Chiari I Malformation and Basilar Invagination in Fibrous Dysplasia: Prevalence, Mechanisms, and Clinical Implications

A Global Bibliometric and Visualization Analysis of Craniovertebral Junction Bony Abnormalities Based on VOSviewer and Citespace

OBJECTIVE

Recent years have witnessed a rapidly growing interest in CVJ bony abnormalities, and a qualitative and quantitative analysis of relevant literatures is necessary. This study aims to identify and summarize the published articles related to craniovertebral junction bony abnormalities, to analyze and visualize the current research trends and major contributors.

METHODS

We collected data from Web of Science, excluding certain article types. Two researchers screened articles for relevance. Data were organized with EndnoteX9, and analyzed using VOSviewer and CiteSpace for co-authorship, co-occurrence, keyword burst, and co-citation analyses to identify research trends and collaborations.

RESULTS

A total of 2,776 articles were included, revealing an increasing trend in annual publications of CVJ bony abnormalities. The USA was the leading country. King Edward Memorial Hospital was the most prolific institution, and Seth GS Medical College had the most citations. The Spine is the most popular journal with the highest number of publications and citations. Professor Goel Atul from India emerged as the most influential pioneer in this field. Keyword analysis highlighted surgical techniques, diagnosis, and anatomy as the primary research hotspots and Fixation, Placement, and Basilar invagination gradually become the new research trend. However, there is a relative weakness in basic research and epidemiology.

CONCLUSIONS

This study provides valuable insights into the current research trends and critical contributors in CVJ bony abnormalities, guiding evidence-based decisions and fostering international collaborations to advance knowledge in this field.

Serum Phosphorus as a Driver of Skeletal Morbidity in Fibrous Dysplasia

CONTEXT

Fibrous dysplasia (FD) results in fractures, pain, and deformities. Abnormal osteoprogenitor cells overproduce FGF23, leading to hyperphosphaturia in most patients and frank hypophosphatemia in a subset. Studies suggest hypophosphatemia is associated with increased FD-related morbidity. However, the relationship between phosphorus and skeletal complications has not been investigated, and the optimal therapeutic target has not been determined.

OBJECTIVE

Characterize the impact of serum phosphorus on FD-related morbidity and identify levels associated with increased skeletal complications.

METHODS

Natural history study with 240 subjects at a clinical research center who had ≥1 fasting phosphorus level, determined as age- and sex-adjusted Z-scores. Subjects were categorized based on frank hypophosphatemia (Z-score ≤ -2; n = 48); low-normophosphatemia (> -2 to ≤ -1; n = 66); and high-normophosphatemia (> -1 to ≤ 2; n = 125). Main outcomes were fractures, orthopedic surgeries, and scoliosis.

RESULTS

Subjects with frank and low-normophosphatemia had increased fracture and surgery rates vs high-normophosphatemia. The prevalence of moderate to severe scoliosis was similarly higher in the frank and low-normophosphatemia groups. In a subanalysis of patients matched for Skeletal Burden Score ≥35, fracture and surgery rates remained higher in the frank hypophosphatemia group, suggesting association between phosphorus and skeletal complications is not explained by differences in FD burden alone.

CONCLUSION

Both frank hypophosphatemia and low-normophosphatemia are associated with increased FD-related complications. This supports FGF23-mediated hypophosphatemia as a driver of skeletal morbidity, which may impact a larger proportion of the FD/McCune-Albright syndrome population than previously recognized. These findings enable clinicians to identify at-risk patients and will inform development of prospective studies to determine optimal therapeutic targets.

Fibrous dysplasia in children and its management

PURPOSE OF REVIEW

The purpose of this review is to provide a comprehensive overview into the diagnosis and management of fibrous dysplasia (FD) in children.

RECENT FINDINGS

FD is a mosaic disorder arising from somatic Gα s variants, leading to impaired osteogenic cell differentiation. Fibro-osseous lesions expand during childhood and reach final disease burden in early adulthood. The mainstay of treatment focuses on surgical correction of skeletal deformities, physiatric care, and medical management of associated hyperfunctioning endocrinopathies. Bisphosphonates may be helpful to treat bone pain, but do not alter lesion quality or progression. Emerging evidence suggests that the RANKL inhibitor denosumab may be effective in improving lesion activity and mineralization, however further studies are needed to determine the potential utility of this and other novel therapies, particularly in children with FD.

