What is Anisocoria?

Anisocoria is a condition characterized by unequal sized pupils. The normal size difference is less than 0.4mm or more. The iris dilator and iris constrictor muscles control the pupil size of the eye regulated by the nervous system. Hence, any damage to the nerves and iris muscles or associated part results in Anisocoria. Although harmless in most cases, it may become a very serious and complicated disorder if untreated.

Infants with ‘Anisocoria’

Although this disease can happen to anyone of any age-group, it’s more common in newborns. It’s not necessary that a child born with different sized pupils may have Anisocoria. However, it can be a case genetic disorder too.

Intermittent Anisocoria

Normal Anisocoria fluctuates from week to week and is more commonly occurring in approximately 20% of the population. But Intermittent Anisocoria is episodic and happens at irregular intervals. The repeated and intermittent spasms result in a condition known as tadpole’s pupils.


There are multiple causes that define the severe and normal conditions of this disorder. There are certain situations to look for such as the severity of the condition where the cause may be an intracranial aneurysm requiring several clinical tests. If iris is absent, then the cause of Anisocoria could be a defect in the efferent nervous pathways which regulates the pupil of the eye. Other causes are described in detail here:

Third Cranial Nerve Damage

The outermost nerve of the nervous system is called third cranial nerve. When the size of the pupil appears abnormally large, then it pinpoints damage to the outer motor nerve. Third cranial nerve controls the movement of the eyes. Hence, when any damage occurs to this nerve, then patient faces genuine difficulties such as double vision, expanded pupil, and eyelid drooping

Parasympathetic Innervation Failure

When parasympathetic innervations get interrupted, it causes dilation of the pupil. Moreover, this process results in mild reactions to direct light. An orbital structure called ciliary ganglion receives parasympathetic substances from the third cranial nerve. The viral infection in ciliary ganglion is the cause of isolated anisocoria.

Horner’s Syndrome

The syndrome causes abnormally smaller pupil which respond rapidly to the light. The ptosis is mild in this case but is one of the major causes of Anisocoria.

Adie’s Pupil

The disease is also known as a tonic pupil and is more common in women than in men. Adie’s pupil is another type of anisocoria that happens due to cranial nerve damage.

Chemical Blockage

Although chemical blockage is rare, any contact between parasympathetic chemicals and the conjunctiva can lead to the deactivation of the iris sphincter muscle and may result in the dilation of the pupil. Atropine containing plants are stimuli to this condition and exposure to these plants can be harmful. That is one of the reasons why hospital personnel suffer more from this condition.

Iris Sphincter Damage

The inflammation and trauma in the sphincter muscle also cause Anisocoria. In this case, the pupil becomes irregular in shape. The damaged muscle is visible under high magnification.

Other Possible Causes

  • Consistent consumption of drugs
  • Inflammation in optic nerves
  • Trauma to the eyes
  • Bleeding in the head
  • Brain tumor
  • Horner’s syndrome
  • Argyll Robertson pupil
  • Tonic pupil/Adie pupil
  • Iritis
  • Meningitis
  • Brain injuries (concussion)
  • Intracerebral hemorrhage (ICH)
  • Subdural hematoma (a condition when blood collects on the surface below skull)
  • Intracranial Hemorrhage
  • Optic Neuritis
  • Epidural hematoma
  • Ptosis
  • Seizures

Relative afferent pupillary defect (RAPD) that is also known as Marcus Gunn Pupil is sometimes misunderstood as a cause of anisocoria, but it is not.

Anisocoria and Horner’s Syndrome

Horner’s syndrome is one of the causes and involves an abnormally small pupil. This syndrome is mostly caused due to ‘carotid artery dissection.’ However, carotid artery dissection is a condition of separation of artery walls that supplies blood and oxygen to the brain.


  • Fever
  • Stiff neck
  • Difficulties in moving eyes
  • Reduced sweating
  • Headache
  • Sometimes the eye pain
  • Drooping eyelids

Risk Factors

  • Genetics or the breed: Being prone to Anisocoria also depends on genetic structure and inheritance.
  • Age: Although Anisocoria can occur at any age, most cases depict the certain age of occurrence.
  • Medical disorders: It can occur due to systemic illness and intrinsic eye problems.
  • Environmental and geographical factors: Occurrence of anisocoria also depends on the geographical and environmental condition in that area.


  • Physical evaluation: First of all, a doctor examines performs physical examination of the eye. Pupil’s shape, size, position, and symmetry are compared to both eyes in bright light, dim light, and dark light. Slit lamp examination provides the detailed physical information about the eyes.
  • Clinical Examinations and tests: Either one-two drops of cocaine or apraclonidine (strong alpha-1 and weak alpha-2) are used to analyze both the eyes. The use of hydroxyamphetamine (Paredrin) facilitates the localization of Horner’s syndrome. Doses of pilocarpine or methacholine are also used in diagnosis.
  • Differential diagnosis: Third nerve palsy (TNP) also called Oculomotor nerve palsy is a condition that arises due to the damage of third cranial nerve. It could be life-threatening if caused by ‘an aneurysm.’ TNP is an instance of the abnormal pupil, whereas another disease that falls under differential diagnosis is ‘Horner’s syndrome.’ Radiology based diagnosis like Computerized tomography (CT), MRI and MRA are used to differentiate other disorders from anisocoria. The pupil ’s dilating agents such as scopolamine patches, nasal vasoconstrictors, glycopyrrolate deodorants, jimson weed and many more normally used in diagnosis procedure.
  • Lab tests: VDRL and FTA-ABS tests can be useful in the detection of syphilis, neurosyphilis and Argyll Robertson pupil. Total count of cells and protein is also the part of this test.

Differential diagnosis of Anisocoria

  • Injury on the iris
  • Adie’s Syndrome
  • Encephalitis
  • Herpes Zoster
  • Botulism
  • Alcohol intoxication
  • Cerebral Aneurysm
  • Brain tumor
  • Diabetes mellitus
  • Iridocyclitis
  • Intracranial Hemorrhage
  • Lead poisoning
  • Neoplastic
  • Retinal disease
  • Ischemia
  • Multiple sclerosis
  • Tuberculosis
  • Syphilis
  • Tuberculosis
  • Trauma
  • Aniridia
  • Keratitis
  • Syringomyelia
  • Tapes Dorsalis
  • Ocular prosthesis
  • Degenerative Neurologic Disorder

Differential diagnosis involves some tests to differentiate anisocoria from the diseases mentioned above. Once a person gets diagnosed with the type of anisocoria, he/she has to undergo many tests. These laboratory tests consist of diagnostic imaging, electrodiagnostic and pharmacological testing. All the tests are essential as to point out the cause of the disease. A thorough ocular examination is mandatory to detect other intrinsic diseases of the eye even after confirmation of Anisocoria.


It is not a matter of serious concern because this condition is mild/benign. But it may cause difficulties in vision and other infections in the eyes. Certain simple treatments are listed here:

  • Maintain a healthy lifestyle: Regular exercise, proper diet, and routine checkups keep the immune system strong and hence controls this condition.
  • Withdrawl of specific drugs: If one is applying topical ointments recklessly then it can cause the mydriasis or miosis. Withdrawal of such drugs can cure the infection readily.
  • Specific Therapies: Some specific ophthalmic therapies are effective and can lessen the impact of this condition.
  • Unavailable or no required treatment: There are some conditions related to Anisocoria that don’t require any treatment such as isolated mydriasis. On the other hand, no treatment is available for iris sphincter and other congenital malformations.



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