Mydriasis

Under one Mydriasis the enlargement or widening of the pupil is understood. The entire iris surface is reduced, the intraocular pressure is increased and the leakage of aqueous humor is reduced. In the Renaissance, dilation of the pupil was even modern and at that time it looked attractive, which is why people dripped various active ingredients into their eyes for cosmetic reasons, such as: B. the juice of deadly nightshade. Today, mydriasis is more used to examine the eye and control a person's state of consciousness; possibly also to determine his death. The opposite of mydriasis is miosis. This describes the condition of the eye when the pupil narrows.

What is mydriasis?

Mydriasis is the expansion or widening of the pupil.

Mydriasis occurs when parasympathetic inhibition of the sphincter muscle or sympathetic stimulation of the dilator muscle occurs. Both are internal eye muscles that interact with one another. The causes are different, e.g. B. it occurs through the intake of pupil-dilating drugs that u. a. are necessary when examining the retina, as this examination can only be carried out in the case of mydriasis.

More precisely, there are three phases in which mydriasis occurs. On the one hand by dark adaptation when the person z. B. comes from bright light into a dark room, on the other hand, with strong excitement, including excitement, fear, pain, shock, joy or through pathological irritation and due to paralysis or inhibition of the sphincter pupillae muscle. Other drugs or intoxicants, such as B. cocaine or amphetamines, in turn, stimulate the sympathetic nervous system, causing the pupillary dilator muscle to become irritated and the pupil to enlarge.

The pupil is always round when there is maximal mydriasis. Dilation of the pupils is completely normal in the dark. Likewise when looking into the distance. The eye reacts under these circumstances with an unconscious pupillary reflex in which the optic nerve and the third cranial nerve are involved. This process takes place biochemically in the sensory cells, more precisely through the rods and cones of the retina. Mydriasis occurs with dark adaptation in both eyes at the same time.

Function & task

The main purpose of mydriasis is the visual process. By changing the pupil, the eye can adjust to different light conditions and recognize the environment at different distances. In this respect, a camera lens is comparable to an eye. Here, too, objects near or far can be fixed sharply and by narrowing or widening the lens, more or less light can enter.

In ophthalmology, dilation of the pupil is a necessary examination tool. To examine the retina of the eye, mydriasis is triggered. For this, the patient is given eye drops that greatly dilate the pupil. They are potted into the conjunctival sac of the eye and cause paralysis, although this is only temporary and lasts for a few hours after the treatment.

This process is necessary because a lamp has to be shone into the eye during the examination and the pupil naturally narrows when exposed to light. If the pupil is wide, the entire retina is examined with a bright lamp and a magnifying glass. The medical name for this is ophthalmoscopy, also known as an ophthalmoscope.

Through this, the ophthalmologist can recognize whether there is mechanical damage or structural changes to the retina, whether metastases have formed somewhere in the eye and whether the head of the optic nerve is injured, B. in optic nerve diseases, brain tumors or glaucoma is the case. The doctor can also check the macula or detect serious inflammation inside the eye.

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Illnesses & ailments

The dilatation of the pupil can also be a symptom of illness. The pathological condition of the pupillary muscles as mydriasis occurs e.g. B. brain death, migraine attacks, severe pain, a cranial nerve disorder such as oculomotor nerve palsy or an injury to the midbrain. Here the sphincter pupillae muscle is inhibited and the fibers and nerves supplying it are blocked. There is an abnormal dilation of the pupil and disruption of the overall pupillary response.

In oculomotor paresis, the oculomotor nerve, i.e. the third cranial nerve, is paralyzed. Together with the abducens nerve and the trochlear nerve, it is responsible for moving the apple of the eye. The cause of this inhibition can be of different types. It can be caused by a stroke or by bleeding in the brain. Vascular disorders or a brain aneurysm also cause paralysis and mydriasis. With a complete oculomotor paresis, all nerve fibers are impaired and all reactions of the eye fail completely. The entire pupil reaction and the near and far position of the eye is disturbed.

In pathophysiology, four different forms of mydriasis are distinguished. Mydriasis spactica causes irritation of the sympathetic nervous system with immediate permanent contraction of the dilatator pupillae muscle. Mydriasis traumatica is a tear in the sphincter papilla. In the case of spinal mydriasis, the centrum ciliospinale is irritated, which influences the pupil width and also the width of the eyelid. In the case of mydriasis paralytica, the entire parasympathetic system is finally paralyzed and the sphincter pupillae muscle relaxes.

Furthermore, a mydriasis develops due to drug exposure. So z. B. by taking plant poison, intoxicants or other pharmacological agents. Parasympatholytics or anticholinergics inhibit the parasympathetic fibers and cause constriction. Active ingredients of this group are z. B. tropicamide, homatropin, scopolamine or atropine. The mydriatic effect is reinforced by sympathomimetics and acts on the sympathetic fibers. Active substances are epinephrine or phenyephrine.