From the Guidelines
Miosis, or small pupils, can be caused by various factors including Horner syndrome, Adie tonic pupil, or a pupil-involving third-cranial-nerve palsy, as well as irregular pupils indicating traumatic sphincter damage, iritis, or congenital abnormalities 1. When evaluating small pupils, it's essential to assess their size, symmetry, and shape, as well as their direct and consensual responses to light. According to the pediatric eye evaluations preferred practice pattern study published in 2023 in the journal Ophthalmology 1, anisocoria greater than 1 millimeter may indicate a pathological process. Some key points to consider when evaluating miosis include:
- Pupillary evaluation in infants and children may be difficult due to hippus, poorly maintained fixation, and/or rapid changes in accommodative status 1
- A relative afferent pupillary defect (RAPD) of 0.3 or more log units is not typically seen in amblyopia, but a subtle RAPD may be seen with dense amblyopia 1
- The presence of a large RAPD should warrant a search for compressive optic neuropathy or other etiologies of visual impairment 1 It's crucial to determine if the small pupils are equal in size and reactive to light to distinguish between normal physiological responses and pathological conditions requiring medical attention.
From the FDA Drug Label
Morphine causes miosis, even in total darkness. Pinpoint pupils are a sign of opioid overdose but are not pathognomonic (e. g., pontine lesions of hemorrhagic or ischemic origins may produce similar findings). The causes of miosis (small pupils) include:
- Opioid use, such as morphine
- Pontine lesions of hemorrhagic or ischemic origins 2
From the Research
Causes of Miosis
The causes of miosis, or small pupils, can be attributed to various factors, including:
- Horner's syndrome, which arises from dysfunction of the oculosympathetic pathway 3
- Adie's syndrome, a rare neurological disorder that can cause tonic pupils, which are characterized by a poor or absent reaction to light 4
- Opioid use, as seen in the study where chronic naloxone administration enhanced morphine-induced miosis 5
- Disorders of the parasympathetic system, such as dorsal midbrain syndrome, third-nerve palsy, and tonic pupil 6
- The use of certain medications, such as pilocarpine, which can induce miosis 7
Neurological Disorders
Neurological disorders, such as Horner's syndrome and Adie's syndrome, can cause miosis due to dysfunction of the nerves that control pupil size.
- Horner's syndrome is characterized by miosis, enophthalmos, protrusion of the third eyelid, and ptosis 3
- Adie's syndrome can cause tonic pupils, which are supersensitive to diluted pilocarpine 4
Pharmacological Causes
Certain medications, such as opioids and pilocarpine, can also cause miosis.