SUMMARY

Management of children with FD has unique challenges related to skeletal growth and age-related lesion progression. Inclusion of children in clinical research is critical to develop effective treatment strategies to treat FD lesions and prevent their development.

Chiari Syndrome: Advances in Epidemiology and Pathogenesis: A Systematic Review

Arnold Chiari syndrome is a rare congenital disease of unknown prevalence and whose origin is still under study. It is encompassed within the posterior cranial malformations, showing a wide spectrum of symptomatology that can range from severe headache, dizziness, and paresthesia to complete asymptomatology. It is for this reason that early diagnosis of the disease is difficult, and it is usually diagnosed in adolescence. Treatment is based on remodeling and decompression of the malformed posterior cranial fossa, although the risk of residual symptoms after surgery is high. The aim of this review is to update all the existing information on this pathology by means of an exhaustive analysis covering all the scientific literature produced in the last 5 years. In addition, it has been carried out following the PRISMA model and registered in PROSPERO with code CRD42023394490. One of the main conclusions based on the results obtained in this review is that the origin of the syndrome could have a genetic basis and that the treatment of choice is the decompression of the posterior cerebral fossa.

Lesion Expansion in Gnathic Fibrous Dysplasia: Natural History, Indicators of Progression, and Response to Bisphosphonates

Fibrous dysplasia (FD) is characterized by expansile fibro-osseous lesions that may occur in association with endocrinopathies as part of McCune-Albright syndrome (MAS). Craniofacial FD is a significant source of morbidity and most commonly involves the gnathic bones. There is a critical need to understand the natural history and risk factors for gnathic FD progression to develop preventative trials and identify candidates for intervention. The purpose of this study was to characterize gnathic FD lesion expansion and to identify risk factors associated with lesion growth. Patients with gnathic FD and serial CT imaging were evaluated. Volumetric analyses of CT scans were performed using MIM Encore software. Generalized mixed model analysis was used to account for intra-subject correlation, with FD lesion volume as the dependent variable. In addition to age, effects of MAS-associated endocrinopathies, sex, disease severity, and bisphosphonate treatment were evaluated. A total of 104 total lesions in 52 patients were characterized longitudinally. Median age at initial scan was 8.8 years (range 3.4-18.8), and median age at final scan was 16.8 years (range 6.9-33.4 years). The median number of scans per subject was 4 (range 2-14). FD lesion volume increased with age (2.50 cm3 /yr, 95% confidence interval [CI] 1.95-3.04, p < 0.001). However, lesion expansion rate decreased over time (-0.05 cm3 /yr, 95% CI -0.07 to 0.04, p < 0.001). Mandibular lesions tended to expand at a greater rate than maxillary lesions (p < 0.001). Growth hormone excess was associated with accelerated expansion rate (p = 0.002). Other MAS-associated endocrinopathies, pubertal status, sex, weight, lesion density, disease severity, and bisphosphonate treatment were not associated with lesion volume or expansion. Gnathic FD lesion expansion is most rapid in younger children and declines as patients approach adulthood. The availability of quantitative natural history data will guide clinicians in identifying patients who are candidates for medical and surgical interventions and clinical trials for preventative therapies. Published 2023. This article is a U.S. Government work and is in the public domain in the USA.

Brain and eye involvement in McCune-Albright Syndrome: clinical and translational insights

McCune-Albright Syndrome (MAS) is a rare mosaic (post-zygotic) genetic disorder presenting with a broad continuum clinical spectrum. MAS arises from somatic, activating mutations in the GNAS gene, which induces a dysregulated Gsα-protein signaling in several tissues and an increased production of intracellular cyclic adenosine monophosphate (cAMP). Overall, MAS is a rare disorder affecting less than 1/100,000 children and, for this reason, data establishing genotype-phenotype correlations remain limited. Affected individuals clinically present with a variable combination of fibrous dysplasia of bone (FD), extra-skeletal manifestations (including cafeí-au-lait spots) and precocious puberty which might also be associated to broad hyperfunctioning endocrinopathies, and also gastrointestinal and cardiological involvement. Central nervous system (CNS) and eye involvement in MAS are among the less frequently described complications and remain largely uncharacterized. These rare complications mainly include neurodevelopmental abnormalities (e.g., delayed motor development, cognitive and language impairment), CNS anomalies (e.g., Chiari malformation type I) and a wide array of ophthalmological abnormalities often associated with vision loss. The pathophysiological mechanisms underlying abnormal neurological development have not been yet fully elucidated. The proposed mechanisms include a deleterious impact of chronically dysregulated Gsα-protein signaling on neurological function, or a secondary (damaging) effect of (antenatal and/or early postnatal) hypercortisolism on early pre- and post-natal CNS development. In this Review, we summarize the main neurological and ophthalmological features eventually associated with the MAS spectrum, also providing a detailed overview of the potential pathophysiological mechanisms underlying these clinical complications.

