Differential Diagnosis: Mid-Dilated Semi-Reactive Pupils + Giddiness + Headache + Bradycardia

PRIORITY 1 - Neurological Emergencies (Most Urgent)

1. Raised Intracranial Pressure (RICP) / Tentorial Herniation

• Bradycardia + hypertension + altered pupils = Cushing's response (triad: bradycardia, hypertension, irregular breathing)
• Uncal herniation compresses CN III → initially sluggish/semi-reactive, then fixed dilated pupil
• Headache is severe, progressive; giddiness from posterior fossa pressure
• Mid-dilated, poorly reactive is the classic early herniation pupil
• Source: Rosen's Emergency Medicine - "sudden onset of a dilated, nonreactive pupil, along with bradycardia and hypertension, is indicative of acute brain herniation from raised intracranial pressure"
• Causes to look for: Subdural/epidural hematoma, SAH, hypertensive ICH, brain tumor, cerebral edema

2. Dorsal Midbrain (Parinaud) Syndrome

• Bilateral mid-dilated pupils, poorly reactive to light but constrict with convergence
• Associated with eyelid retraction (Collier sign), upgaze palsy, convergence-retraction nystagmus
• Headache and giddiness from the underlying lesion
• Causes: Pinealoma, obstructive hydrocephalus, dorsal midbrain infarct, AVM
• Source: Wills Eye Manual - "Parinaud syndrome/dorsal midbrain lesion: Bilateral mid-dilated pupils that react poorly to light"

3. Subarachnoid Hemorrhage (SAH)

• "Thunderclap" worst headache of life
• Can cause bradycardia via vagal activation and raised ICP
• Pupils may be unequally dilated or semi-reactive if there is blood tracking near CN III

PRIORITY 2 - Toxic/Pharmacological Causes

4. Acute Angle-Closure Glaucoma

• Classic mid-dilated, fixed (or semi-reactive) pupil - the most textbook ophthalmic cause
• Severe headache (often periorbital), nausea/vomiting, blurred vision with halos
• Bradycardia can occur via oculo-cardiac reflex (vagal stimulation from acute IOP rise)
• Giddiness from the acute pain response and vagal activation
• Source: Kanski's Clinical Ophthalmology - "A non-reactive mid-dilated vertically oval pupil is classic"
• Source: Rosen's - listed under "headache + bradycardia" differentials

5. Cardiac Glycoside (Digoxin) Toxicity

• Bradycardia is a hallmark
• Headache, fatigue, visual disturbances (altered color perception, halos, photophobia)
• Giddiness/vertigo is common
• Pupils can be mid-dilated due to CNS effects
• Source: Goodman & Gilman's - "Headache, fatigue, and sleeplessness can be early symptoms of CG toxicity... visual effects: altered color perception and coronas"

6. Beta-Blocker Toxicity

• Bradycardia is the dominant feature
• Headache, giddiness, and fatigue
• Pupils may be mid-position (beta-2 blockade affects some pupillomotor tone)

7. Calcium Channel Blocker Toxicity

• Bradycardia, hypotension, headache, dizziness
• Pupils variable but can appear mid-dilated

PRIORITY 3 - Specific Toxidromes

8. Sympathomimetic Withdrawal / Clonidine Effect

• Clonidine (central alpha-2 agonist): bradycardia + hypotension + miosis initially, but mid-dilated if mixed sympathetic effect
• Giddiness from hypotension

9. Myasthenic Crisis (vs. Cholinergic Crisis)

• In cholinergic crisis (e.g., AChE inhibitor overdose): typically miosis + bradycardia
• However, in myasthenic crisis, paradoxical mydriasis (mid-dilated) can be seen alongside bradycardia from vagal effects
• Source: Rosen's - "mydriasis is much more likely to be seen in myasthenic crisis"

Systematic Summary Table

Immediate Workup

1. CT head (non-contrast) - rule out ICH, herniation, SAH
2. ECG - bradycardia type (sinus? AV block? digoxin pattern?)
3. IOP measurement - if red eye or periorbital pain, rule out acute glaucoma
4. Drug history - digoxin, beta-blockers, CCBs, organophosphates, opioids
5. Serum digoxin level, electrolytes, renal function
6. MRI brain with contrast if dorsal midbrain syndrome suspected

Red Flag: Mid-dilated semi-reactive pupils + bradycardia + headache = treat as increased ICP/herniation until proven otherwise. Stabilize airway, get CT head urgently.

Diagnostic Algorithm: Bradycardia + Abnormal Pupils

STEP 1 - First Assessment (Simultaneous)

• Airway, Breathing, Circulation
• GCS score
• Blood pressure (critical branch point)
• Pupil size + reactivity + symmetry
• Drug/medication history
• 12-lead ECG immediately

STEP 2 - Branch by Consciousness

BRANCH A: GCS Altered / Unconscious

• Uncal herniation → ipsilateral fixed dilated pupil + contralateral hemiplegia
• Tonsillar/cerebellar herniation → bilateral pinpoint or mid-sized unreactive pupils
• Bilateral fixed dilated → catastrophic brainstem failure
• (Source: Goldman-Cecil Medicine)

BRANCH B: Conscious Patient - Stratify by Pupil Size

🔵 PINPOINT PUPILS (<2 mm) + Bradycardia

(Sources: Tintinalli's EM, Forensic Medicine & Toxicology 2026, Harriet Lane Handbook)

🟡 MID-DILATED PUPILS (4-6 mm), Semi-Reactive + Bradycardia

(Sources: Wills Eye Manual, Kanski's Ophthalmology, Rosen's EM, Goodman & Gilman's)

🔴 FIXED/FULLY DILATED PUPILS (>6 mm), Non-reactive + Bradycardia

STEP 3 - Mandatory Workup for ALL Cases

Key Mnemonic Summary

Bottom line: Pupil size is your fastest non-invasive clue to the anatomical level or toxicological class of the problem. Bradycardia narrows it further - it rules out pure sympathomimetic toxidromes and points toward opioid/cholinergic/structural CNS causes.The algorithm above is organized as a 3-step clinical decision pathway:

• Altered GCS → focus on BP to separate Cushing's triad (raised ICP/herniation) from toxicological causes
• Conscious → branch entirely by pupil size into three arms:
  • Pinpoint → Opioid, Cholinergic (OP poisoning), Clonidine, Pontine bleed
  • Mid-dilated, semi-reactive → Acute angle-closure glaucoma, Dorsal midbrain/Parinaud, early herniation, Digoxin toxicity
  • Fixed/fully dilated → Anticholinergic, CCB/Beta-blocker OD, severe hypoxia