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Pycnodysostosis is a genetic lysosomal disease characterized by osteosclerosis of the skeleton, short stature and brittle bones. It is very rare, the exact prevalence is unknown but it is less than 1/100,000. The disease is discovered at variable ages, ranging from 9 months to 50 years. The condition is most often diagnosed in childhood, but sometimes the condition is not detected until adulthood, usually as a result of a fracture or a routine examination. The most frequent clinical or radiological manifestations of the disease are osteosclerosis, short stature or dwarfism, acroosteolysis of the distal phalanges, fragile bones associated with spontaneous fractures and dysplasia of the clavicles. Patients present with characteristic cranial malformations: a voluminous skull with wormian bones present and persistence of the anterior fontanelle, and a small mandible. Dental abnormalities such as decayed, poorly located or abnormally shaped (pointed or conical) teeth and delayed tooth eruption may be observed. Nails are sometimes irregular and cracked. Very rarely, the disease is associated with anemia, hepatosplenomegaly, hematologic alterations, respiratory distress and sleep apnea. The short stature is variable but moderate (1.35m to 1.50m). Inheritance is autosomal recessive and pycnodysostosis is due to mutations in the gene encoding cathepsin K (localized to 1q21), a lysosomal enzyme secreted by osteoclasts, which allows the division of proteins of the bone matrix (collagen type I, osteonectin or osteopontin). Diagnosis is clinical and should be confirmed by radiographic examination of the entire skeleton and skull. Differential diagnoses include osteoporosis, osteopetrosis, cleidocranial dysplasia and idiopathic acroosteolysis (see these terms). Management is symptomatic and multidisciplinary. It includes orthopedic monitoring, treatment of fractures, of which consolidation is sometimes slow, and static vertebral surveillance to detect frequent spondylolisthesis. The prognosis is favorable; the disease is not progressive.
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