Pilar Sheath Acanthoma
Pilar sheath acanthoma is a rare benign adnexal growth made up of an acanthotic (thickened) proliferation of epithelial cells that differentiate toward the hair follicle isthmus and infundibular epithelium.
How the Follicle Differentiates
- Cell of origin: The lesion arises from epithelial germ cells at the junction of the follicular infundibulum and isthmus. Unlike trichofolliculoma (which differentiates fully and produces small secondary hairs), pilar sheath acanthoma stops differentiating at the outer root sheath stage, forming solid, finger-like epithelial lobules that radiate from a central cystic wall.
- Pattern of keratinization: The lobules keratinize in a trichilemmal pattern, changing abruptly from pale cells to compact keratin without a granular layer of keratohyalin granules, which confirms their origin in the outer root sheath of the isthmus.
Histopathologic Criteria
Microscopic Features (H&E)
Under light microscopy, the lesion sits mainly in the superficial and deep dermis:
- Central Dilated Cavity: A large cyst-like space opens directly onto the skin surface through a punctum and is filled with laminated keratin.
- Radiating Epithelial Lobules: Enlarged acanthotic lobules of irregular width extend from the wall of the central cavity deep into the dermis.
- Clear Cell Change: Cells in the center of the lobules are often enlarged, with abundant pale cytoplasm rich in glycogen (PAS-positive).
- Absence of Secondary Hair Shafts: The key feature separating it from trichofolliculoma is the complete lack of rudimentary follicles or malformed vellus hairs entering the central cavity.
Imaging and Dermoscopy
- Dermoscopy:
- Brown/yellow central keratin plug: a compact mass of keratin in the center of the lesion.
- Raised white halo or border: surrounds the central pore and reflects the acanthotic epithelium.
- Radial (crown-like) vessels: small parallel vessels running toward the center without crossing the central pore.
- High-frequency skin ultrasound (50 MHz): a mixed anechoic/hyperechoic cystic structure in the upper dermis that opens onto the epidermis, surrounded by well-defined hypoechoic nodules (the acanthotic lobules).
Clinical Presentation
- Appearance: a solitary, firm, skin-colored or pale pink papule or nodule, 3–10 mm across, with a small central pore holding a tightly packed keratin plug.
- Typical sites: mostly on the face of middle-aged and older adults, especially the upper lip, nose, cheeks and forehead. Patients usually come in for cosmetic reasons or because the keratin plug keeps coming back in the same spot after being squeezed out.
- Behavior: entirely benign, grows slowly over years, and causes no pain or itch.
Differential Diagnosis of Facial Papules With a Central Pore
| Condition | Core Microscopic Structure | Distinguishing Clinical Features |
|---|---|---|
| Pilar sheath acanthoma | Central cavity + thick radiating acanthotic lobules, no hair shafts | Firm facial papule with a keratin-filled pore, no hairs |
| Trichofolliculoma | Central cyst + many secondary vellus hairs | Tuft of fine hairs emerging from a central pore |
| Dilated pore of Winer | Very large pore + thin, atrophic epithelial wall with few lobules | Enlarged pore containing a hard blackhead-like plug |
| Keratoacanthoma | Large keratin-filled crater + raised, volcano-like epithelial lip | Grows very rapidly (2–6 weeks), easily confused with SCC |
Clinical and Cosmetic Treatment Protocol at Haute-Derma
Because the lobules of pilar sheath acanthoma extend deep into the dermis, simply squeezing the lesion or burning off the surface lets it grow back quickly from the lobules left behind. Haute-Derma follows a standardized microsurgical/laser protocol:
Dermoscopic Assessment and Differential Diagnosis: We use 20x dermoscopy to exclude abnormal arborizing vessels (suggesting BCC) or a protruding tuft of vellus hairs (trichofolliculoma).
Microsurgical or Laser Removal
Depending on the size and depth of the lesion, our doctors choose one of two approaches:
CO2 / Er:YAG 2940 nm Laser Ablation (for Lesions < 5 mm)
- Cleansing and plug removal: a mild keratin-softening agent and microsurgical instruments are used to extract the entire keratin plug from the pore.
- Layer-by-layer laser ablation:
- A CO2 (10,600 nm) or Er:YAG (2940 nm) laser is used in ultra-pulse mode.
- The treated area is widened to the edge of the acanthotic mass and ablated deeply along the axis of the central pore, removing the entire wall and all radiating lobules down to healthy dermis.
- Hemostasis and wound base: a defocused CO2 beam gently controls bleeding at the wound base while preserving as much of the surrounding healthy collagen as possible.
Elliptical Micro-Excision (for Lesions > 5 mm or Suspicious Lesions)
- A very small ellipse is cut close around the lesion, and the whole cyst with its lobules is removed down to the deep dermis or subcutis.
- The wound is closed with buried intradermal sutures of fine cosmetic monofilament (6-0/7-0) to avoid a noticeable facial scar. The entire specimen is sent for H&E histopathology.
Recovery and Aftercare
- A serum containing PDRN (polydeoxyribonucleotide), EGF and FGF is applied to speed re-epithelialization and prevent post-inflammatory hyperpigmentation (PIH).
- Red 633 nm LED light therapy supports wound healing during the first 3–5 days.