Craniofacial Fibrous Dysplasia: Clinical and Therapeutic Implications

PURPOSE OF REVIEW

This study aims to review diagnosis, potential complications, and clinical management in craniofacial fibrous dysplasia.

RECENT FINDINGS

Fibrous dysplasia (FD) is a rare mosaic disorder in which normal bone and marrow are replaced with expansile fibro-osseous lesions. Disease presents along a broad spectrum and may be associated with extraskeletal features as part of McCune-Albright syndrome (MAS). The craniofacial skeleton is one of the most commonly impacted areas in FD, and its functional and anatomical complexities create unique challenges for diagnosis and management. This review summarizes current approaches to diagnosis and management in FD/MAS, with emphasis on the clinical and therapeutic implications for the craniofacial skeleton.

GNAS mutation analysis assists in differentiating chronic diffuse sclerosing osteomyelitis from fibrous dysplasia in the jaw

Chronic diffuse sclerosing osteomyelitis of the mandible (DSOM) and fibrous dysplasia (FD) are distinct lesions with overlapping clinicopathological features that complicate their diagnosis. This study aimed to evaluate the efficacy of GNAS mutation analysis in differentiating between these two conditions. DNA samples from patients with DSOM (n = 35) and FD (n = 29) were collected to analyze the presence of GNAS mutations in exons 8 and 9, the two previously reported hotspot regions, using polymerase chain reaction and direct sequencing. Twenty-four of 29 patients (83%) with FD showed missense mutations in codon 201 in exon 8, whereas no mutation was detected in exon 9. No mutations were found in any of the 35 cases with DSOM. We also identified one case with an uncertain diagnosis due to overlapping clinicopathological features of DSOM and FD. A Q227H mutation was detected in this case, that confirmed the diagnosis of FD. Taken together, the findings indicate that mutational analysis of the GNAS is a reliable approach to differentiate between DSOM and FD of the jaw.

Long-term imaging course of Chiari malformation type I due to fibrous dysplasia/McCune-Albright syndrome

INTRODUCTION

McCune-Albright syndrome (MAS) and fibrous dysplasia (FD) have been reported to cause Chiari type I malformation (CM1) and skull base invagination (BI).

CASE

A 6-year-old girl was diagnosed with MAS and FD. She was diagnosed with CM1 at age 8 years, and the syringomyelia had gradually increased by age 20 years. We performed foramen magnum decompression and C1 laminectomy, and the syringomyelia stopped spreading after surgery.

DISCUSSION

This patient underwent long-term radiological observation and morphological evaluations, which revealed that the skull thickening was progressing, while the posterior cranial fossa volume (PCFV) remained unchanged for 14 years. Therefore, although PCFV did not decrease, it was considered to be relatively inadequate due to the increase in brain volume with growth, resulting in posterior fossa overcrowding, causing CM1.

CONCLUSION

In patients with FD/MAS, long-term evaluation of bone thickening, odontoid position, and PCFV is necessary.

Reduction of giant parietooccipital fibrous dysplasia using dynamic mirror image guidance: a case report and review of the literature

BACKGROUND

Craniofacial fibrous dysplasia (CFD) typically occurs in the facial bones and anterior cranial vault and can produce both disfigurement and functional limitations for patients disfigurement. Treatment consists of reducing the abnormal bone. Bone contouring can become challenging when the exposure does not extend to the corresponding normal contralateral structures for comparison or when normal landmarks are not available, which may compromise the overall aesthetic outcome. We describe a technique using dynamic mirroring to accurately contour the involved part of the cranium in a case of giant CFD.

