Mydriasis occurs when there is parasympathetic inhibition of the sphincter muscle or sympathetic stimulation of the dilatator muscle. Both are inner eye muscles that interact. The causes are different, e.g. B. It occurs through the intake of pupil-dilating drugs, which are necessary, among other things, for an examination of the retina , since this examination can only be carried out in the case of a mydriasis.

More specifically, there are three phases in which mydriasis occurs. On the one hand by dark adaptation, when humans z. B. stepping out of bright light into a dark room, on the other hand in the case of strong excitement, including excitement , fear , pain , shock, joy or through pathological irritation and due to paralysis or inhibition of the sphincter pupillae muscle. Other medications or intoxicants such as B. cocaine or amphetamines , in turn cause a stimulation of the sympathetic nervous system , whereby the musculus dilatator pupillare is irritated and the pupil enlarges.

The pupil is always round when there is maximum mydriasis. Dilation of the pupils 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 precisely through the rods and cones of the retina. Mydriasis occurs in both eyes at the same time during dark adaptation.

function & task

Mydriasis primarily 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 sharply fixed and by narrowing or widening the lens can let in more or less light. 

In ophthalmology, dilation of the pupil is a necessary examination tool. In order to be able to examine the retina of the eye, a mydriasis is triggered. For this, the patient is given eye drops that greatly dilate the pupil. They are placed in the conjunctival sac of the eye and cause paralysis, but this is temporary and lasts a few hours beyond the treatment.

This process is necessary because the examination involves shining a lamp into the eye and the pupil naturally narrows when exposed to light. If the pupil is dilated, the entire retina is examined with a bright lamp and a magnifying glass. The medical term for this is ophthalmoscopy , also known as ophthalmoscopy.

This allows the ophthalmologist to see 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, which can be caused by e.g. 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.

Diseases & Ailments

However, the dilation of the pupil can also be a symptom of diseases. The pathological condition of the pupillary muscles as mydriasis occurs e.g. B. in 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 disturbances in the overall pupillary response. 

In the case of oculomotor nerve palsy, the oculomotor nerve, i.e. the third cranial nerve, is paralysed. 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 the result of a stroke or cerebral hemorrhage . Vascular disorders or a brain aneurysm also cause paralysis and mydriasis. In the case of a complete oculomotor nerve palsy, all nerve fibers are affected and there is a total failure of all reactions of the eye. The complete reaction of the pupil and the near and far position of the eye are disturbed.

In the pathophysiology, four different forms of mydriasis are distinguished. Mydriasis spactica leads to sympathetic irritation 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 ciliospinal center is irritated, which affects the width of the pupil and the width of the eyelid fissure. In the case of paralytic mydriasis, the entire parasympathetic nervous system is finally paralyzed and the sphincter pupillae muscle relaxes.

Furthermore, a mydriasis develops due to the effects of medication. such as B. by taking plant toxins, intoxicants or other pharmacological agents. Parasympatholytics or anticholinergics inhibit the parasympathetic fibers and cause constriction. Active substances of this group are e.g. B. tropicamide, homatropine, scopolamine or atropine. The mydriatic effect is enhanced by sympathomimetics and acts on the sympathetic fibers. Active ingredients are epinephrine or phenyephrine.