What could cause loss of all colors except green and purple in the left eye?

Acute Monocular Color Vision Loss Requires Urgent Ophthalmologic Evaluation for Retinal or Optic Nerve Pathology

This patient needs immediate ophthalmologic assessment, as isolated preservation of only green and purple vision in one eye suggests severe acquired color vision deficiency from retinal vascular occlusion, optic nerve disease, or advanced glaucoma—all conditions requiring urgent intervention to prevent permanent vision loss or protect the fellow eye.

Most Likely Differential Diagnoses

Retinal Artery Occlusion (Most Urgent)

This represents an ophthalmic emergency requiring evaluation within hours 1. Central or branch retinal artery occlusion typically presents with:

• Sudden, painless, unilateral vision loss occurring over seconds 1
• Profound color vision disturbance (the selective preservation of only green/purple suggests severe cone dysfunction)
• Classic fundoscopic findings: retinal whitening with cherry-red spot at fovea, boxcar segmentation of blood flow 1
• Visual acuity typically 20/200 to counting fingers, though can vary 1
• Relative afferent pupillary defect proportional to ischemia 1

Critical action: Transfer to stroke center immediately—do not delay for ancillary testing 1. The patient needs cardiovascular workup as this represents a stroke equivalent.

Optic Neuritis (Second Most Likely)

Demyelinating optic nerve inflammation causes 2:

• Color desaturation (particularly red desaturation, though severe cases affect all color perception)
• Pain with eye movements
• Relative afferent pupillary defect
• Often associated with multiple sclerosis 3

Color perception impairment following optic neuritis shows permanent dysfunction even after apparent recovery, with reduced cone contrast scores for red, green, and blue 3.

Advanced Glaucoma

Primary open-angle glaucoma can cause acquired color vision defects 4, 5. The mechanism involves:

• Swelling and dysfunction of L/M-cones (red/green sensitive cones) 5
• Produces tritan-like (blue-yellow) deficiency but affects all cone types 5
• Associated with optic disc changes, RNFL thinning, and visual field defects 4

However, glaucoma typically progresses bilaterally (though asymmetrically) and would be unusual to present acutely with such profound unilateral color loss 4.

Pathophysiology of Selective Color Preservation

The preservation of only green and purple perception suggests:

• Severe dysfunction of red-sensitive (L) cones while some middle-wavelength (M/green) and short-wavelength (S/blue) cone function persists
• Acquired color vision deficiency typically produces tritan (blue-yellow) defects in most ocular diseases 6, 7
• The unusual pattern described (only green/purple) suggests either:
  • Severe, non-selective cone damage with residual function in M and S cones
  • Retinal ischemia affecting different cone populations variably
  • Possible cortical processing abnormality (though less likely given monocular presentation)

Immediate Evaluation Required

History Elements to Obtain 1, 2:

• Timing: Seconds (arterial occlusion) vs. hours-days (optic neuritis)
• Pain: Absent (arterial occlusion) vs. present with eye movement (optic neuritis) vs. severe with headache (acute angle-closure glaucoma)
• Vascular risk factors: Hypertension, diabetes, hyperlipidemia, smoking, atrial fibrillation
• Giant cell arteritis symptoms if age >50: Scalp tenderness, jaw claudication, temporal headache, constitutional symptoms 2
• Neurologic symptoms: Multiple sclerosis history, prior optic neuritis 3

Urgent Examination 1:

• Visual acuity measurement
• Relative afferent pupillary defect assessment
• Intraocular pressure
• Dilated fundoscopy: Look for retinal whitening, cherry-red spot, emboli, boxcar blood flow, optic disc swelling or pallor 1
• Visual field testing if feasible

Ancillary Testing (Do Not Delay Transfer) 1:

• OCT: Shows inner retinal thickening acutely in arterial occlusion, later thinning 1
• Fluorescein angiography: Demonstrates arterial flow delays
• ESR/CRP if giant cell arteritis suspected 2

Critical Pitfalls to Avoid

1. Do not delay stroke center transfer for ancillary ophthalmic testing in suspected arterial occlusion 1
2. Do not miss giant cell arteritis—requires immediate empiric steroids before temporal artery biopsy to prevent fellow eye involvement 2
3. Do not assume good visual acuity rules out serious pathology—color vision can be profoundly affected even with preserved acuity in some conditions 3
4. Recognize that this monocular, acute presentation is not congenital color blindness 8

The selective preservation of only green and purple vision in one eye is highly abnormal and demands urgent ophthalmologic evaluation to identify treatable causes and prevent vision loss in the fellow eye.