OBSERVATIONS

A 49-year-old male presented with a giant deforming fibrous dysplasia of the right mastoid and parieto-temporo-occiput that was causing functional limitations due to the size of the bony mass. This was managed with multidisciplinary bony reduction. Several neurovascular structures were in proximity to the areas of planned drilling of the expansile lesion, and dynamic mirroring of the uninvolved left skull was utilized to maximize safety and symmetry of reduction. High-speed drilling of the right occipital bone was performed until the navigation system alerted the surgeon that symmetric depth had been achieved. There were no complications from the procedure and this technique maximized the limits of symmetric reduction without significantly increasing surgical complexity or duration.

LESSONS

Dynamic mirroring of bony structures in the posterior cranium is not commonly employed in neurosurgical practice. This technique may help improve the aesthetic outcomes of bony reduction in craniofacial dysplasia and a variety of similarly managed bony lesions, contour cranioplasties, and in unilateral craniosynostosis surgery.

Chiari Malformation (Update on Diagnosis and Treatment)

Chiari Malformation Type I (CMI) is a congenital malformation diagnosed by MRI findings of at least 5 mm of cerebellar ectopy below the foramen magnum. CM1 is frequently associated with syringomyelia. Herein, we discuss the history of CMI and syringomyelia, including early pathological and surgical studies. We also describe recent investigations into the pathogenesis and pathophysiology of CMI and their practical implications on management and surgical intervention. We also highlight the recent development of the Common Data Elements for CMI, providing a framework for ongoing investigations. Finally, we discuss current controversies of surgical management in CMI.

Long Bone Fractures in Fibrous Dysplasia/McCune-Albright Syndrome: Prevalence, Natural History, and Risk Factors

Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) is a rare bone and endocrine disorder arising along a broad spectrum. Long-bone fractures are a common, painful, and potentially disabling complication. However, fracture prevalence and risk factors have not been well-established, making it difficult to predict which patients are at risk for a severe course. Clinical and imaging data were reviewed from two large, well-phenotyped cohorts (National Institutes of Health [NIH] in the United States and the Leiden University Medical Center [LUMC] in the Netherlands) to identify long-bone fractures at FD sites. Skeletal burden score was quantified using bone scintigraphy. Multiple linear regressions were performed to identify clinical associations with fractures. A total of 419 patients were included (186 NIH, 233 LUMC); 194 (46%) had MAS endocrinopathies. Median age at last follow-up was 30.2 years (range 3.2-84.6, interquartile range [IQR] 25.5), and median skeletal burden score was 16.6 (range 0-75, IQR 33). A total of 48 (59%) patients suffered one or more lifetime fracture (median 1, range 0-70, IQR 4). Median age at first fracture was 8 years (range 1-76, IQR 10). Fracture rates peaked between 6 and 10 years of age and decreased thereafter. Lifetime fracture rate was associated with skeletal burden score (β = 0.40, p < 0.01) and MAS hyperthyroidism (β = 0.22, p = 0.01). Younger age at first fracture was associated with skeletal burden score (β = -0.26, p = 0.01) and male sex (β = -0.23, p = 0.01). Both skeletal burden score >25 and age at first fracture ≤7 years were associated with a higher total number of lifetime fractures (median 4, range 1-70, IQR 5 versus median 1, range 1-13, IQR 1) (p < 0.01). In conclusion, higher skeletal burden score and MAS hyperthyroidism are associated with long-bone fractures in FD/MAS. Both skeletal burden score ≥25 and age at first fracture ≤7 years are associated with a higher lifetime long-bone fracture risk and may predict a more severe clinical course. These results may allow clinicians to identify FD/MAS patients at risk for severe disease who may be candidates for early therapeutic interventions. © 2021 American Society for Bone and Mineral Research (ASBMR). This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Periorbital inflammation associated with craniofacial fibrous dysplasia: Report of three cases and review of the literature

