Since Ekman's initial description of this condition as “congenital osteomalacia” in 1788, several taxonomic schemes have been proposed to categorize OI, including one by Weil.48 Looser23 divided OI into congenita and tarda forms, based on age at presentation. Subsequently, Sillence, et al.,42 proposed a four-tiered classification system that emphasized, in addition to age, the mode of inheritance and morphological findings (Table 1). Regardless of taxonomy, the hallmark of all forms of OI is bone fragility and osteopenia caused by defective type I collagen synthesis.7,13,43 Additional (but less consistent) clinical features characteristic of this disorder include blue sclerae, laxity of ligaments, presenile deafness, and impaired dentition.
|I||dominant inherited||A||variable bone fragility; blue||autosomal|
|OI||sclerae; presenile hearing||dominant|
|w/ blue||loss/deafness; normal dentition;|
|sclerae||presentation in adolescence|
|or early adulthood|
|B||IA + dentinogenesis imperfecta|
|II||lethal perinatal||severe bone fragility; blue||autosomal|
|OI||sclerae; stillbirth or neonatal||recessive,|
|death; crumpled/deformed or||de novo|
|thin/beaded long bones &||dominant|
|III||progressively||A||progressively deforming bone||autosomal|
|deforming||disease; normal sclerae (blue||recessive|
|OI||in infancy); severe spinal deformity;|
|skull deformity with|
|Wormian bones; marked ligamentous|
|laxity; normal dentition;|
|presentation in childhood|
|B||IIIA + dentinogenesis imperfecta|
|IV||dominant in||A||variable bone fragility (> type||autosomal|
|herited OI||I); normal sclerae; moderate||dominant|
|w/ normal||spinal deformity; normal dentition;|
|sclerae||variable age of presentation|
|B||IVA + dentinogenesis imperfecta|
Other osteochondrodysplasias (heritable disorders of bone and cartilage development) may present with craniovertebral junction pathology that is indistinguishable from that associated with OI, based on clinical and radiological criteria. Among these, the spondyloepiphyseal dysplasias appear to be the most closely related to OI in terms of pathogenesis. This group of conditions arises from defects in type II collagen that are nearly identical to the type I collagen deficiencies present in some forms of OI.7,29 In both disorders, genetically programmed anomalies in collagen chain synthesis impair formation of the collagen triple helix, yielding structurally and functionally abnormal collagen in bone and other connective tissues.7,29 Another rare but morphologically similar condition that may affect the craniovertebral junction is the Hajdu—Cheney syndrome (known as “cranio-skeletal dysplasia”14 or “acro-osteolysis”6). Although the biochemical defect underlying this disorder remains obscure, some investigators have postulated that an impairment in collagen synthesis and maturation may precipitate dysplastic bone formation in this condition.6,49 Thus, it may be surmised that OI, spondyloepiphyseal dysplasia, and the Hajdu—Cheney syndrome, in addition to exhibiting similar clinical features, may share related pathogenic mechanisms.
Basilar invagination is a potentially devastating complication of OI and the related osteochondrodysplasias. Unfortunately, the rarity of these conditions and the relative infrequency of associated craniovertebral junction involvement has precluded the systematic study of the natural history of such lesions, as well as their response to treatment. However, based on the available literature we hypothesize the following: 1) the natural history of basilar invagination associated with these conditions is one of progressive deformity and neurological dysfunction, ending ultimately in the patient's death;5,15,17,20,22,30,32,34,37,38 2) posterior decompression alone (suboccipital craniectomy and upper cervical laminectomy) is inadequate treatment for this type of pathology and may actually hasten the progression of ventral brainstem compression;11,15,17,30,34 and 3) ventral decompression with subsequent dorsal fusion may be effective in ameliorating symptoms and halting progressive basilar invagination.15 The veracity of the last postulate is unclear, because published experience has been limited to single case reports or small series with generally short duration of follow up.
In this article, we review our experience with basilar invagination associated with OI and related osteochondrodysplasias and present a comprehensive strategy for medical and surgical management of the craniovertebral junction anomalies associated with these rare conditions.
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