eMedicine Specialties > Orthopedic Surgery > Pediatrics

Achondroplasia: Differential Diagnoses & Workup

Author: Shital Parikh, MBBS, MS, Consulting Staff, Department of Orthopedics, Buchanan General Hospital
Coauthor(s): Preeti Batra, MBBS, MD, Staff Physician, Department of Radiology, VS Hospital, India
Contributor Information and Disclosures

Updated: Jan 26, 2009

Differential Diagnoses

Diastrophic Dysplasia
Spondyloepiphyseal Dysplasia

Other Problems to Be Considered

Thanatophoric dwarfism
Achondrogenesis
Chondroectodermal dysplasia (Ellis-van Creveld syndrome)
Metatrophic dwarfism
Asphyxiating thoracic dysplasia
Chondrodysplasia punctata (Conradi disease)
Pseudoachondroplastic dysplasia
Metaphyseal chondrodysplasia (Schmid type)

Workup

Laboratory Studies

  • Cytogenetics
    • Plasma can be analyzed for the FGFR3 mutation in the mother when a short-limb skeletal dysplasia is diagnosed prenatally on ultrasound.11,15 This can be confirmatory for achondroplasia and can help the family to make educated decisions.
    • DNA testing can be performed when both of the parents are affected. Infants with affected genes from both the parents (double homozygous) are either stillborn or die shortly after birth.

Imaging Studies

  • Radiographs of the skull, spine, and extremities reveal the characteristic features.
    • A lateral skull radiograph demonstrates midface hypoplasia, enlarged calvaria, frontal prominence, and shortening of the base of the skull. The size of the foramen magnum is diminished (see Image 2).
    • A lumbar spine (anteroposterior [AP]) view reveals distinct narrowing on the interpedicular distances from proximal to distal in L1-L5. Normally, the interpedicular distance from the cephalocaudad direction should increase. However, if this distance decreases or remains unchanged, it is abnormal. The changes should be observed in 3 consecutive vertebrae (see Image 3).
    • The lateral view reveals shortening of the pedicles and vertebral bodies with significant posterior scalloping. Various degrees of thoracolumbar kyphosis may be present before walking age (see Image 4). This may be associated with wedging of T12 or L1. Once ambulation is established, kyphosis generally improves, and lumbar lordosis develops. The inclination of the sacrum becomes increasingly horizontal. Scoliosis of more than 20° develops in some patients. The curves are relatively short and are located in the thoracolumbar or lumbar region. Rotation is not a prominent feature. Degenerative changes can be seen in the vertebral column, particularly at the anterior margins of the vertebral bodies located in the thoracolumbar area and in the cervical spine. However, such changes are compatible with the patient's age.
    • The pelvis is typically broad and short, and the ilium has a square appearance. The sacrosciatic notch is short, and the acetabular roof is horizontal (see Image 5). The femoral neck is short with trochanteric overgrowth, giving an appearance of coxa vara. However, true coxa vara is not seen. Arthritic changes of the hip, even in older patients, usually are not observed. This can be attributed to the reduction of major joint lever arms and the comparative lightness (50 kg) of the patient.
    • The long bones have metaphyseal flaring and are short and thick. During the first year, the proximal metaphyses of the femur and the humerus have oval areas of radiolucency. Sites of major muscle attachments, such as the deltoid and patellar tendon tuberosity, are prominent. The distal femoral physes have an inverted-V (chevron) shaped configuration. Bowing usually affects the tibia more than the femur. The fibula is typically longer than the tibia. The humerus is markedly shortened, and the radial head frequently is dislocated. The ulna is typically short with an elongated styloid process. The proximal and middle phalanges of the hand are broader, with greater shortening than the distal phalanges and metacarpals.
  • Primary radiographic criteria for diagnosis are as follows:
    • Decrease in interpedicular distance in the lumbar spine
    • Square short ilia
    • Short, broad neck of femur
    • Shortening of long tubular bones, with metaphyseal flaring
    • Brachydactyly
  • Secondary radiographic criteria for diagnosis are as follows:
    • Anteroposterior shortening of lumbar pedicles
    • Dorsal concavity of lumbar vertebra
    • Long distal fibula
    • Short distal ulna
    • Long ulnar styloid
  • CT scanning17,18
    • The size of the foramen magnum can be measured most accurately using CT scanning.
    • The spinal canal is narrowed developmentally, particularly in the lower lumbar segments. The cross-sectional anatomy can be evaluated noninvasively using CT scan.
    • CT scanning can be used to develop a 3-dimensional image of the rib cage, which can be used to calculate lung volumes and can substantiate a successful surgical chest expansion.
  • MRI19
    • Given the incidence and potential severity of neurologic symptoms associated with foramen magnum stenosis, a baseline MRI is strongly recommended in infancy. Cervicomedullary compression at the foramen magnum, fusion of C1, or isolated subaxial cervical stenosis can be demonstrated (see Image 6). In addition, MRI can show myelomalacia, intramedullary cyst, or angulation at the craniocervical junction.
    • MRI may be used to establish the cause of neurocranial enlargement. Dilated ventricles without hydrocephalus, and communicating and noncommunicating forms of hydrocephalus may be observed.
    • MRI is also recommended for preoperative evaluation of lumbar spinal stenosis, especially to determine whether associated disc herniations exist and the proximal level of compression. The average area of L1 is decreased by 39%, and that of L5 is decreased by 27%. Kyphosis correlates strongly with neurologic symptoms, and MRI can demonstrate apical wedging and neurologic involvement.
  • Prenatal ultrasonography
    • Heterozygous achondroplasia is associated with normal or near-normal femur lengths until 20-24 weeks of pregnancy. Thereafter, the growth rate of the femur decreases. Hence, ultrasonography may not be useful for diagnosing achondroplasia in the first half of the pregnancy.
    • Later in the pregnancy, ultrasonography can detect short-limb dysplasia. However, differentiation among various skeletal dysplasias is difficult.
  • Ultrasonography of brain
    • Ultrasound can be used in the neonate to detect ventricle size and other abnormalities.
    • It cannot be used once the sutures and fontanelles close. MRI is the imaging modality of choice at that time.
  • Cine-phase contrast and magnetic resonance (MR) angiography20
    • These studies are useful to study the pathophysiology of brain stem compression and hydrocephalus.
    • Gated cine-phase contrast (PC) cerebrospinal fluid (CSF) flow studies can be used to evaluate CSF dynamics across the foramen magnum.
    • MR angiography (venography) can be used to depict obstructed venous drainage due to jugular foramen stenosis.21
    • Steno-occlusive disease of the internal jugular vein, reduced blood flow in the superior sagittal sinus, and compensatory enlargement in the emissary veins can be depicted.