Fibrous dysplasia (FD) is a mosaic skeletal disorder in which the craniofacial bones are commonly affected. Normal structures are replaced by expansile, highly vascular, fibro-osseous tissue. The typical clinical course is a gradual, asymptomatic expansion of the osseous structures. However, in the periorbital region, even minor structural changes may cause functional impairment, such as diplopia and hyposmia. Furthermore, rapidly evolving secondary lesions, such as fluid-filled cysts, can sometimes develop. In the midface and periorbital regions, such acute change may be associated with severe pain, vision loss, and, signs of inflammation. Here we describe three patients with craniofacial FD who presented with recurrent episodes of periorbital inflammation mimicking orbital cellulitis. All presented with pain, edema, erythema, and warmth, with varying degrees of functional impairment. On imaging, all had cystic changes in the FD lesion, including two with aneurysmal bone cysts (ABCs). Two were initially diagnosed with periorbital cellulitis and treated with antibiotics; in two, the radiographic findings were misdiagnosed as osteomyelitis. Recurrent episodes were recognized as not infectious and effectively managed with corticosteroids. Given the vascular nature of FD and the association of ABCs, it is likely the findings in these patients represent inflammation associated with vascular leak in the relatively confined space of the tissues overlying the periorbital bones. Recognition of this entity can lead to more rapid and appropriate treatment.

Severe Fibrous Dysplasia in McCune-Albright Syndrome: A Need for Continuous Surveillance

INTRODUCTION

McCune-Albright syndrome (MAS) is a rare condition, in which GNAS mutations affect multiple organs. Fibrous dysplasia (FD), affecting only one or multiple skeletal territories, may severely affect craniofacial structures. Concomitant occurrence of acromegaly aggravates skull deformity, leading to eye, ear, and posterior cranial fossa compromise.

CASE PRESENTATION

A 30-year-old man diagnosed with MAS at the age of 3 developed almost all known complications of the syndrome. The craniofacial component of his polyostotic FD increased over time, aggravated by difficult to control acromegaly. Acute onset of severe headache and neurologic compromise, caused by subarachnoid haemorrhage, caused his demise. Post-mortem examination revealed a meningeal artery aneurysm caused by disruption of the intracranial vasculature by severe bone disease. Adrenal histology revealed nodular hyperplasia without clinical evidence of hypercortisolism.

DISCUSSION

The post-mortem findings described aid understanding of the multiorgan involvement of MAS, providing new insights into possible pathogenetic mechanisms underlying the systemic effects of GNAS mutations, and highlight a need for systematic surveillance for cerebrovascular changes in craniofacial FD that may be amenable to intervention to avoid catastrophic outcome.

Burosumab treatment for fibrous dysplasia

BACKGROUND

Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) is a rare mosaic disorder of Gα_s activation. Fibroblast Growth Factor 23 (FGF23)-mediated hypophosphatemia is a feature of FD/MAS that has been associated with poor skeletal outcomes. Standard therapy includes oral phosphorus and vitamin D analogs; however, treatment is limited by potential adverse renal and gastrointestinal effects. Burosumab is a monoclonal antibody to FGF23 approved to treat patients with X-linked hypophosphatemia and tumor-induced osteomalacia. There is currently no safety or efficacy data to support burosumab use in patients with FD/MAS.

CASE DESCRIPTION

A 7-year-old boy with severe FD/MAS presented with persistent hypophosphatemia and skeletal complications despite conventional treatment with oral phosphate and calcitriol. He was started on burosumab and achieved sustained normalization of serum phosphorus and marked improvement in alkaline phosphatase levels. This was accompanied by an encouraging clinical response, including decreased bone pain, improved muscle strength, and improved ambulation. No adverse effects of burosumab therapy were observed.

CONCLUSIONS

This is the first reported case of burosumab treatment in a patient with FD/MAS. The encouraging biochemical and clinical response in this patient highlights the need for future studies to explore the safety and efficacy of burosumab in the FD/MAS pediatric population.

Optic disc edema in fibrous dysplasia/McCune-Albright syndrome: Prevalence, etiologies, and clinical implications

BACKGROUND

Fibrous dysplasia (FD) is a rare disorder of expansile fibro-osseous lesions that may be associated with extraskeletal features as part of McCune-Albright syndrome (MAS). Optic disc edema is a potentially serious ophthalmologic finding that has been rarely reported in patients with FD/MAS. The purpose of this study was to investigate the prevalence and potential clinical associations of optic disc edema in a large cohort.

