Generality
The pupil is a circular opening positioned at the center of the iris, which allows light to penetrate the back of the eyeball.
Appearance and structure
The pupil is approximately 3 mm from the vertex of the cornea. On observation, this small hole in the center of the iris appears black, since most of the light, which manages to pass through the cornea and crystalline, is absorbed by the tissues present inside the eye. A certain amount of light, however, manages to be reflected and, in particular situations, makes the pupil appear "luminous".
Under normal conditions, the pupil diameter tends to vary between 2 and 5 mm. The pupil dilates to let in more light (miosis) and shrinks when it has to let in a smaller amount (mydriasis).
Constriction and pupillary dilation
The iris consists of a loose connective tissue, rich in pigmented cells, and is coated anteriorly with endothelial tissue. In the stroma, a ring of smooth muscle cells forms the constrictor muscle (or sphincter) of the pupil . The posterior face of the iris, on the other hand, presents smooth muscle cells arranged radially, which constitute the dilator muscle of the pupil .
- The cells that make up the constrictor muscle are arranged to form concentric rings around the pupil and when they contract, the pupil diameter decreases, causing constriction ( miosis ). The activity of the constrictor muscle is regulated by parasympathetic neurons.
- The dilator muscle is organized in radius and innervated by the orthosympathetic; when it contracts, it causes an increase in the pupillary diameter ( mydriasis ). Therefore, the contraction of the radial muscles generates dilation of the pupil.
Why does the pupil change size without our realizing it?
The pupil's response is an involuntary reflex: in the case of intense light, its constriction allows the photosensitive cells of the retina to not be damaged. On the contrary, under weak lighting conditions, its expansion serves to capture as much light as possible.
Mydriasis
The word "midriasi" derives from the Greek " amadros ", which means dark, and indicates the dilation of the pupil.
From the physiological point of view, the pupillary hole widens transiently due to the adaptation of the eye to darkness. This reaction can also follow intense emotion, such as anxiety, excitement or fear.
Pathological Mydriasis
Fixed mydriasis is non-light reactive dilation; it occurs a few seconds after a cardiac arrest and can persist for several hours, even after the recovery of the blood circulation. Acute glaucoma also causes pupil dilation and loss of the pupillary reflex in the affected eye; this pathological event represents an eye emergency and requires an urgent medical evaluation.
In stroke, the presence of mydriasis is indicative of compressive phenomena on the third cranial nerve (oculomotor). Pupillary dilation is also observed in the presence of ocular lesions, cranial traumas, infectious and toxic states.
Mydriasis can be induced by antihistamine, sympathomimetic, anticholinergic, barbiturate, estrogen and antidepressant drugs. Atropine and other mydriatic substances (such as tropicamide and cyclopentolate) are instilled in the eye to induce dilation of the pupillary diameter and allow the ophthalmologist to evaluate the fundus.
Miosi
The term "miosis" derives from the Greek " meiosis", or decrease, and indicates the decrease in the pupillary diameter.
The pupils shrinkage occurs physiologically during near vision or in response to a very intense light stimulus.
Pathological miosis
The miosis is observed in various pathological conditions: iridocyclitis, uveitis, corneal foreign body and lesions of the eyeball or orbit.
The pupil narrowing may also indicate the presence of cerebral hemorrhages, encephalitis and other pathological states of the nervous system.
The miosis can be induced by drugs, such as pilocarpine, timolol and reserpine. In some cases, point-like pupils may signal intoxication from narcotics (such as heroin, codeine and morphine) or from toxic chemicals.
anisocoria
The difference in amplitude of the two pupils is called anisocoria. Within certain limits, this asymmetry can be physiological. However, when observed in conjunction with gaze activity or light stimulus, anisocoria is always an expression of pathological states of the eye or nervous system.
A different pupillary diameter between the two eyes may be due to paralysis of the third cranial nerve (following aneurysm or brain tumor), syphilis and Horner syndrome. The anisocoria can also be found in the case of subarachnoid hemorrhage, meningitis, encephalitis, epilepsy, head trauma and intoxications. Furthermore, it may depend on congenital iris defects or dysfunctions in pupil constriction (as in Adie's syndrome). Sometimes, the anisocoria is induced by drugs, such as scopolamine and eye drops based on pilocarpine or tropicamide.
Watch out for the signs!
Consult a doctor immediately or go to the emergency room if the change in pupil size coincides with one of the following situations:- It occurred following an ocular or cranial trauma.
- It is associated with headache, nausea, vomiting, blurred vision and diplopia.
- It is associated with fever, photosensitivity, neck soreness, and a headache that is sharpened by leaning forward.
- It is accompanied by severe pain in the eye and / or sudden loss of vision.
