Craniosynostosis and hypophosphatasia

Craniosynostosis in primary metabolic bone disorders: a single-institution experience

PURPOSE

The incidence of metabolic bone diseases in pediatric neurosurgical patients is rare. We examined our institutional experience of metabolic bone diseases along with a review of the literature in an effort to understand management for this rare entity.

METHODS

Retrospective review of the electronic medical record database was performed to identify patients with primary metabolic bone disorders who underwent craniosynostosis surgery between 2011 and 2022 at a quaternary referral pediatric hospital. Literature review was conducted for primary metabolic bone disorders associated with craniosynostosis.

RESULTS

Ten patients were identified, 6 of whom were male. The most common bone disorders were hypophosphatemic rickets (n = 2) and pseudohypoparathyroidism (n = 2). The median age at diagnosis of metabolic bone disorder was 2.02 years (IQR: 0.11-4.26), 2.52 years (IQR: 1.24-3.14) at craniosynostosis diagnosis, and 2.65 years (IQR: 0.91-3.58) at the time of surgery. Sagittal suture was most commonly fused (n = 4), followed by multi-suture craniosynostosis (n = 3). Other imaging findings included Chiari (n = 1), hydrocephalus (n = 1), and concurrent Chiari and hydrocephalus (n = 1). All patients underwent surgery for craniosynostosis, with the most common operation being bifronto-orbital advancement (n = 4). A total of 5 patients underwent reoperation, 3 of which were planned second-stage surgeries and 2 of whom had craniosynostosis recurrence.

CONCLUSIONS

We advocate screening for suture abnormalities in children with primary metabolic bone disorders. While cranial vault remodeling is not associated with a high rate of postoperative complications in this patient cohort, craniosynostosis recurrences may occur, and parental counseling is recommended.

Physiology and Clinical Manifestations of Pathologic Cranial Suture Widening

Cranial sutures are complex structures integrating mechanical forces with osteogenesis which are often affected in craniofacial syndromes. While premature fusion is frequently described, rare pathological widening of cranial sutures is a comparatively understudied phenomenon. This narrative review aims to bring to light the biologically variable underlying causes of widened sutures and persistent fontanelles leading to a common outcome. The authors herein present four syndromes, selected from a literature review, and their identified biological mechanisms in the context of altered suture physiology, exploring the roles of progenitor cell differentiation, extracellular matrix production, mineralization, and bone resorption. This article illustrates the gaps in understanding of complex craniofacial disorders, and the potential for further unification of genetics, cellular biology, and clinical pillars of health science research to improve treatment outcomes for patients.

Hypophosphatasia: from birth to adulthood

Hypophosphatasia (HPP) is an inherited disease caused by a low activity of tissue-nonspecific alkaline phosphatase, a hydrolase that removes phosphate groups from many molecules. Decreased alkaline phosphatase activity leads to the accumulation of three main metabolites, i.e., pyridoxal 5´-phosphate (PLP), inorganic pyrophosphate (PPi), and phosphoethanolamine. Impairment in PLP dephosphorylation induces seizures, while PPi accumulation inhibits bone mineralization. Clinically, HPP has a wide spectrum of presentations, ranging from neonatal death to an apparent lack of symptoms. This disease is classified into six subtypes according to the age at onset of first signs or symptoms. The clinical manifestations of the disease include rickets-like bone changes, bone demineralization, fragility fractures, reduced muscular strength, chest deformity, pulmonary hypoplasia, nephrolithiasis, nephrocalcinosis, and chondrocalcinosis. Treatment of HPP consists of enzyme replacement therapy. Before this therapy was approved, treatment was palliative and associated with high morbidity and mortality. Asfotase alfa has changed the prognosis of the disease by reducing bone deformity and improving bone mineralization, lung function, and muscle weakness, among other benefits. In adults, teriparatide and anti-sclerostin antibody have been used off-label in selected cases, demonstrating benefit in accelerating fracture healing and in concomitant treatment of osteoporosis. This review summarizes the main aspects of HPP and identifies the particularities of the disease in adult patients.

