Patch-Type Granuloma Annulare

Case Overview

By Thomas Davis, MD, FAAD
Educational dermpath case series for dermatology residents

Patient: 69-year-old female
Lesion Location: Right rib cage and left hip

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Take a moment to review the clinical and histologic images below to see how Dr. Davis arrived at this diagnosis.

Clinical Presentation: One-year history of asymptomatic, bilaterally distributed erythematous patches on the trunk
A biopsy of the patch lesion was performed to establish a definitive diagnosis.

 

Key Histologic Findings

On examination, several defining features stand out:

• Interstitial lymphohistiocytic infiltrate throughout the dermis
• Slight increase in dermal mucin deposition
• Absence of epidermal changes (no atrophy, hyperplasia, or interface alterations)
• Lack of plasma cells or prominent eosinophils

 

[Image 1: Low-power histologic view showing subtle interstitial infiltrate]

 

[Image 2: Medium power showing interstitial lymphohistiocytic infiltrate]

 

[Image 3: High power showing histiocytes and increased dermal mucin between collagen bundles.]

 

Differential Diagnosis: 3 Common Mimickers

When evaluating an interstitial dermatosis like this, several differentials come to mind. Let’s walk through the 3 most likely contenders and highlight what sets them apart.

1️⃣ Interstitial Mycosis Fungoides

Etiology: Cutaneous T-cell lymphoma with interstitial growth pattern presenting in early patch stage

Histology:

• Interstitial lymphoid infiltrate within the dermis
• Epidermotropism may be subtle or absent in early lesions
• Lymphocytes are typically atypical with hyperchromatic, cerebriform nuclei
• Mucin deposition can be present

Key Distinction:
While both entities show interstitial infiltrates, interstitial MF displays atypical lymphocytes with hyperchromatic, cerebriform nuclei. Patch GA shows bland histiocytes rather than atypical lymphocytes, and lacks epidermotropism. The clinical context is also critical—MF patches are often more extensive and may show progression over time.

 

2️⃣ Inflammatory Morphea

Etiology: Early-stage morphea with prominent inflammatory infiltrate preceding dermal sclerosis

Histology:

• Interstitial and perivascular lymphoplasmacytic infiltrate
• Plasma cells are a key component (usually prominent)
• Early collagen thickening and homogenization in the deep dermis
• Loss of appendageal structures may be evident
• Mucin may be present but is not a defining feature

Key Distinction:
Inflammatory morphea characteristically shows plasma cells within the infiltrate and early signs of dermal sclerosis with collagen bundle thickening and straightening, particularly in the deep dermis. Patch GA lacks plasma cells and does not show collagen alteration or sclerosis. Instead, histiocytes are seen “splitting” between normal-appearing collagen bundles with associated mucin.

 

3️⃣ Interstitial Granulomatous Dermatitis (IGD)

Etiology: Reaction pattern associated with autoimmune diseases (rheumatoid arthritis, lupus), infections, or medications

Histology:

• Dense interstitial infiltrate of histiocytes arranged in cords between collagen bundles
• Scattered neutrophils and nuclear debris (leukocytoclasis) are common
• Focal collagen degeneration with “floating” histiocytes
• Mucin is typically minimal or absent
• May show palisading around altered collagen

Key Distinction:
IGD displays a denser histiocytic infiltrate with characteristic neutrophils and nuclear debris creating a “dirty” appearance, along with obvious collagen degeneration. Patch GA shows a more subtle, cleaner interstitial infiltrate of bland histiocytes with prominent mucin deposition and intact, well-preserved collagen bundles. IGD is also more commonly associated with underlying systemic diseases.

 

The Final Diagnosis:

Patch-Type Granuloma Annulare

This is patch-type granuloma annulare, an uncommon but increasingly recognized variant of classic GA. The diagnosis is confirmed by the interstitial pattern of bland mononuclear histiocytes dissecting between preserved collagen bundles with increased dermal mucin and absence of epidermal changes or palisaded granulomas.

Key Takeaways for Residents:

• Patch GA is clinically often misdiagnosed as MF, morphea, or eczematous dermatitis due to its subtle patch-like presentation
• Unlike classic GA, palisaded granulomas are absent or rare in patch GA—don’t wait to see them before considering the diagnosis
• The triad of interstitial histiocytes + increased mucin + absence of epidermal changes is your diagnostic key
• Studies show patch GA predominantly affects women over 50 with bilateral trunk and proximal extremity involvement (Khanna & North, 2020)
• Consider mucin stains (Alcian blue, colloidal iron) when the diagnosis is suspected to highlight the increased dermal mucin

 

[Image: High-power view highlighting bland histiocytes within collagen with mucin stain inset]

 

📚 Quick Summary

Feature	Patch GA	Interstitial MF	Inflammatory Morphea	IGD
Infiltrate composition	Bland histiocytes	Atypical lymphocytes	Lymphocytes + plasma cells	Dense histiocytes + neutrophils
Dermal mucin	✅ Prominent	✅ Can be present	❌ Minimal	❌ Absent/minimal
Epidermal changes	❌ Absent	✅ Epidermotropism (variable)	❌ Absent	❌ Absent
Collagen alteration	❌ Preserved bundles	❌ Normal	✅ Sclerosis/thickening	✅ Degeneration
Plasma cells	❌ Absent	❌ Rare	✅ Prominent	❌ Rare
Neutrophils/debris	❌ Absent	❌ Absent	❌ Absent	✅ Present
Palisading	✅ May be present	❌ Absent	❌ Absent	✅ May be present
Behavior	Benign	Malignant	Benign/inflammatory	Reactive/secondary

 

💬 Final Thought

When you see subtle interstitial histiocytes with mucin but no obvious granulomas, don’t dismiss granuloma annulare from your differential—you might just be looking at its stealthier patch variant. Remember: not all GA makes rings, and not all patches are lymphoma!

 

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📖 Reference:
Khanna U, North JP. Patch-type granuloma annulare: Clinicopathologic study of 23 cases with increased dermal mucin. J Cutan Pathol. 2020;47(7):614-622.