METHODS

Clinical records were reviewed from subjects in an ongoing FD/MAS natural history study. Computed Tomography scans were evaluated for the presence of structural craniofacial abnormalities associated with optic disc edema, including Chiari I malformation and space-occupying lesions. Craniomorphometric analyses were performed to determine optic canal diameter and intracranial volume. Statistical analyses were performed to compare clinical and radiographic features between subjects with and without optic disc edema.

RESULTS

Optic disc edema was diagnosed in 7/187 subjects, for a prevalence of 3.7%. All subjects with optic disc edema were diagnosed before age 18 years and had mild, non-progressive disease. Radiographic structural abnormalities, including Chiari I malformation, aneurysmal bone cysts, and arachnoid cysts, were associated with higher odds of optic disc edema (odds ratio [OR] 24.3; 95% confidence interval [CI], 4.2 to 121.4; p < 0.01) (OR 18.0; 95% CI, 3.4 to 108.2; p < 0.01). Treatment with leuprolide, a gonadotropin releasing hormone analog, was also associated with optic disc edema (OR 26.0; 95% CI 3.3 to 177.5; p < 0.05). There was no significant association of optic disc edema with other MAS endocrinopathies, medications, optic canal diameter, or intracranial volume.

CONCLUSION

Optic disc edema is an uncommon but potentially serious complication of craniofacial FD, which may occur more frequently in pediatric patients and those with structural craniofacial abnormalities. The potential association of leuprolide therapy with optic disc edema in this population warrants further study.

Utility of Optical Coherence Tomography in the Diagnosis and Management of Optic Neuropathy in Patients with Fibrous Dysplasia

Optic neuropathy (ON) is a highly disabling complication of fibrous dysplasia (FD). The optimal test for identifying and monitoring ON in FD is unknown. Optical coherence tomography (OCT) is an imaging modality that detects retinal nerve fiber layer (RNFL) thinning, a sign of optic nerve atrophy. The purpose of this study was to (i) assess the ability of OCT RNFL thickness measurements to identify ON in FD; (ii) compare the performance of RNFL thickness to computed tomography measurements; and (iii) examine changes in RNFL thickness over time to assess disease progression. A retrospective cohort study was performed to assess subjects (n = 70) who underwent neuro-ophthalmologic examination, including OCT. The diagnostic utility of RNFL thickness was determined using receiver operator characteristic (ROC) curve analysis, and the accuracy was compared with computed tomography measurements. The relationship between RNFL thickness and age was assessed cross-sectionally, using generalized estimating equation methodology, and longitudinally, using a generalized mixed model. Eleven subjects were identified with ON. RNFL thickness identified ON (area under curve = 0.997, p < 0.0001) with sensitivity and specificity of 100% and 95%, respectively, when using the diagnostic criterion of ≤71 μm. RNFL thickness outperformed computed tomography measurements of optic canal narrowing and optic nerve stretch. Subjects with ON exhibited a greater decrease in RNFL thickness with each year of age (-0.70 μm/year, p < 0.001) than subjects with normal vision (-0.16 μm/year, p < 0.05). When assessed longitudinally, subjects with normal vision demonstrated an increase in RNFL thickness until approximately age 20 years that decreased thereafter. In contrast, subjects with ON exhibited an earlier decrease in RNFL thickness during adolescence. In conclusion, RNFL thickness of ≤71 μm accurately identified ON in this population. By establishing the difference in rate of RNFL thinning in patients with and without ON, clinicians may distinguish between patients at risk for ON and intervene before irreversible damage. © 2020 American Society for Bone and Mineral Research.

DIAGNOSIS OF ENDOCRINE DISEASE: Mosaic disorders of FGF23 excess: Fibrous dysplasia/McCune-Albright syndrome and cutaneous skeletal hypophosphatemia syndrome

Fibrous dysplasia/McCune-Albright Syndrome (FD/MAS), arising from gain-of-function mutations in Gαs, and cutaneous skeletal hypophosphatemia syndrome (CSHS), arising from gain-of-function mutations in the Ras/MAPK pathway, are strikingly complex, mosaic diseases with overlapping phenotypes. Both disorders are defined by mosaic skin and bone involvement, and both are complicated by increased FGF23 production. These similarities have frequently led to mis-diagnoses, primarily in patients with CSHS who are often assumed to have FD/MAS. The intriguing similarities in skeletal involvement in these genetically distinct disorders have led to novel insights into FGF23 physiology, making an understanding of FD/MAS and CSHS relevant to both clinicians and researchers interested in bone and endocrine disorders. This review will give an overview of FD/MAS and CSHS, focusing on the roles of mosaicism and FGF23 in the pathogenesis and clinical presentation of these disorders.

