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Vision Loss with Normal Fundus — Approach

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Differential Diagnosis: Normal Fundus + Decreased Vision

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Step-by-Step Approach

Step 1 — Confirm True Visual Loss

• Measure corrected visual acuity (Snellen at 6m or Jaeger at 14 in)
• Pinhole test: improvement → refractive/media cause (cornea, lens, vitreous), not neurological
  • (Adams & Victor): "If the acuity improves to near normal with a pinhole, one can conclude that the reduced vision relates to a refractive error or a defect in the optical media rather than a neurological cause."

Step 2 — Characterize the Visual Loss

Step 3 — Examine the Afferent Pathway

• Relative Afferent Pupillary Defect (RAPD): present in unilateral or asymmetric optic nerve disease; absent in functional loss and symmetric disease
• Color vision testing (Ishihara, FM100 hue): disproportionately impaired in optic nerve disease vs. macular disease
• Visual field testing (confrontation / automated perimetry):
  • Central scotoma → optic nerve or macula
  • Altitudinal → ischemic optic neuropathy
  • Bitemporal → chiasm
  • Homonymous → retrochiasmal

Step 4 — Ophthalmic Investigations

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Key Diagnoses in Detail

Retrobulbar Optic Neuritis

• Mean age 32 years; 77% female; 92% have ocular pain on eye movement
• Demyelinating event is retrobulbar → fundus normal initially; optic disc pallor develops over months
• Virtually all recover vision spontaneously; IV methylprednisolone speeds recovery but does not affect 6-month acuity
• ONTT: 50% develop clinically definite MS at 15 years
• Brain MRI mandatory — if ≥2 demyelinating plaques, initiate disease-modifying therapy

Posterior Ischemic Optic Neuropathy (PION)

• Rare; due to severe anemia + hypotension (cardiac/spinal surgery, major haemorrhage, dialysis)
• Fundus normal unless inflammation extends anteriorly to reach the globe
• Management: immediate transfusion + haemodynamic correction

Leber Hereditary Optic Neuropathy (LHON)

• Mitochondrial DNA mutation (most commonly m.11778G>A, m.3460G>A, m.14484T>C)
• Young men; painless, bilateral sequential subacute central visual loss
• Fundus may appear normal early; later disc pallor
• Later stages show circumpapillary telangiectasias (sometimes) but early fundus can be completely normal

Toxic / Nutritional Optic Neuropathy

• Agents: ethambutol, isoniazid, chloramphenicol, alcohol (methanol, chronic), tobacco, deficiency states (B12, folate, thiamine)
• Bilateral central/cecocentral scotomas; fundus normal early
• Stop offending agent + nutritional correction

Cancer-Associated Retinopathy (CAR)

• Paraneoplastic, most often small cell lung cancer
• Autoantibodies against recoverin → photoreceptor dysfunction
• Photopsias, ring scotoma, impaired dark adaptation; fundus initially normal; ERG extinguished or severely reduced

Stargardt Disease / Cone Dystrophy

• Juvenile-onset macular dystrophy; fundus may look normal or near-normal early despite severe central acuity loss
• Fundus autofluorescence / multifocal ERG abnormal before visible fundus changes

Functional (Non-organic) Visual Loss

• Consider when: inconsistent acuity, no RAPD, normal VEP, normal ERG, non-physiological field pattern (tubular fields, spiral fields)
• Prism test, fogging test, OKN drum testing useful to unmask preserved function

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Summary Algorithm

Vision loss + Normal Fundus
         ↓
  Pinhole test
  ├── Improves → Refractive/media cause
  └── No improvement → Neurological workup
              ↓
        RAPD present?
        ├── YES → Optic nerve disease
        │         (neuritis, toxic, ischemic, LHON)
        │         → VEP, MRI, VF, OCT
        └── NO  → Bilateral optic/retinal or functional
                  → ERG (CAR, dystrophy), mfERG
                  → Consider functional loss

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• Wills Eye Manual, 8e — Normal Fundus With Decreased Vision
• Harrison's Principles of Internal Medicine 22e — Transient or Sudden Visual Loss (Ch. 34)
• Adams & Victor's Principles of Neurology 12e — Approach to the Problem of Visual Loss (Ch. 12)
• Bradley & Daroff's Neurology in Clinical Practice — Approach to the Patient with Visual Loss (Ch. 16)

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Normal Fundus + Vision Loss — The "Big 3" for Neurology Viva

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PART 1: NORMAL FUNDUS + MONOCULAR VISION LOSS

Branch A — RAPD Present → Retrobulbar Optic Neuritis / PION

Retrobulbar Optic Neuritis

Posterior Ischemic Optic Neuropathy (PION)

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Branch B — No RAPD, Transient → Amaurosis Fugax

1. Carotid atherosclerosis (most common) — emboli from ipsilateral ICA
2. Cardioembolism — AF, valvular disease
3. Retinal artery vasospasm / retinal migraine
4. Hypoperfusion — severe ICA stenosis (also "bright-light amaurosis" — loss in bright light due to delayed photopigment regeneration)
5. Others: hypercoagulable states, GCA (giant cell arteritis), hyperviscosity

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Branch C — Functional/Psychogenic → VEP Normal

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PART 2: NORMAL FUNDUS + BILATERAL VISION LOSS

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Branch A — Bitemporal Hemianopia → Chiasmal Compression (Pituitary)

