Tadpole Pupil

Key Points at a Glance

Key Points of This Disease

• A rare paroxysmal disease in which the pupil deforms into a tadpole shape due to segmental spasm of the iris dilator muscle.
• It predominantly affects healthy young women, with only about 45 cases reported in the literature.
• 79% of attacks resolve spontaneously within 5 minutes, and 91% occur spontaneously.
• 93% are unilateral, and may be accompanied by ipsilateral ocular or facial abnormal sensations (72%).
• Horner syndrome coexists in about 42% of cases, so a thorough examination is necessary at onset.
• No specific treatment is required; it is a benign condition with a good prognosis.

1. What is tadpole pupil?

Tadpole pupil (tadpole-shaped pupil) is a rare paroxysmal condition in which part of the iris is stretched into a narrow, pointed shape, causing the entire pupil to deform like a tadpole. It is essentially the same condition as episodic unilateral mydriasis and results from peripheral transient spasm of the pupillary dilator muscle due to sympathetic stimulation.

The disease name originates from the first case series reported by Thompson et al. in 1983 ¹. Only about 45 cases have been reported in the literature, making it an extremely rare condition ². It is more common in healthy young women.

Q How rare is tadpole pupil?

A

Only about 45 cases have been reported in the literature, making it an extremely rare condition. It is more common in healthy young women, and no randomized controlled trials have been conducted.

2. Main symptoms and clinical findings

Subjective symptoms

• Photophobia and blurred vision: Caused by transient unilateral mydriasis and accommodative disturbance.
• Strange ocular or facial sensations: Accompanied by ipsilateral ocular or facial abnormal sensations during attacks (72% of cases).
• Headache: Headache often occurs on the same side as the eye symptoms.

Clinical Findings (Findings Confirmed by Doctor’s Examination)

The following characteristic findings are observed during an attack.

• Tadpole-shaped pupil: The pupil becomes oval-shaped, with the end near the corneal margin becoming narrow. The narrowed part (“tail”) can appear in any direction on the clock face.
• Segmental poor light reflex: In the dilated iris segment, constriction in response to light is poor. The remaining part of the pupil constricts normally, reflecting abnormal activity of the iris dilator muscle (not the sphincter).
• Vermiform movements of the iris: Vermiform movements may be observed in the iris during an attack, and it is important to confirm them.

Characteristics of Attacks

• Duration: 79% resolve within 5 minutes. Often lasts minutes to hours, but may persist for weeks ¹².
• Frequency and course: Repeats several times a day to several times a year, lasting about 1 year. There are also clusters of 10–50 times a day.
• Spontaneous occurrence: 91% occur spontaneously without triggers ².
• Laterality: Unilateral in 93%. Alternating sides in 23%. Rarely bilateral attacks.
• Outcome: Most cases resolve spontaneously, but occasional recurrence may occur.

Q How long does an attack last?

A

79% of attacks resolve spontaneously within 5 minutes. However, many last minutes to hours, and rarely up to weeks. Clusters of 10–50 attacks per day may occur.

3. Causes and Risk Factors

The pathophysiology of tadpole pupil is thought to be a transient, peripheral spasm of the pupillary dilator muscle due to sympathetic stimulation. Segmental spasm of the dilator muscle causes part of the pupil to dilate, resulting in an irregular, teardrop-shaped deformity.

Hypotheses regarding the pathogenesis include the following two.

• Denervation hypersensitivity: Due to the high rate of association with Horner syndrome (42% in Thompson 1983, 46% in the Udry 2019 review), denervation hypersensitivity of the ocular sympathetic nerves is thought to be involved ¹². In Horner syndrome, α1 receptors increase due to denervation hypersensitivity.
• Elevated catecholamines: Cases where exercise or waking from sleep trigger attacks suggest the involvement of elevated circulating catecholamines.

Triggers and associated conditions have been reported as follows, but no known risk factors have been identified.

• Triggers: Migraine, menstruation, waking from sleep, exercise, strabismus surgery, seizures associated with hyponatremia
• Associated conditions: Horner syndrome (most frequent, associated in about 42% of cases), Adie pupil

Rarely, there are severe cases associated with aneurysms or cases that progress to Horner syndrome.

What to do when an attack occurs

Even if an attack occurs, the pupil deformity usually disappears spontaneously within 5 minutes. Stay calm and observe. Taking photos or videos with a smartphone during an attack can be very helpful for the doctor in making a diagnosis.

Q Are there factors that trigger tadpole pupil attacks?

A

Migraine, menstruation, waking from sleep, and exercise have been reported as triggers, but no specific risk factors have been established. 91% of attacks occur spontaneously without a trigger.

4. Diagnosis and examination methods

Diagnosis is clinical, based on the medical history and the characteristic clinical finding of paroxysmal contraction of the iris dilator muscle. Since it is rare for a doctor to directly witness an attack, recording the attack with a smartphone or other device is useful for diagnosis.

