranial nerve. This process takes place biochemically in the sensory cells, more specifically by the rods and cones of the retina. Mydriasis occurs simultaneously in both eyes when dark adaptation is present.
Function and task
Primarily, mydriasis serves the visual process. By changing the pupil, the eye can adapt to different lighting conditions and recognize the environment at different distances. In this respect, a camera lens is comparable to an eye. There, too, objects near or far can be focused, and by narrowing or widening, the lens can let in more or less light. In ophthalmology, pupil dilation is a necessary means of examination. To be able to examine the retina of the eye, mydriasis is induced. For this purpose, the patient is given eye drops that greatly dilate the pupil. They are dripped into the conjunctival sac of the eye and cause paralysis, which, however, is only temporary and lasts for a few hours beyond the treatment. This procedure is necessary because the examination involves shining a lamp into the eye, and the pupil naturally constricts when exposed to light. Once the pupil is wide, a bright lamp and a magnifying glass are used to examine the entire retina. The medical term for this is ophthalmoscopy, also known as ophthalmoscopy. Through this, the ophthalmologist can see if there is mechanical damage or structural changes to the retina, if metastases have formed somewhere in the eye, and if the head of the optic nerve is injured, which is the case, for example, with optic nerve diseases, brain tumors, or glaucoma. This also allows the doctor to check the macula or detect serious inflammation inside the eye.
Diseases and complaints
However, pupil dilation can also be a symptom of disease. The pathologic condition of the pupillary muscles as mydriasis occurs, for example, in brain death, in migraine attacks, in severe pain, in a cranial nerve disorder such as oculomotor palsy, or in midbrain injury. In this case, the sphincter pupillae muscle is inhibited and the fibers and nerves supplying it are blocked. There is abnormal dilation of the pupil and disturbances in the overall pupillary response. In oculomotor palsy, the oculomotor nerve, the third cranial nerve, is paralyzed.Together with the abducens nerve and the trochlear nerve, it is responsible for the movement of the eyeball. The cause of this inhibition can be of different kinds. It can be caused by a stroke or by cerebral hemorrhage. Vascular disorders or brain aneurysm also cause paralysis and mydriasis. In complete oculomotor palsy, all nerve fibers are affected and there is a total failure of all reactions of the eye. In this case, the complete pupillary reaction and the near and far position of the eye are disturbed. In pathophysiology, four different forms of mydriasis are distinguished. In mydriasis spactica there is sympathetic stimulation with immediate permanent contraction of the dilator pupillae muscle. Mydriasis traumatica is a tear in the sphincter papilla. In spinal mydriasis, the centrum ciliospinale is irritated, which affects pupil width and also the width of the palpebral fissure. Finally, in mydriasis paralytica, the entire parasympathetic nervous system is paralyzed and the sphincter pupillae muscle becomes flaccid. Furthermore, mydriasis occurs due to drug exposure. For example, due to ingestion of herbal poison, intoxicants or other pharmacological agents. Parasympatholytics or anticholinergics inhibit the parasympathetic fibers and cause constriction. Agents in this group include tropicamide, homatropine, scopolamine, or atropine. The mydriatic effect is enhanced by sympathomimetics and acts on the sympathetic fibers. Active ingredients are epinephrine or phenyephrine.