Fibrous Dysplasia/McCune-Albright Syndrome: A Rare, Mosaic Disease of Gα s Activation

Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) is a rare disorder of striking complexity. It arises from somatic, gain-of-function mutations in GNAS, leading to mosaic Gα s activation and inappropriate production of intracellular cyclic adenosine monophosphate (cAMP). The clinical phenotype is largely determined by the location and extent of affected tissues, and the pathophysiological effects of Gα s activation within these tissues. In bone, Gα s activation results in impaired differentiation of skeletal stem cells, leading to discrete skeletal lesions prone to fracture, deformity, and pain. Extraskeletal manifestations include a variable combination of hyperpigmented macules and hyperfunctioning endocrinopathies. Distinctive age-related changes in disease development has key effects on histologic, radiographic, and clinical features. FD/MAS thus presents along a uniquely broad clinical spectrum, and the resulting challenges in diagnosis and management can be difficult for clinicians. This review presents FD/MAS in the context of a mosaic disorder of Gα s activation, providing an intellectual framework within which to understand, evaluate, and treat this interesting disease. It includes a comprehensive summary of current understanding of FD/MAS pathogenesis, and a detailed discussion of clinical presentation and management. Critical areas of unmet need are highlighted, including discussion of key challenges and potential solutions to advance research and clinical care in FD/MAS.

Neuraxial dysraphism in EPAS1-associated syndrome due to improper mesenchymal transition

Objective

To investigate the effect of somatic, postzygotic, gain-of-function mutation of Endothelial Per-Arnt-Sim (PAS) domain protein 1 (EPAS1) encoding hypoxia-inducible factor-2α (HIF-2α) on posterior fossa development and spinal dysraphism in EPAS1 gain-of-function syndrome, which consists of multiple paragangliomas, somatostatinoma, and polycythemia.

Methods

Patients referred to our institution for evaluation of new, recurrent, and/or metastatic paragangliomas/pheochromocytoma were confirmed for EPAS1 gain-of-function syndrome by identification of the EPAS1 gain-of-function mutation in resected tumors and/or circulating leukocytes. The posterior fossa, its contents, and the spine were evaluated retrospectively on available MRI and CT images of the head and neck performed for tumor staging and restaging. The transgenic mouse model underwent Microfil vascular perfusion and subsequent intact ex vivo 14T MRI and micro-CT as well as gross dissection, histology, and immunohistochemistry to assess the role of EPAS1 in identified malformations.

Results

All 8 patients with EPAS1 gain-of-function syndrome demonstrated incidental posterior fossa malformations—one Dandy-Walker variant and 7 Chiari malformations without syringomyelia. These findings were not associated with a small posterior fossa; rather, the posterior fossa volume exceeded that of its neural contents. Seven of 8 patients demonstrated spinal dysraphism; 4 of 8 demonstrated abnormal vertebral segmentation. The mouse model similarly demonstrated features of neuraxial dysraphism, including cervical myelomeningocele and spinal dysraphism, and cerebellar tonsil displacement through the foramen magnum. Histology and immunohistochemistry demonstrated incomplete mesenchymal transition in the mutant but not the control mouse.

Conclusions

This study characterized posterior fossa and spinal malformations seen in EPAS1 gain-of-function syndrome and suggests that gain-of-function mutation in HIF-2α results in improper mesenchymal transition.

Fibrous dysplasia of occipital and temporal bone. A case report

Fibrous dysplasia is a rare non-malignant condition where fibrous tissue replaces the normal bone architecture. Involvement of temporal and occipital bones is exceptionally rare and is associated with unique complications. A 10-year-old boy presented with right retroauricular enlargement and pain. Imaging studies and biopsy revealed fibrous dysplasia of the temporal and occipital bones. There was no hearing loss or sequelae arising from posterior fossa compression. The patient was discharged with follow-up instructions. Only 10 cases of occipital bone fibrous dysplasia have been reported in the medical literature. Occipital bone fibrous dysplasia can be complicated with Chiari malformation and syringomyelia while temporal bone involvement is associated with hearing loss. These potential developments require close follow-up that includes detailed neurologic examination, imaging and audiology.

