Dilated Pupils (Enlarged Black of Eye) Causes and Other Symptoms

The pupils are the dark centers of the eye where light can enter the inner eyeball. The size of the pupil’s change with varying light conditions to allow more or less light into the eyeball. This is controlled by the iris. When the pupil’s are enlarged it is dilated and when it is small it is constricted. While these changes are not abnormal with different light conditions, sometimes the pupils are abnormally enlarged.

What are dilated pupils?

Dilated pupils are where the black center part of the eye is abnormally enlarged. This is known as mydriasis. While it is not abnormal for the pupils to be enlarged in dark conditions, it should usually respond to light and constrict accordingly. When it fails to respond to light and are enlarged, then the pupils are said to be dilated. Usually both pupils constrict or dilate at the same time but in some cases, only one pupil changes in size. This is seen in anisocoria.

There are a number of different causes of dilated pupils. It may be a symptom of certain diseases and disorders, as well as occur with the use of certain medication, illicit substances and toxins.. Pupils that are dilated and unresponsive to light are one of the signs of death, as the part of the brain that regulates the size of the pupil shuts down. However, this also depends on other signs such as an absent pulse, inactivity of the heart and cessation of breathing. Therefore dilated pupils on its own may not be a sign of death.

dilated pupils

Causes of Dilated Pupils

The brain can control the contraction and relaxation of the muscles of the iris which in turn changes the pupil size. Dilatation of the pupils are controlled by the sympathetic nervous system which is part of the autonomic system. This means that actions like dilatation of the pupils is involuntary.

Physiologic dilatation occurs with reduced light intensity, strong emotions and when the ‘fight or flight’ response is initiated. This is not considered to be abnormal. Pathologic causes is where pupil dilation is due to some diseases. Iatrogenic causes is where the pupils become dilated due to the action of certain medication.

Parts of the Human Eye

Parts of the Human Eye


Sometimes drugs are administered for the purposes of dilating the pupils in order to examine the inner eye. These drugs are known as mydriatic agents and are usually administered in the form of eye drops. It contains substances like atropine, scopolamine and phenylephrine.

Pupil dilatation may also occur as a side effect with the use of other drugs, like amphetamines, certain anthistamines, eye drops containing tetrahydrozoline used to relieve eye redness and with some decongestants like those containing pseudoephedrine.

Illicit Substances

A number of illicit substances can cause dilated pupils. This includes marijuana, cocaine, LSD and methamphetamines. With some of these substances, dilated pupils may only be seen with higher doses. It may also occur with withdrawal from certain illicit substances as is seen with narcotics such as heroin. Excessive alcohol intake can also cause changes in pupillary response. Dilated pupils especially with other symptoms similar to the use of illicit substances may be seen with alcohol poisoning and can be fatal without immediate medical attention.

Poisons and Toxins

Several poisons and toxins may cause pupil dilated. Some of these substances are naturally occurring like in plant material while others are synthetic. Toxic mushrooms, benzene and chloroform may cause dilated pupils. Deadly nightshade (Atropa belladonna) is a plant that contains atropine, scopolamine and hyoscyamine, which are known mydriatic agents. At one time drops made from this plant was used to dilate the pupil for cosmetic purposes.

Diseases and Disorders

  • Brain aneurysm: An abnormally enlarged and weakened portion of a blood vessel in the brain that can burst in time.
  • Brain hemorrhage: Bleeding within the skull cavity that houses the brain (intracranial hemorrhage) that may arise with an injury, following surgery to the area or a burst aneurysm.
  • Brain tumor: An abnormal growth in the brain tissue. It can be benign (non-cancerous) or malignant (cancerous).
  • Cerebral edema: Swelling within the brain usually associated with inflammation. It can occur with trauma, infections, exposure to toxins and from bleeding within the brain.
  • Glaucoma: A rise in the pressure within the eyeball leading to damage of the optic nerve. It is more common in the elderly, particularly in the backdrop of diabetes mellitus or trauma to the eye.
  • Oculumotor nerve palsy: Damage or disease of the oculomotor nerve (third cranial nerve) which controls the iris and therefore the size of the pupil.
  • Raised intracranial pressure: Increase in the fluid pressure within the brain usually due to a rise in the amount of cerebrospinal fluid (CSF) within the intracranial cavity or bleeding. Swelling of the brain may also cause it.
  • Stroke: Death of a portion of the brain tissue due to an interruption in the blood supply. Commonly occurs when a blood clot blocks an artery supplying blood to the brain or with a break in these arteries.
  • Traumatic head injury: Blunt or sharp force trauma to the head that may occur with a fall, assault, contact sports or motor vehicle accidents.

Other Signs and Symptoms

Dilated pupils is a symptom and may cause in a number of different conditions. There may be a range of associated symptoms depending on the underlying cause. Some of these symptoms are due to the inability to regulate the light that enters the eye. These other eye-related symptoms include:

  • Visual disturbances
  • Light sensitivity
  • Eye pain
  • Eye redness
  • Excessive tearing

Apart from eye-related symptoms there may also be other symptoms present that are not related to the eye but caused by the same condition that is responsible for the dilated pupils.

  • Slurred speech
  • Loss of consciousness
  • Confusion
  • Poor coordination
  • Loss of sensation
  • Muscle weakness or paralysis
  • Mouth dryness
  • Vomiting
  • Profuse sweating
  • Headaches
  • Seizures

The presence of these other symptoms varies from one causative condition to the other and also among individuals. It is important to consult with a medical professional as further investigations may be required for a definitive diagnosis.

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