Differential diagnoses are as follows.

• Benign episodic unilateral mydriasis ³⁴⁵
• Horner syndrome
• Adie pupil (tonic pupil)
• Argyll Robertson pupil
• Unilateral exposure to mydriatic agents
• Ocular migraine
• Iris coloboma
• Iris sphincter injury (post-traumatic or post-surgical)
• Anterior uveitis
• Cyclic oculomotor palsy

Pharmacological testing

Two types of eye drop tests are used for differentiation.

Test	Purpose	Interpretation
Low-dose pilocarpine (0.125%) eye drop test	Exclusion of Adie pupil	Excessive miosis present → Adie pupil
1% apraclonidine (Iopidine) eye drop test	Detection of Horner syndrome	Mydriasis and reversal of anisocoria after 30–60 minutes → Horner syndrome

Principle of the apraclonidine test: In Horner syndrome, denervation supersensitivity leads to an increase in α1 receptors, so apraclonidine, which normally causes miosis as an α2 receptor agonist, produces mydriasis via α1 receptors, reversing the anisocoria. Note that the apraclonidine test is not covered by insurance.

Other tests

• Systemic evaluation: Perform brain and chest imaging as needed.
• Complete neuro-ophthalmic examination: Assess iris color (heterochromia → congenital Horner, iris atrophy → trauma, transillumination defects), eyelid symmetry, and eye movements.

5. Standard treatment

No specific treatment is required for tadpole pupil; symptomatic management is sufficient. Medical or surgical intervention is not necessary. There are no known complications, and it is important to reassure patients that if Horner syndrome is ruled out, there is no long-term adverse effect on vision.

If headache is present, treat the headache.

Exclusion of Horner syndrome is important

The episodes do not adversely affect vision, but approximately 42% of cases are associated with Horner syndrome. If Horner syndrome is suspected, a thorough workup is necessary because it may indicate serious causes such as lung cancer, mediastinal tumor, or carotid artery dissection.

Q Is treatment necessary?

A

No special treatment is required; only symptomatic management is sufficient. Most cases are benign and resolve spontaneously. However, since Horner syndrome coexists in approximately 42% of cases, it is important to rule out its presence using the apraclonidine test mentioned in the Diagnosis and Examination Methods section.

6. Pathophysiology and Detailed Mechanism

The exact pathophysiological mechanism of tadpole pupil remains unknown, but current understanding is as follows.

Anatomical background of the oculosympathetic pathway: The sympathetic nervous system originates in the hypothalamus and consists of a three-neuron chain, ultimately innervating the iris dilator muscle and the superior tarsal muscle via the long ciliary nerves. In tadpole pupil, transient abnormal firing (spasm) of peripheral sympathetic nerves in this pathway is thought to occur.

Mechanism of segmental spasm: Because only a segment of the iris dilator muscle selectively spasms, part of the pupil dilates, causing an irregular, teardrop-shaped deformation. The dilated segment shows poor constriction to light, while the rest of the pupil constricts normally. This indicates that the lesion is in the iris dilator muscle, not the sphincter.

Relationship with Horner syndrome: Since Horner syndrome coexists in approximately 42–46% of cases ¹², a hypothesis has been proposed that denervation hypersensitivity of the oculosympathetic pathway is involved. Increased α1 receptors after denervation may predispose to strong spasms even with mild sympathetic stimulation.

Catecholamine hypothesis: Attacks are triggered by situations that elevate circulating catecholamines, such as exercise or awakening from sleep, suggesting that catecholamines may trigger dilator muscle spasm ². The exact pathophysiology is an area awaiting further case accumulation and research.

References

Footnotes

1. Thompson HS, Zackon DH, Czarnecki JS. Tadpole-shaped pupils caused by segmental spasm of the iris dilator muscle. Am J Ophthalmol. 1983;96(4):467-477. PMID: 6624828. doi:10.1016/s0002-9394(14)77910-3 ↩ ↩² ↩³ ↩⁴

2. Udry M, Kardon RH, Sadun F, Kawasaki A. The Tadpole Pupil: Case Series With Review of the Literature and New Considerations. Front Neurol. 2019;10:846. PMID: 31481920. doi:10.3389/fneur.2019.00846 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

3. Jacobson DM. Benign episodic unilateral mydriasis. Clinical characteristics. Ophthalmology. 1995;102(11):1623-1627. PMID: 9098253 ↩

4. Woods D, O’Connor PS, Fleming R. Episodic unilateral mydriasis and migraine. Am J Ophthalmol. 1984;98(2):229-234. PMID: 6476048 ↩

5. Seibold A, Barnett J, Stack L, Lei C. Benign Episodic Mydriasis as a Cause of Isolated Anisocoria. Clin Pract Cases Emerg Med. 2023;7(2):113-114. PMID: 37285496. PMC: PMC10247168. doi:10.5811/cpcem.1316 ↩