Other Tests

  • Somatosensory evoked potential (SSEP) abnormalities have been reported for 44% of neurologically intact persons with achondroplasia, and are probably related to brain stem compression at the level of the foramen magnum.
  • Pulmonary function tests are useful for preoperative evaluation when respiratory symptoms are present. Typically, the vital capacity is decreased, averaging 68% for affected males and 72% for affected females.
  • A sleep study may be performed if symptoms suggest airway obstruction. The cause of airway obstruction can be established to guide the treatment. Differentiating between central sleep apnea (due to brain stem or upper cervical cord compression) and obstructive sleep apnea (due to midface hypoplasia) is helpful.

Procedures

  • Intracranial pressure monitoring: Reports exist of sudden blindness associated with an increase in the intracranial pressure (ICP). Hence, ICP monitoring is recommended in persons with achondroplasia with moderate ventriculomegaly, as demonstrated by MRI. This is performed with a percutaneous spinal catheter. Treatment is recommended when the ICP is greater than 15 mm Hg.

Histologic Findings

Biopsy from the growth plates of ilium and proximal fibula reveal an essentially normal structure. Glycosaminoglycan determination is normal. The proportion of proteoglycan aggregates increases in the fibular head. The defect is mainly quantitative and lies in the proliferative zone of the growth plate.

More on Achondroplasia

Overview: Achondroplasia
Differential Diagnoses & Workup: Achondroplasia
Treatment & Medication: Achondroplasia
Follow-up: Achondroplasia
Multimedia: Achondroplasia
References
Further Reading
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References

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Further Reading

Guidelines:
Health supervision for children with achondroplasia. American Academy of Pediatrics - Medical Specialty Society.  2005 Sep.  13 pages.  NGC:004494
 

Clinical trials:
Issues Surrounding Prenatal Genetic Testing for Achondroplasia

Study of Skeletal Disorders and Short Stature

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Keywords

achondroplasia, rhizomelic dwarfism, short-limb dwarfism, short-trunk dwarfism, chondrodystrophia fetalis, classic chondrodystrophy, dyschondroplasia fetalis, chondrodysplasia, micromelia, skeletal dysplasia, little people, achondroplastic, skeletal dysplasia

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Contributor Information and Disclosures

Author

Shital Parikh, MBBS, MS, Consulting Staff, Department of Orthopedics, Buchanan General Hospital
Disclosure: Nothing to disclose.

Coauthor(s)

Preeti Batra, MBBS, MD, Staff Physician, Department of Radiology, VS Hospital, India
Disclosure: Nothing to disclose.

Medical Editor

Mininder S Kocher, MD, MPH, Associate Professor of Orthopedic Surgery, Harvard Medical School/Harvard School of Public Health; Associate Director, Division of Sports Medicine, Department of Orthopedic Surgery, Children's Hospital Boston
Mininder S Kocher, MD, MPH is a member of the following medical societies: American Academy of Orthopaedic Surgeons, American Association for the History of Medicine, American Medical Association, American Orthopaedic Society for Sports Medicine, and Massachusetts Medical Society
Disclosure: Smith & Nephew Endoscopy Consulting fee Consulting; ConMed Linvatec Consulting fee Consulting; Covidian Consulting fee Consulting; EBI Biomet Consulting fee Consulting

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: Nothing to disclose.

Managing Editor

George H Thompson, MD, Director, Pediatric Orthopedics, Rainbow Babies and Children's Hospital
George H Thompson, MD is a member of the following medical societies: American Academy of Orthopaedic Surgeons, American Orthopaedic Association, Pediatric Orthopaedic Society of North America, and Scoliosis Research Society
Disclosure: Nothing to disclose.

CME Editor

Dinesh Patel, MD, FACS, Associate Clinical Professor of Orthopedic Surgery, Harvard Medical School; Chief of Arthroscopic Surgery, Department of Orthopedic Surgery, Massachusetts General Hospital
Dinesh Patel, MD, FACS is a member of the following medical societies: American Academy of Orthopaedic Surgeons, American Association of Physicians of Indian Origin, American College of International Physicians, and American College of Surgeons
Disclosure: Nothing to disclose.

Chief Editor

Dennis P Grogan, MD, Clinical Professor, Department of Orthopedic Surgery, University of South Florida College of Medicine; Chief of Staff, Department of Orthopedic Surgery, Shriners Hospital for Children of Tampa
Dennis P Grogan, MD is a member of the following medical societies: American Academy of Orthopaedic Surgeons, American Medical Association, American Orthopaedic Association, American Orthopaedic Foot and Ankle Society, Eastern Orthopaedic Association, Irish American Orthopaedic Society, Pediatric Orthopaedic Society of North America, and Scoliosis Research Society
Disclosure: Nothing to disclose.

 
 
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