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Achondroplasia

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Topic updated on 11/22/14 9:05am
Introduction
  • Also known as classic dwarfism
    • most common cause of disproportionate dwarfism
  • Genetics
    • autosomal dominant (AD)
    • caused by mutation of FGFR3 (fibroblast growth factor receptor 3, on chromosome 4P)   
      • an "activation" mutation that leads to abnormal chondroid production by chondroblasts in the proliferative zone during enchondral bone formation at the physis  
      • a quantitative, not qualitative, cartilage defect
      • a sporadic mutation in >80%
        • risk increases with advanced paternal age
  • Associated conditions
    • spinal manifestations
      • lumbar stenosis
        • caused by short pedicles
        • most likely to cause disability 
      • thoracolumbar kyphosis 
        • may cause neurologic symptoms
      • foramen magnum and upper cervical stenosis
        • may cause periods of apnea
    • Pseudoachondroplasia
      • an autosomal dominant condition that is clinically similar to achondroplasia
      • genetics
        • caused by a defect in the cartilage oligomeric matrix protein (COMP) on chromosome 19
      • differs from achondroplasia by
        • normal facies on physical exam
        • associated with cervical instability due to odontoid hypoplasia 
        • absence of spinal stenosis
Presentation 
  • Symptoms
    • normal intelligence 
    • delayed motor milestones
    • symptoms of spinal stenosis
      • pseudoclaudication and standing discomfort
      • numbness and paresthesias
      • subjective weakness
  • Physical exam
    • classic rhizomelic dwarfism  
      • adult height ~ 50 inches
      • humerus shorter than forearm and femur shorter than tibia
      • normal trunk
    • facial features
      • frontal bossing
      • button noses
      • small nasal bridges
    • extremities
      • trident hands (inability to approximate extended middle and ring finger)  
      • bowed legs
      • radial head subluxation
      • muscular hypotonia
    • spine
      • thoracolumbar kyphosis (often resolves at weight-bearing age)
      • excessive lordosis (due to short pedicles)
Imaging
  • Radiographs
    • CTLS spine
      • recommended views
        •  AP and lateral  
      • lumbar spine findings
        • short pedicles 
        • decreased interpedicular distance from L1-S1 
        • T12/L1 vertebral wedging
        • posterior vertebral scalloping  
    • pelvis and extremities
      • recommended views
        • AP pelvis and weight-bearing hip-to-ankle AP  
      • findings
        • champagne glass pelvis (pelvis is wider than deep) 
  • MRI
    • indications
      • to evaluate spinal stenosis  
      • suspected foramen magnum stenosis  
Treatment
  • Spinal kyphosis
    • nonoperative
      • bracing
        • indicated as first line of treatment in mild curves
    • operative
      • anterior strut corpectomy and posterior fusion
        • indications
          • bracing has failed
          • kyphosis of > 60° by age 5
  • Lumbar stenosis
    • nonoperative
      • weight loss, physical therapy, corticosteroid injections
    • operative
      • multilevel laminectomy and fusion
        • indications
          • spinal stenosis with severe symptoms
          • nonoperative management has failed
  • Foramen magnum stenosis
    • operative
      • urgent decompression
        • indications
          • when cord compression is present
  • Genu varum
    • operative
      • tibial osteotomies or hemiepiphysiodeses
        • indications
          • symptoms are severe
          • nonoperative modalities have failed
  • Short stature
    • operative
      • limb lengthening through a metaphyseal corticotomy
        • indications
          • controversial due to high rate of complications

 

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Qbank (5 Questions)

TAG
(OBQ12.82) Which of the following radiographs represents the condition associated with the genetic mutation G380R in the transmembrane domain of fibroblast growth factor 3 (FGFR3)? Topic Review Topic
FIGURES: A   B   C   D   E  

1. Figure A
2. Figure B
3. Figure C
4. Figure D
5. Figure E

PREFERRED RESPONSE ▶
TAG
(OBQ10.161) Dwarfism caused by a defect of fibroblast growth factor receptor-3 (FGFR3) is associated with all of the following traits EXCEPT: Topic Review Topic

1. Rhizomelic limb shortening
2. Normal intelligence
3. Frontal bossing
4. Cervical spine instability
5. Spinal stenosis

PREFERRED RESPONSE ▶
TAG
(OBQ09.10) Which of the following conditions is associated with a mutation in fibroblast growth factor receptor-3 (FGFR3)? Topic Review Topic

1. Marfan syndrome
2. Gaucher's disease
3. Fibrous dysplasia
4. Achondroplasia
5. Diastrophic dysplasia

PREFERRED RESPONSE ▶
TAG
(OBQ06.139) A 7-year-old presents to your office for general orthopedic evaluation at the request of his primary care physician. A clinical picture of the patient with his taller sibling is shown in Figure A. What physeal zone is affected by the mutation leading to this patient's condition? Topic Review Topic
FIGURES: A          

1. Zone of hypertrophy
2. Reserve zone
3. Zone of proliferation
4. Primary spongiosa
5. Secondary spongiosa

PREFERRED RESPONSE ▶
TAG
(OBQ06.226) Achondroplasia results from abnormal chondrocyte function in the physis. What receptor is defective and what region of the physis is affected? Topic Review Topic

1. FGFR-3, zone of proliferation
2. FGFR-2, zone of proliferation
3. FGFR-3, zone of hypertrophy
4. FGFR-2, zone of hypertrophy
5. COMP, zone of hypertrophy

PREFERRED RESPONSE ▶




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