Mydriasis occurs when parasympathetic inhibition of the sphincter muscle or sympathetic stimulation of the dilator muscle occurs. Both are inner eye muscles that interact with each other. The causes are different, z. For example, it occurs by taking pupil-dilating drugs, which are necessary, inter alia, in a study of the retina, since this investigation can be performed only with a mydriasis.
More specifically, there are three phases in which a mydriasis occurs. On the one hand by dark adaptation, if the human z. B. from bright light in a dark room, on the other hand in strong excitement, including excitement, anxiety, pain, fright, joy or by pathological irritation and due to a paralysis or inhibition of the sphincter pupillae muscle. Other drugs or intoxicants, such as. As cocaine or amphetamines, in turn cause a stimulation of the sympathetic, whereby the dilator pupillare muscle is irritated and the pupil enlarges.
The pupil is always round when it comes to a maximum mydriasis. The pupil dilation is perfectly normal in the dark. Likewise with a look into the distance. Under these circumstances, the eye reacts with an unconscious pupillary reflex involving the optic nerve and the third cerebral nerve. This process takes place biochemically in the sensory cells, more precisely by the rods and cones of the retina. The mydriasis occurs in a dark adaptation in both eyes at the same time.
First and foremost, the mydriasis serves the visual process. By changing the pupil, the eye can adjust to different light conditions and recognize the environment at different distances. In this point, a camera lens is similar to an eye. Even there, objects can be sharply fixed in the near or far and narrowing or narrowing the lens can allow more or less light.
In ophthalmology, pupil dilation is a necessary means of examination. To examine the retina of the eye, a mydriasis is triggered. For this, the patient gets eye drops that greatly dilate the pupil. They are pimped into the conjunctival sac of the eye and cause a paralysis, which, however, is only temporary and lasts several hours beyond the treatment.
This procedure is necessary because the examination with a lamp in the eye must be shone 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 referred to as ophthalmoscopy.
Through this, the ophthalmologist can detect whether there are mechanical damage or structural changes of the retina, anywhere metastases have formed in the eye and whether the head of the optic nerve is injured, which z. B. in optic nerve diseases, brain tumors or green star the case. The doctor can also check the macula or detect serious inflammation inside the eye.
The dilation of the pupil can also be a symptom of disease. The pathological state of the pupil musculature as mydriasis occurs, for. As in brain death, in migraine attacks, in severe pain, a cranial nerve disorder such as the Okulomotoriusparese or in a midbrain injury. In doing so, the sphincter pupillae muscle is inhibited and the fibers and nerves supplying it are blocked. There is an abnormal enlargement of the pupil and disturbances of the whole pupil reaction.
In oculomotor palsy, the oculomotor nerve, the third cerebral nerve, is paralyzed. Together with the nervus abducens and the trochlear nerve, it is responsible for the movement of the eye-apple. The cause of this inhibition can be different. It may be due to a stroke or brain bleeding. Vascular disorders or a brain aneurysm also cause paralysis and mydriasis. In complete oculomotor palsy, all nerve fibers are impaired and a total failure of all reactions of the eye occurs. The entire pupil reaction and the near and far position of the eye are disturbed.
In pathophysiology four different forms of mydriasis are distinguished. A mydriasis spactica causes sympathetic stimulation with immediate permanent contraction of the dilator pupillae. A mydriasis traumatica is a tear in the sphincter papilla. In a mydriasis spinalis, the center ciliospinale is irritated, which affects the pupil width and also the width of the eyelid. In a mydriasis paralytica, finally, the entire parasympathetic nervous system is paralyzed and there is a relaxation of the sphincter pupillae muscle.
Furthermore, a mydriase is produced due to medical effects. So z. B. by the ingestion of plant poison, intoxicants or other pharmacological agents. Parasympatholytics or anticholinergics inhibit the parasympathetic fibers and cause a bottleneck. Active substances of this group are z. Tropicamide, homatropin, scopolamine or atropine. The mydriatic effect is enhanced by sympathomimetics and acts on the sympathetic fibers. Active ingredients are epinephrine or phenyephrine.Tags: