What are the muscles responsible for pupil dilation?

From the Guidelines

The pupil dilator muscle, or dilator pupillae, plays a crucial role in controlling pupil dilation by contracting to pull the iris outward, causing the pupil to enlarge, and is innervated by the sympathetic nervous system. When assessing the function of this muscle, a comprehensive pupillary examination is essential, including evaluating pupil size, shape, symmetry, and response to light, as outlined in the pediatric eye evaluations preferred practice pattern 1. This examination can help identify potential issues with the optic nerve or other areas in the anterior visual pathway, which may be indicated by an afferent pupil defect. Key aspects of the examination include:

• Observing pupils in dim light to assess for differences in size, with a difference of more than 1 millimeter potentially being clinically significant
• Using the swinging-light test to assess for the presence of a relative afferent pupillary defect
• Shining a light directly into each eye to observe pupillary reactivity
• Monitoring for abnormal responses, such as pupillary dilation when the light is shined on the eye, which can indicate an afferent pupil defect. The balance between the dilator pupillae and the pupil constrictor muscle (sphincter pupillae) is vital for regulating pupil size in response to light conditions and emotional states, and certain medications or conditions, such as Horner's syndrome, can affect this balance 1.

From the FDA Drug Label

Atropine acts as a competitive antagonist of the parasympathetic (and sympathetic) acetylcholine muscarinic receptors. Topical atropine on the eye induces mydriasis by inhibiting contraction of the circular pupillary sphincter muscle normally stimulated by acetylcholine This inhibition allows the countering radial pupillary dilator muscle to contract which results in dilation of the pupil.

The pupil dilator muscles are the radial pupillary dilator muscles, which contract to cause dilation of the pupil when the circular pupillary sphincter muscle is inhibited by atropine 2 2.

From the Research

Pupil Dilator Muscles

• The pupil dilator muscle is responsible for dilating the pupil, and its function can be affected by various factors, including the use of mydriatic agents 3, 4, 5, 6, 7.
• Studies have compared the effectiveness of different mydriatic agents, such as tropicamide and phenylephrine, in dilating the pupil 3, 4, 5, 6, 7.
• The results of these studies suggest that the combination of tropicamide and phenylephrine is effective in dilating the pupil, with some studies showing that it can achieve a faster and more significant dilation compared to other agents 3, 4, 5.
• The use of mydriatic agents can also affect the retinal vessel diameter and oxygen saturation, but some studies have found that the addition of phenylephrine to tropicamide does not significantly influence these parameters 7.
• The effectiveness of mydriatic agents can vary depending on the individual and the specific condition being treated, and further research is needed to fully understand their effects on the pupil dilator muscle and other ocular structures 3, 4, 5, 6, 7.

Mydriatic Agents

• Tropicamide and phenylephrine are commonly used mydriatic agents that can be administered in various forms, including eye drops and injectable solutions 3, 4, 5, 6, 7.
• The combination of tropicamide and phenylephrine has been shown to be effective in dilating the pupil, with some studies suggesting that it can achieve a faster and more significant dilation compared to other agents 3, 4, 5.
• The use of mydriatic agents can be influenced by various factors, including the individual's age, the specific condition being treated, and the presence of any underlying ocular or systemic diseases 3, 4, 5, 6, 7.

Clinical Applications

• Mydriatic agents are commonly used in ophthalmic practice to dilate the pupil for various diagnostic and therapeutic procedures, including cataract surgery and retinal examinations 3, 4, 5, 6, 7.
• The choice of mydriatic agent and the specific dosing regimen can depend on the individual's specific needs and the desired outcome, and should be determined by a qualified healthcare professional 3, 4, 5, 6, 7.
• Further research is needed to fully understand the effects of mydriatic agents on the pupil dilator muscle and other ocular structures, and to develop more effective and safe treatments for various ocular conditions 3, 4, 5, 6, 7.