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Pupillary Abnormalities (Horner’s vs Argyll Robertson): The Differences — MRCP Part 1

TL;DR:

 For MRCP Part 1, pupillary abnormalities are tested through pattern recognition and lesion localisation. Horner’s syndrome produces a small pupil with ptosis and a preserved light reflex due to sympathetic pathway damage. The Argyll Robertson pupil shows light–near dissociation and points to midbrain pathology, classically neurosyphilis.


Why pupillary abnormalities matter in MRCP Part 1

Pupillary abnormalities are a high-yield neuro-ophthalmology topic in MRCP Part 1 because they integrate anatomy, neurology, and systemic disease. Examination questions rarely require advanced investigations; instead, they reward a structured clinical approach: pupil size → reaction to light → reaction to accommodation → localisation.

Horner’s syndrome and the Argyll Robertson pupil appear repeatedly in MRCP-style questions because each represents a “classic” lesion with a recognisable pattern. Confusing these two is a common reason candidates lose easy marks.


Exam-focused approach to pupils

When faced with a pupillary abnormality in the exam, follow this order:

  1. Is the pupil small or large?

  2. Does it react to light?

  3. Does it react to accommodation?

  4. Is there ptosis or anhidrosis?

  5. Which autonomic pathway is involved?

This approach allows rapid differentiation between sympathetic and parasympathetic lesions — the key skill tested in MRCP Part 1.


Horner’s syndrome

Pathophysiology (what the exam expects)

Horner’s syndrome results from interruption of the sympathetic pathway supplying the eye. The lesion may occur anywhere from the hypothalamus to the cervical sympathetic chain.

Core clinical features

  • Miosis (small pupil)

  • Partial ptosis (loss of Müller muscle tone)

  • Anhidrosis (variable, depending on lesion level)

Pupillary reflexes

  • Light reflex: preserved

  • Accommodation: preserved

This is a critical discriminator: the parasympathetic pathway is intact.

High-yield associations

  • Pancoast tumour (apical lung cancer)

  • Carotid artery dissection (especially if painful)

  • Lateral medullary (Wallenberg) syndrome

In MRCP Part 1, acute neck pain plus Horner’s syndrome should immediately suggest carotid artery dissection.


Argyll Robertson pupil

Pathophysiology

The Argyll Robertson pupil reflects damage to the midbrain light reflex pathway, sparing the accommodation fibres.

Core clinical features

  • Small, irregular pupils

  • Absent light reflex

  • Preserved accommodation

This phenomenon is known as light–near dissociation.

Classic association

  • Neurosyphilis (especially tabes dorsalis)

Although neurosyphilis is now uncommon, the Argyll Robertson pupil remains a favourite MRCP Part 1 diagnosis because of its distinctive physiology.


One-glance comparison (high-yield table)

Feature

Horner’s syndrome

Argyll Robertson pupil

Pupil size

Small (miosis)

Small, irregular

Ptosis

Mild

Absent

Light reflex

Normal

Absent

Accommodation

Normal

Preserved

Autonomic pathway

Sympathetic

Parasympathetic reflex arc

Classic cause

Pancoast tumour

Neurosyphilis

Study desk setup for MRCP Part 1 exam preparation

The 5 most tested subtopics

  1. Light–near dissociation Seen in Argyll Robertson pupil; occasionally diabetic neuropathy may mimic this.

  2. Sympathetic lesions cause miosis Horner’s pupils are small, not large.

  3. Ptosis patterns Mild ptosis suggests Horner’s; complete ptosis suggests third nerve palsy.

  4. Painful Horner’s syndrome Strongly associated with carotid artery dissection.

  5. Bilateral small pupils Consider systemic causes (e.g. opioids) before focal neurology.


Mini-case (MRCP style)

Question: A 48-year-old man presents with left-sided neck pain and drooping of the eyelid. Examination reveals a small left pupil with a normal response to light and accommodation. Facial sweating is reduced on the same side.

Most likely diagnosis? Horner’s syndrome

Explanation: The triad of miosis, ptosis, and anhidrosis with preserved light reflex indicates sympathetic pathway disruption. Neck pain is a red flag for carotid artery dissection — a classic MRCP Part 1 association.


Common pitfalls (exam traps)

  • Mistaking Argyll Robertson pupil for third nerve palsy

  • Forgetting that Horner’s pupils do react to light

  • Ignoring anhidrosis as a localising sign

  • Assuming all light–near dissociation equals neurosyphilis

  • Overlooking pain as a diagnostic clue in Horner’s syndrome


Practical study checklist

  • Always describe pupil size first

  • Test and state light and accommodation separately

  • Link each pattern to a classic cause

  • Practise short clinical stems using MRCP-style questions

  • Reinforce recall with timed practice papers

For targeted question practice, use the Crack Medicine Qbank:👉 https://www.crackmedicine.com/qbank/

For full exam simulation, attempt a timed paper here:👉 https://www.crackmedicine.com/mock-tests/

You can also revisit the parent hub for structured revision:👉 https://www.crackmedicine.com/mrcp-part-1/


FAQs

What is light–near dissociation?

Loss of the pupillary light reflex with preserved accommodation, classically seen in the Argyll Robertson pupil.

Does Horner’s syndrome affect the light reflex?

No. The parasympathetic pathway is intact, so the light reflex is preserved.

Is Argyll Robertson pupil still relevant for MRCP Part 1?Yes. Despite being rare clinically, it remains a classic exam diagnosis linked to neurosyphilis.

How can I quickly differentiate pupillary abnormalities in exams?

Assess pupil size, then light and accommodation responses, and finally decide whether the lesion is sympathetic or parasympathetic.


Ready to start?

To consolidate pupillary abnormalities, revise this topic alongside neuro-ophthalmology content in the MRCP Part 1 overview, practise targeted questions in the Free MRCP MCQs, and test recall under pressure using a full mock test.


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