• Insidious onset — often missed because binocular overlap hides temporal field loss; test each eye separately
• Suprasellar lesion → upper temporal quadrants affected first (inferior nasal fibres cross more anteriorly)
• Junctional scotoma (anterior chiasm): ipsilateral central scotoma + contralateral superotemporal defect (von Willebrand knee fibres)
• Investigation: MRI pituitary with gadolinium; visual field perimetry; pituitary hormone profile
• Pituitary apoplexy = neurosurgical emergency; start IV hydrocortisone immediately

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Branch B — Homonymous Hemianopia → Post-chiasmal Stroke

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Branch C — Complete Blindness + Normal Pupils → Cortical Blindness (Anton Syndrome)

• Patient blames poor vision on "dim lighting" or "not wearing glasses"
• Confabulates visual experiences — "I can see fine"
• Walks directly into objects without adjusting behaviour
• Reflects unawareness in thinking/speaking centres of the absence of visual cortex input
• May co-exist with Korsakoff-type memory dysfunction if bilateral hippocampal involvement
• (Localization in Clinical Neurology): "thalamic involvement in posterior circulation infarcts may be critical for the genesis of Anton syndrome in some patients"

1. Bilateral PCA stroke (most common) — thromboembolism, top-of-basilar syndrome
2. PRES (posterior reversible encephalopathy syndrome) — hypertensive crisis, eclampsia, immunosuppressants
3. Post-contrast/arteriography (transient)
4. Hypoxic-ischemic encephalopathy (cardiac arrest)
5. Trauma
6. Migraine (transient), seizures (postictal)
7. CJD — Heidenhain variant (visual onset)
8. Posterior cortical atrophy (Alzheimer variant)

• Absent ocular pathology
• Preserved pupillary light reflexes ← the key bedside test
• Normal optic fundi
• MRI: bilateral occipital infarcts/lesions
• VEP: absent or grossly abnormal (unlike functional blindness)

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MASTER COMPARISON TABLE

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ONE-LINE VIVA SUMMARIES

• Retrobulbar ON: Pain on eye movement + RAPD + normal fundus + delayed VEP → MS association
• PION: Perioperative anaemia/hypotension + sudden monocular loss + normal fundus → emergency transfusion
• Amaurosis fugax: Curtain descending over one eye × minutes + carotid disease → TIA equivalent; image carotids urgently
• Functional: Tubular fields + normal VEP + inconsistent acuity + no RAPD
• Bitemporal HH: Test each eye separately; check above/below, pituitary MRI, hormone profile
• Homonymous HH: Post-chiasmal = fundus always normal; localise by congruity and pattern
• Anton syndrome: Blind + normal pupils + denies blindness + normal fundus = bilateral occipital infarction

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Why They Look Different

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How They Fit Together — The Unified Framework

VISION LOSS + NORMAL FUNDUS
         ↓
    PINHOLE TEST
    ├── Improves → Refractive/media cause (STOP here — not neurological)
    └── No improvement ↓
    
    MONOCULAR?                        BILATERAL?
         ↓                                 ↓
    RAPD present?              What field pattern?
    ├─ YES → Optic nerve            ├─ Bitemporal HH → Chiasmal (pituitary)
    │   (neuritis, PION,            ├─ Homonymous HH → Post-chiasmal stroke
    │    toxic, LHON)               └─ Complete + normal pupils → Cortical (Anton)
    │   → VEP, MRI, OCT
    ├─ NO + transient → Amaurosis fugax
    └─ NO + persistent → Functional
         → VEP normal

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The Key Reconciliation Points

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The Complete Single Algorithm (Reconciled)

Normal Fundus + Vision Loss
         ↓
  ┌─ PINHOLE improves → Refractive error / media (cornea, lens, vitreous)
  │
  └─ PINHOLE no change → Neurological cause
              ↓
    ┌─── MONOCULAR ──────────────────────────────────────┐
    │  RAPD present                                      │
    │  ├─ Subacute + pain → Retrobulbar optic neuritis   │
    │  ├─ Perioperative anaemia → PION                   │
    │  ├─ Painless + bilateral later → LHON              │
    │  └─ Symmetric + toxin exposure → Toxic ON          │
    │     → VEP (delayed P100), MRI orbit+brain, OCT     │
    │                                                    │
    │  No RAPD + TRANSIENT → Amaurosis fugax             │
    │     → Carotid Doppler, Echo, ECG                   │
    │                                                    │
    │  No RAPD + PERSISTENT → Functional/psychogenic     │
    │     → VEP NORMAL, tubular fields, inconsistent VA  │
    └────────────────────────────────────────────────────┘
    
    ┌─── BILATERAL ──────────────────────────────────────┐
    │  Bitemporal HH → Chiasmal compression              │
    │     → MRI pituitary, hormone profile               │
    │                                                    │
    │  Homonymous HH → Post-chiasmal stroke (PCA)        │
    │     → MRI DWI, carotid/cardiac workup              │
    │                                                    │
    │  Complete blindness + normal pupils → Cortical     │
    │     (Anton syndrome — denies blindness)            │
    │     → MRI bilateral occipital                      │
    │                                                    │
    │  Bilateral central scotomas + no RAPD              │
    │     → ERG (CAR, cone dystrophy, Stargardt)         │
    └────────────────────────────────────────────────────┘

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