Mild (<0.5 mm) anisocoria in young children is usually normal, particularly if it is variable. Anisocoria associated with other disorders, particularly Horner syndrome and third nerve palsy, is not an isolated finding. If a patient has ptosis along with anisocoria, referral to a pediatric ophthalmologist is indicated for evaluation of possible Horner syndrome or third nerve palsy. If the pupil does not react at all, referral is also indicated.

What Shouldn’t Be Missed

Horner syndrome, particularly in older children, may result from serious diseases such as neuroblastoma. These patients require evaluation to look for these problems. Children with iritis secondary to juvenile idiopathic arthritis (JIA) may have no symptoms of ocular discomfort despite severe inflammation. In some of these patients, abnormal pupils due to scarring of the iris may be the first abnormality noted.

• 1. Physiological anisocoria. Mildly asymmetric pupils may occur in otherwise normal infants. This may be familial. The anisocoria is more noticeable in dim light. The hallmark of physiological anisocoria is variability, with the pupils sometimes appearing equal. Physiological anisocoria does not cause any problems with development of vision.
  
• 2. Horner syndrome. Horner syndrome occurs due to interruption of the oculosympathetic chain that begins in the hypothalamus, travels through the spinal cord to the thorax, and ascends along the internal carotid artery to the orbit. Lesions anywhere along this pathway may cause Horner syndrome. The syndrome is characterized by anisocoria (pupil smaller on the affected side), mild ptosis, and anhidrosis (decreased sweating on the affected side of the face) (Figure 18–1). It typically does not cause vision problems. It is important because of its association with other systemic conditions.
  
• 3. Iritis. Most patients with iritis (intraocular inflammation) have marked eye discomfort and seek medical attention because of this complaint. For unknown reasons, children with iritis associated with JIA usually do not experience significant eye pain. Because of this, even severe inflammation may go unnoticed until substantial eye damage is present. Children with JIA sometimes present with nonreactive pupils due to scarring of the iris to the lens capsule (Figure 18–2).
  
• 4. Pharmacological. Several eye drops or exposure to certain plants may affect the pupil. This diagnosis can usually be established with a careful history, examination, and confirmatory drop testing (Table 18–1).
  
• 5. Trauma. Direct ocular injuries may result in damage to the muscles in the iris that control pupil size (Figure 18–3). In these patients, there is usually a recognized history of trauma.
  
• 6. Third cranial nerve palsy. Patients with third cranial nerve palsies usually have an enlarged pupil on the affected side, along with marked strabismus and ptosis. Congenital third nerve palsies sometimes present with a smaller pupil on the affected side. Partial third cranial nerve palsies present with variable ptosis and eye movement abnormalities, in which case the anisocoria is helpful in establishing a diagnosis (Figure 18–4).