Anisocoria - Causes, Symptoms, Diagnosis, and Treatment

What Is a Pupil?

The pupil of an eye is the black centremost part, which helps in admitting the external source of light into the retina. Additionally, it also plays a vital role in regulating the amount of light entering the retina and perceiving the formed images. The normal size of the pupil in an adult varies depending upon the intensity of the light entering the eyes. The size ranges from 2 mm to 4 mm in the light and from 4 mm to 8 mm in the dark.

What Is Anisocoria?

Anisocoria is an abnormal condition defined by variation in the size of the pupils of the eyes. Anisocoria is a frequently occurring eye condition. It shows no sexual or age preferences.

What Causes Anisocoria?

Anisocoria occurs either physiological or is associated with other causative factors. Physiologic anisocoria is the most common form. However, the exact cause of physiological anisocoria is not known.

The various factors linked to the development of pathological anisocoria include:

• Horner's syndrome presents with a triad of drooping of the upper eyelids (ptosis), increased constriction of the pupil (miosis), and inadequate sweating (anhidrosis). It is associated with defect in pupillary dilator muscle resulting in anisocoria.

• Adie's pupil is a syndrome causing dilation of the pupil resulting from the damage of the ciliary or the intrinsic muscles and nerves of the eye.

• Pharmacological Anisocoria: Medications including blood dilators like Scopolamine patches and Glycopyrrolate deodorants cause excessive pupil dilation (mydriasis). Other medicines like opiates, Pilocarpine, and prostaglandins cause excessive pupil constriction (miosis). Anisocoria caused by medicines is also known as pharmacological anisocoria.

• There is associated paralysis of the third cranial nerve (oculomotor nerve). Compressive traumas or lesions affect the parasympahtetic fibers affecting the pupil, hence resulting in anisocoria.

• There is associated trauma or complication related to the pigmented part of the eye (iris).

• Developmental defects of the iris, like coloboma and ectopic pupil (dislocation of the pupil) can lead to anisocoria presentation in the children.

• Mechanical Anisocoria: It is caused due to the damage caused to the iris and its surrounding structures.

• Autoimmune gangliopathy, a disorder of antibody production against the autonomic ganglia, affects the sympathetic and parasympathetic innervation of the pupil, resulting in pupillary abnormalities, including anisocoria.

What Is the Mechanism Behind the Development of Anisocoria?

The defects in the nerve pathways of the eye contribute to the variations in pupil size. In addition, the miosis and mydriasis of the pupil also result from the stimulation of parasympathetic and sympathetic pathways.

Based on the behavior of pupils against light, anisocoria is of three types:

• There is inhibition of the pupil constriction on one side. As a result, the affected pupil remains dilated than the other eye. The difference is more pronounced in light conditions.

• There is inhibition of dilatation of the pupil on one side. Hence, the defective pupil remains more constricted than the normal eye. The difference is more pronounced in darker conditions.

• Variation in pupil sizes is not affected by dark or light conditions. More common in physiologic anisocoria.

What Are the Clinical Presentations of Anisocoria?

Anisocoria, as such, is asymptomatic. Various signs and symptoms exhibited by isolated anisocoric patients include:

• Headache.

• The blurring of the vision.

• Double vision.

• Weakness in eyesight.

• Numbness.

• Ptosis.

Patient with underlying ocular conditions presents with other symptoms apart from the anisocoria, such as:

• Patients with Horner's syndrome show a classic triad of ptosis (drooping of the upper eyelids), miosis (short-sightedness), and anhidrosis (lack of sweat due to dysfunctioning of sweat glands).

• Anisocoria due to Adie's pupils is more common in females between 20 years to 40 years. It has a unilateral presentation and is associated with diminishing deep tendon reflexes.

• Patients suffering from anisocoria from third nerve palsy also present with loss of eye accommodation and "down and out" gaze on the affected side.

