Introduction
The size of the pupils is precisely regulated by the body in everyday life. In darkness the pupils dilate to capture as much light as possible, while in healing the pupils narrow. Pupils of different sizes are innate and harmless in 10-20% of the population. Also in the rest of the population the pupil width between the eyes can vary by 0.2-0.4mm. In some cases, however, pupils of different sizes can also be signs of disease.
Causes
The size of the pupil is controlled by muscles. The opening in the iris can be narrowed by one muscle and widened by another. These muscles are controlled by nerves of the brain (cranial nerves).
Disorders in the area of these nerves lead to functional failures of pupil mobility. As a result, disorders of the nerve fibers that enlarge the pupil are associated with a pupil that is too small. Conversely, disorders of the nerves that constrict the pupil lead to a too large pupil.
You can find more interesting information on this topic at Which drugs or medications influence the pupil? The third cranial nerve (Nervus oculomotrius) is responsible for narrowing the pupil. This nerve also has many other functions.
On the one hand, it is able to move a large part of the eye muscles. On the other hand, it is responsible for opening the eyelid. In summary, damage to the third cranial nerve can lead to a drooping upper lid, double vision and an enlarged pupil.
The cause of an enlarged pupil can also be a vascular sacculation (aneurysm) in the area of the base of the brain. This is a medical emergency and should be investigated immediately using imaging techniques such as CT, MRI or angiography. In addition, certain drugs or chemicals can make the pupil dilate so that the pupil does not narrow even in bright light.
Pupil dilating drugs, for example, have a dilating effect: Patches against seasickness, certain garden chemicals and also special eye drops (e.g. atropine). Fortunately, the effect of such chemicals or medicines diminishes after a certain time, so that the original pupil width is restored after hours or days. One of the side effects of antibacterial eye drops containing gentamicin is the pupil dilating effect.
The so-called sympathetic nervous system (=”sympathetic nervous system“) dilates the pupil, for example in darkness or in situations of excitement. This nervous system is also responsible for the position of the upper eyelid. When the “sympathetic nervous system” fails, not only is the pupil on the affected side narrowed, but the upper eyelid also hangs slightly lower.
These failure symptoms are summarized as Horner syndrome. Although Horner’s syndrome is harmless to the eye itself, it can nevertheless be a significant indication of a disruption of the sympathetic nerve fibers. These nerve fibers have a long path along many important structures and any damage along this long path can trigger Horner’s syndrome.
Therefore it is an important task of the physician to find out where the damage is located. Possible causes of a Horner’s syndrome are, for example, a stroke, a tumor in the lung or an injury in the area of the carotid artery. Sometimes Horner’s syndrome is the result of a birth trauma.
It is also possible that the cause of this syndrome cannot be identified and the patient still has no particular health problems. The Horner syndrome is composed of
- Narrowed pupil (miosis)
- Drooping upper eyelid (ptosis)
- Withdrawal of the eyeball into the eye socket (enophthalmos)
Likewise, head injuries caused by external forces such as a fall or blow can be the cause of pupils of different sizes. Brain haemorrhages can occur inside the skull, which do not show any external symptoms.
The inflow of a large amount of blood into the brain tissue can compress various structures. If the (cranial) nerves responsible for dilating and narrowing the pupil are compressed on one side while the bleeding on the opposite side does not cause compression, pupils of different sizes may occur.Life-threatening symptoms of a severe cerebral hemorrhage include clouding of consciousness, unconsciousness and seizures. Pupils of different sizes in small children are very often not pathological.
As in adults, the difference between the pupils can vary, so that sometimes one pupil is larger than the other. More rarely, Horner syndrome can be the reason for the inequality of the pupils. This clinical picture usually occurs in children as a result of a birth trauma.
Other diseases that are also causative in adults, such as cerebral nerve paralysis, can also be the reason. However, these diseases are rare in infants. During the examination, the parents are first interviewed in the sense of an interview.
Possible questions can be When did the inequality of the pupils become apparent? Is it always present? Do you have photos with you?
Did complications occur during the birth? Are there any other abnormalities? Afterwards, the pupil width is assessed in a bright and a darkened room.
This examination should take longer than for adults. Often a clearly fluctuating inequality of the pupil width then occurs. Afterwards, the pupil reaction is examined and an attempt is made to trigger a close reaction of the pupils. Since a certain degree of cooperation from the child is required, the diagnosis can prove difficult. As in adults, the course of therapy and the prognosis depend on the cause of the disease, provided that the abnormalities of the pupils are due to disease.