Best practice management guidelines for fibrous dysplasia/McCune-Albright syndrome: a consensus statement from the FD/MAS international consortium

Fibrous Dysplasia / McCune Albright syndrome (FD/MAS) represents a wide spectrum of diseases due to somatic gain-of-function mutations of the GNAS gene. The mutation leads to overactivity in the target tissues and to a wide phenotype of clinical features that vary in severity and age of onset. The rarity of the disease and its variable presentation to multiple specialities often leads to misdiagnosis and inappropriate variability in investigations and treatments. To address this, our international consortium of clinicians, researchers, and patients’ advocates has developed pragmatic clinical guidelines for best clinical practice for the definition, diagnosis, staging, treatment and monitoring for FD/MAS to empower patients and support clinical teams in both general and specialised healthcare settings. With the lack of strong evidence to inform care, the guidelines were developed based on review of published literature, long-standing extensive experience of authors, input from other healthcare professionals involved in the care of FD/MAS patients and feedback from patients and patient groups across the globe. This has led to the formulation of a set of statements to inform healthcare professionals, patients, their families, carers and patient groups of the best practice of care. It is anticipated the implementation of these recommendations will lead to improvement in the care of patients with FD/MAS internationally.

Fibrous Dysplasia of Bone and McCune-Albright Syndrome: A Bench to Bedside Review

Fibrous dysplasia is an uncommon mosaic disorder in which bone is replaced by structurally unsound fibro-osseous tissue. It is caused by the sporadic post-zygotic activating mutations in GNAS, resulting in dysregulated Gα_S-protein signaling in affected tissues. This manifests on a broad clinical spectrum ranging from insignificant solitary lesions to severe disease with deformities, fractures, functional impairment, and pain. Fibrous dysplasia may present in isolation or in association with hyperfunctioning endocrinopathies and café-au-lait macules, known as McCune-Albright Syndrome. This review summarizes the current understanding of pathophysiology in fibrous dysplasia, describes key pre-clinical and clinical investigations, and details the current approach to diagnosis and management.

The Clinical Spectrum of McCune-Albright Syndrome and Its Management

McCune-Albright syndrome (MAS) is a rare, mosaic disorder presenting along a broad clinical spectrum. Disease arises from somatic-activating GNAS mutations, leading to constitutive Gαs activation and ligand-independent signaling of the Gαs-coupled protein receptor. The phenotype is largely determined by location and extent of tissues in which the GNAS mutation is expressed, as well as the pathophysiologic effects of Gαs activation within these tissues. Patients pre-sent clinically with a variable combination of fibrous dysplasia of bone (FD), café-au-lait skin macules, and hyperfunctioning endocrinopathies. In bone, Gαs leads to impaired differentiation of skeletal stem cells and formation of discrete, expansile FD lesions, resulting in fractures, pain, and functional impairment. A systematic approach to diagnosis and management is critically important to optimize outcomes for patients with FD/MAS. There are no medical therapies capable of altering the disease course in FD; however, screening and treatment for endocrinopathies can mitigate some skeletal morbidities. This review summarizes current understanding of MAS pathophysiology, describes the spectrum of clinical features, and includes a detailed discussion of the recommended approach to diagnosis and management.

Clinical and translational pharmacological aspects of the management of fibrous dysplasia of bone

Fibrous dysplasia (FD) is a genetic, noninheritable rare bone disease caused by a postzygotic activating mutation of the α subunit of the stimulatory G‐protein causing increased abnormal bone formation leading to pain, deformity and fractures. To date, no cure has been identified for FD/McCune–Albright syndrome (MAS) and treatment is symptomatic and aimed at decreasing pain and/or local bone turnover. Various drugs have been used to achieve clinical improvement in FD/MAS patients including bisphosphonates and denosumab, however further translational studies are also warranted to address unresolved pathophysiological issues and explore novel pharmacological targets for the management of FD/MAS.

In this article, we review literature on the medical treatment of FD/MAS, discuss the unresolved pathophysiological issues and explore novel pharmacological targets for the management of FD/MAS.