What is the most likely diagnosis and recommended evaluation and management for a patient with year‑long progressive hard, crusted, painful nodules (described as feeling like glass) on the eyebrows, ears, cheeks, legs, torso, and extremities, with associated ankle edema and dark blue/black/brown spots on the feet?

Most Likely Diagnosis: Cutaneous Amyloidosis (Nodular Type)

Based on the distinctive clinical presentation of hard, crusted, painful nodules described as feeling like "glass" under the skin with progressive multifocal distribution over one year, nodular cutaneous amyloidosis is the most likely diagnosis, though cutaneous polyarteritis nodosa and other panniculitides remain important differential considerations.

Clinical Reasoning

The patient's description of lesions feeling like "glass" under the skin is highly characteristic and pathognomonic for amyloid deposits. The progressive nature starting from eyebrows and spreading to multiple body sites, combined with the hard, crusted texture and severe pain, strongly suggests nodular cutaneous amyloidosis (NCA) 1.

Key Distinguishing Features Supporting Amyloidosis:

• "Glass-like" quality: This specific tactile description is classic for amyloid deposits
• Progressive multifocal distribution: Starting focally (eyebrows) then spreading to face, torso, extremities
• Waxy, crusted appearance: Consistent with amyloid deposition
• Severe pain with pressure: Amyloid nodules can be extremely painful and friable
• Ankle edema with dark spots on feet: May suggest systemic involvement or vascular compromise

Critical Differential Diagnoses to Exclude

1. Cutaneous Polyarteritis Nodosa

• Presents with painful subcutaneous nodules, livedo reticularis, and ulcers 2, 3
• Can cause similar distribution on extremities
• Associated with systemic symptoms (fever, malaise, myalgias)
• Key difference: Less likely to have the "glass-like" quality; more associated with livedo reticularis

2. Nodular Vasculitis/Erythema Induratum

• Painful nodules primarily on lower legs (dorsal aspect) 4
• Key difference: Typically limited to lower extremities, not the widespread distribution described

3. Cutaneous Fungal Infections (in immunocompromised)

• Aspergillus, Fusarium, or Mucor can cause painful nodules 5
• Key difference: Would expect fever, systemic illness, and immunocompromised state

4. Nocardia or Atypical Mycobacteria

• Can present as subcutaneous nodules 5
• Key difference: Usually "cold" abscesses, not typically described as "glass-like"

Recommended Diagnostic Evaluation

Immediate Priority:

1. Skin biopsy with special stains (most critical):

   • Standard H&E to assess dermal deposits
   • Congo red stain (essential for amyloid diagnosis - shows apple-green birefringence under polarized light)
   • Immunohistochemistry for kappa/lambda light chains
   • S100, CD68 staining if histiocytosis considered

2. Laboratory workup to assess for systemic amyloidosis 1:

   • Complete blood count (CBC)
   • Complete metabolic panel (CMP) with renal function
   • Serum protein electrophoresis (S-PEP)
   • Urine protein electrophoresis (U-PEP)
   • 24-hour urine collection with creatinine clearance and protein
   • Serum immunoglobulins and β2-microglobulin
   • Serum free light chains (kappa/lambda ratio)

3. Imaging studies:

   • Chest/abdomen/pelvis CT to assess for lymphadenopathy or organomegaly
   • Consider PET scan if systemic disease suspected

4. Additional biopsies if systemic amyloidosis suspected:

   • Abdominal fat pad biopsy (less invasive screening)
   • Bone marrow biopsy
   • Rectal biopsy

If Initial Workup Negative for Amyloidosis:

1. Vasculitis evaluation:

   • ESR, CRP
   • ANCA panel
   • Hepatitis B/C serology
   • Cryoglobulins
   • Deep skin biopsy including subcutaneous tissue to assess vessels

2. Infectious workup (if immunocompromised or atypical features):

   • Tissue culture for bacteria, fungi, mycobacteria
   • Fungal stains (GMS, PAS)
   • Acid-fast bacilli stain
   • PCR for mycobacteria

Management Approach

If Nodular Cutaneous Amyloidosis Confirmed:

Initial Management 1:

• Hematology/oncology referral (mandatory) - to exclude plasma cell dyscrasia or multiple myeloma
• Dermatology consultation for specialized treatment

Treatment Options (based on extent and progression):

1. For localized, mild disease:

   • High-potency topical corticosteroids
   • Close monitoring every 3-6 months

2. For progressive, disfiguring disease 1:

   • CO2 laser ablation (recommended for symptomatic relief)
   • Surgical excision of individual lesions
   • Consider systemic therapy if underlying plasma cell disorder identified

3. Long-term surveillance (critical):

   • Annual systemic amyloid re-evaluation including CBC, CMP, S-PEP, U-PEP, 24-hour urine collection
   • Monitor for progression to systemic amyloidosis (can occur years later)
   • Biannual follow-up with hematology/oncology initially

If Cutaneous Polyarteritis Nodosa Confirmed:

Treatment Algorithm 2, 3:

1. Mild disease:

   • NSAIDs for symptomatic relief
   • Close observation

2. Moderate-to-severe disease:

   • Systemic corticosteroids (prednisone 0.5-1 mg/kg/day)
   • Taper over several months based on response

3. Refractory or steroid-dependent disease:

   • Add immunosuppressive agents (azathioprine, methotrexate, or mycophenolate)
   • Consider biologics in severe cases

If Nodular Vasculitis Confirmed:

First-line treatment 4:

• Potent topical corticosteroids with compression stockings
• Screen for tuberculosis (QuantiFERON or tuberculin skin test)
• If latent TB positive: antitubercular therapy
• Systemic steroids or potassium iodide for refractory cases

Critical Pitfalls to Avoid

1. Delaying biopsy: The "glass-like" quality is so distinctive that biopsy should be performed urgently to confirm amyloidosis

2. Inadequate systemic workup: Even with "localized" cutaneous amyloidosis, systemic involvement can develop - comprehensive evaluation is mandatory 1

3. Missing plasma cell dyscrasia: NCA can be the presenting sign of multiple myeloma or other plasma cell disorders

4. Assuming infection without evidence: While fungal/mycobacterial infections are in the differential, the chronic progressive nature over one year without fever makes infection less likely unless immunocompromised

5. Inadequate biopsy depth: For vasculitis evaluation, superficial biopsies may miss the diagnosis - need deep incisional biopsy including subcutaneous fat

6. Failure to consider mixed histiocytosis: Rosai-Dorfman disease can present with similar nodules but typically has massive lymphadenopathy 6, 7

Immediate Next Steps

Prioritize skin biopsy with Congo red stain and immunohistochemistry, followed by comprehensive laboratory evaluation for systemic amyloidosis and hematology/oncology referral regardless of initial findings, given the high risk of progression and association with plasma cell disorders.