= FONG DISEASE = ILIAC HORNS = FAMILIAL / HEREDITARY OSTEO-ONYCHODYSPLASIA = OSTEO-ONYCHODYSOSTOSIS = HOOD SYNDROME = ELBOW-PATELLA SYNDROME

= rare autosomal-dominant disorder characterized by symmetrical meso- and ectodermal anomalies

Etiology: ? enzymatic defect in collagen metabolism

Genetics: mutation of LMX1B gene encoding a transcription factor important to development of kidneys + limbs, particularly embryologically dorsal limb structures including nail + patella

Age: evident in 2^nd + 3^rd decades

• Diagnosed only late in life (if at all)!

• bilateral spooning / splitting / ridging of fingernails
• abnormal gait; abnormal pigmentation of iris
• renal dysfunction ← abnormal glomerular basement membrane: proteinuria, hematuria, failure later in life
• 4 key features:
  1. Nail abnormality (98%):
     Site: 1^st–5^th digit (in decreasing order of frequency)
     • absent / hypoplastic / split nails
     • hypoplasia of both thumbnails
     • PATHOGNOMONIC triangular lunula
     • lack of skin creases over dorsum of DIP joints
  2. Patella abnormality (74%):
     • fragmentation / hypoplasia / absence of patella
     • recurrent lateral dislocations (frequent)
     • “squared” appearance of knees upon genuflection ← lateral subluxation
       Cx: knee joint instability, restriction, pain, arthritis
  3. PATHOGNOMONIC bilateral posterior iliac horns (= conical exostosis) in 68–80%
     • occasionally capped by an epiphysis
       Site: at attachment of gluteus medius to posterior iliac wing
  4. Elbow abnormality (33%):
     • pterygia = skin webs → restricted range of motion / dislocation
     • radial head / capitellum hypoplasia with subluxation / dislocation of radial head dorsally and increased carrying angle of elbow (DDx: congenital dislocation of radial head)
• flared iliac crest with protuberant anterior iliac spines
• flexion contractures of hip, knee, elbow, fingers, foot with inability of full extension
• deltoid, triceps, quadriceps hypoplasia
• mandibular cysts (occasionally)
• scoliosis
• Hand
  • clinodactyly of 5^th finger
  • short 5^th metacarpal
• Knee
  • genu valgum ← asymmetrical development of femoral condyles
  • prominent tibial tubercles
• Kidney
  • accelerated age-related decline in renal function
  • mild proteinuria (nephrotic-range in 5–10%)
  • renal osteodystrophy
    Cx: renal failure and death
• Eye
  • open-angle glaucoma

DDx:

1. Seckel syndrome = bird-headed dwarfism
2. Popliteal pterygium syndrome (absence of patella, toenail dysplasia)