• In patients with trigeminal nerve cephalgias, the anisocoria of the eyes is also accompanied by unilateral head pain, lacrimation, and rhinorrhea (runny nose).

How Can Anisocoria Be Diagnosed?

The diagnosis of anisocoria comprises a complete history, physical examination, lab tests, and imaging.

• Complete history regarding the time of onset, presence of any coexisting symptoms, entire drug history, history of trauma, family history, ophthalmic history including previous surgeries, and any other intraocular conditions is asked from the patient.

• Physical evaluation of the ocular structures is essential. Slit-lamp examination of the eye is performed to ascertain intraocular pathologies.

• Detailed evaluation of the pupil is conducted under light and dark conditions. Size of the pupil, accommodation capability and physical structure like size, shape, reactivity, and position are examined. In addition, neurological tests investigate defective neural, sensory, or motor reflexes.

• Lab tests, including serological tests, cultures, and immune assays, are performed in case of suspicion of neural infection.

• Imaging procedures like magnetic resonance imaging (MRI) and computed tomography (CT) determine the presence of ballooning in the arterial wall (aneurysm), tumor, or bleeding spots in the head and neck region.

What Are the Conditions That Share Similarities With Anisocoria?

The conditions that mimic anisocoria are of three types:

• Pupil's Reaction to the Light:

  • Cholinergic medicines like Pilocarpine cause miosis of the pupil.

  • Posterior synechiae lead to incomplete dilation of the pupil.

• Greater Anisocoria in Bright Light:

  • Migraine often is associated with dilation of the pupil accompanied by pain.

  • Cycloplegic eye drops of Atropine and Tropicamide lead to irreversible dilation of the pupil.

  • Sympathomimetic drugs such as Cocaine and Amphetamine also cause mydriasis of the pupil. However, pupil dilation due to sympathomimetics is not as severe as cycloplegics.

  • Intraocular trauma or surgeries damaging the papillary muscles result in potential dilation of the pupil.

• Anisocoria in Both Dim and Bright Light:

  • Physiologic anisocoria represents a stable difference in pupil size for a long time and without any history of other systemic conditions.

What Are the Complications Associated With Anisocoria?

Anisocoria itself does not cause any severe complications.

• Mild impairment in light sensitivity occurs due to the reduced pupil size.

• Difficulty in performing cataract surgery and cataract-induced diminishing vision is present.

How Can a Patient With Anisocoria Be Treated?

Management of anisocoria aims at treating the underlying ocular or systemic condition.

• Physiologic anisocoria mandates mere observation.

• Wearing eyewear and Pilocarpine drops are suggested in the case of Adie's pupil.

• Pharmacological drugs causing anisocoria are deferred from further use.

• Emergency management is a prerequisite for treating potentially severe conditions like third nerve palsy, aneurysms, and severe Horner's syndrome.

• Surgery of the condition is rarely required. However, it is essential to correct structural defects related to trauma.

Is It Possible to Prevent Anisocoria?

Anisocoria being asymptomatic is often neglected.

• Patients' education regarding immediate consultation with the ophthalmologist and neuro-ophthalmologist after initiation of symptoms is crucial.

• Symptoms like headaches, blurred vision, and blood in the sputum are candidates for prompt diagnosis and treatment.

• Multidisciplinary effort executed by the healthcare workers is crucial for early diagnosis of the basal problem and subsequent referral.

What Is the Prognosis of Anisocoria?

The prognosis of anisocoria is primarily affected by the severity and presence of other conditions associated with it.

• Isolated anisocoria cases hardly affect the visual acuity of the patients.

• Patients with ruptured aneurysms and carotid nerve dissection (Horner's syndrome) have a higher mortality rate owing to the condition.

Conclusion

Anisocoria is a frequent pupillary condition. Although completely isolated anisocoria is rarely vision-threatening, initial conditions associated with it need immediate attention and management. Therefore, early diagnosis and early referral are vital in managing patients with anisocoria.