Purtscher Flecken vs Cotton-wool Spots

Differentiating Purtscher flecken from cotton-wool spots (also called soft exudates) is primarily done on fundus examination (clinical appearance), with OCT providing supportive but often overlapping information in the acute phase. Both represent areas of retinal ischemia/whitening, but they arise from slightly different levels of microvascular occlusion and have distinct classic features.

On Fundus Examination (Key Differentiator)

This is where the distinction is clearest and most reliable:

• Cotton-wool spots:
  • Fluffy, white or off-white patches with indistinct, feathery, or blurred borders.
  • Often appear superficial and may obscure or hide underlying retinal vessels (vessels disappear into the lesion).
  • Oriented along the retinal nerve fiber layer (arcuate or following nerve fiber bundles).
  • Typically smaller and more rounded/irregular in shape.
  • Common in many conditions (e.g., diabetic retinopathy, hypertension, HIV, etc.).
• Purtscher flecken:
  • Polygonal (many-sided, angular, plaque-like) areas of retinal whitening.
  • Sharp, well-demarcated borders with a characteristic clear zone (perivascular clearing) of normal-looking retina immediately adjacent to the lesion (usually within ~50 µm of nearby arterioles/venules).
  • Located between (not over) retinal arterioles and venules — vessels remain clearly visible and are not obscured by the whitening.
  • Confined to the posterior pole, often near the optic disc or macula.
  • Considered pathognomonic for Purtscher/Purtscher-like retinopathy (though present in only ~50–65% of cases).

In short: If the white patch is fluffy, vessel-obscuring, and feathery → likely cotton-wool spot. If it’s polygonal, sharply bordered with a clear rim next to vessels, and vessels are visible through/around it → Purtscher flecken.

On OCT (Optical Coherence Tomography)

OCT findings in the acute phase are quite similar for both, so it is not the primary way to differentiate them — it mainly confirms inner retinal involvement and ischemia/edema.

• Both lesions typically show hyperreflectivity in the inner retinal layers (nerve fiber layer to inner nuclear layer), reflecting axoplasmic stasis, edema, or infarction.
• Cotton-wool spots → hyperreflectivity mainly in the nerve fiber layer (superficial), often with focal thickening.
• Purtscher flecken → hyperreflectivity in the inner retina (may extend deeper toward inner nuclear layer in some descriptions), sometimes associated with features of paracentral acute middle maculopathy (PAMM) like band-like hyperreflectivity in the inner nuclear layer or perivenular changes on en face OCT (due to deeper capillary plexus ischemia).
• Additional acute OCT signs in Purtscher contexts: macular edema, subretinal fluid, retinal thickening, or later outer retinal atrophy/photoreceptor loss.
• In practice, OCT helps rule out other differentials (e.g., full-thickness changes in commotio retinae) but doesn’t reliably separate the two lesions when they coexist (as they often do in Purtscher retinopathy).

Quick Comparison Table