Imaging patterns in pediatric hypophosphatasia

Hypophosphatasia is a rare genetic disorder of calcium and phosphate metabolism due to ALPL gene mutations, which leads to abnormal mineralization of the bones and teeth. Hypophosphatasia is characterized by low serum alkaline phosphatase activity and a number of clinical signs, including failure to thrive, bone pain and dental issues. The diagnosis is suspected based on clinical, laboratory and imaging findings and confirmed by genetic testing. Diagnosis in children is often delayed due to a lack of disease awareness, despite specific imaging findings that are a cornerstone of the diagnosis. The recent approval of enzyme replacement therapy (bone-targeted recombinant tissue nonspecific alkaline phosphatase) has given imaging an important role in monitoring treatment efficacy. The aim of this pictorial essay is to review the imaging features of hypophosphatasia at diagnosis and during follow-up, including whole-body magnetic resonance imaging patterns.

Hypophosphatasia and cleidocranial dysplasia-a case report and review of the literature: the role of the neurosurgeon

Hypophosphatasia (HPT) and cleidocranial dysplasia (CCD) are rare genetic disorders characterized by both defective ossification and bone mineralization. Patients usually present with craniosynostosis and cranial defects which in many cases require surgical repair. There is only 1 reported case of combined HPT and CCD in the literature. Our reported case involves a 3.5-year-old girl with concomitant homozygous CCD and heterozygous HPT. The child had an extended cranial defect since birth which improved with the administration of Strensiq and was followed until preschool age. Bone defects were relatively minor on revaluation. Due to the limited final defect, we decided not to intervene. In HPT-CCD patients, bone defects are overestimated due to osteomalacia, and thus, management strategy should be less aggressive. They should undergo surgical repair with cranioplasty with the use of cement and/or titanium meshes in case of extended final defects.

A two-year follow-up of asfotase alfa replacement in a patient with hypophosphatasia: clinical, biochemical, and radiological evaluation

Hypophosphatasia (HPP) is a rare disease with a high mortality rate in its severe forms. It is caused by mutations within the gene encoding the tissue-nonspecific alkaline phosphatase (TNSALP), an enzyme responsible for bone mineralization. In 2015, the Food and Drug Administration approved the use of asfotase alfa, the first medication showing benefit in the treatment of HPP. We describe a case with a 2-year follow-up of the first Brazilian child treated with asfotase alfa. A 5-year-old boy, born to consanguineous parents, was diagnosed with HPP at the age of 20 months. During prenatal ultrasonography, polyhydramnios and shortening of long bones were detected. After birth, he presented delayed motor development, repeated respiratory infections, and bone deformities. At the age of 2 years and 8 months, he started walking and had already lost his primary teeth. He had reduced levels of alkaline phosphatase (ALP), elevated levels of pyridoxal 5'-phosphate (PLP), and a p.Ala33Val (c.98C>T) missense mutation in homozygosis in the TNSALP gene. His parents and sister also had reduced ALP levels, high PLP levels, and the same mutation in heterozygosis. His father and sister were healthy, and his mother was diagnosed with rickets in childhood, which resulted in short physical stature and lower limb deformities. The patient was started on asfotase alfa at the age of 2 years and 10 months. After 2 years of treatment, he improved his motor skills, had no further episodes of severe respiratory infection, and showed improved radiological findings of rickets, without any severe side effect.

Hypophosphatasia

Hypophosphatasia (HPP) is a group of inherited disorders characterised by the impaired mineralisation of bones and/or teeth and low serum alkaline phosphatase (ALP) activity. It is caused by a mutation in the ALPL gene encoding the tissue-non-specific isoenzyme of ALP (TNSALP) resulting in a loss of function. The disease is highly heterogenous in its clinical expression ranging from stillbirth without mineralised bone to the mild form of late adult onset with symptoms and signs such as musculoskeletal pain, arthropathy, lower-extremity fractures, premature loss of teeth or an incidental finding of reduced serum ALP activity. A classification based on the age at diagnosis and the presence or absence of bone symptoms was historically used: perinatal, prenatal benign, infantile, childhood, adult and odontohypophosphatasia. These subtypes are known to have overlapping signs and complications. Three forms of HPP distinguishable by their genetic characteristics have been described: severe, moderate and mild. Severe forms of HPP (perinatal and infantile severe) are recessively inherited, whereas moderate HPP may be dominantly or recessively inherited. The biochemical hallmark of HPP is persistently low serum ALP for age and increase in natural substrates of TNSALP, pyridoxal 5'-phosphate and phosphoethanolamine supported by radiological findings. The diagnosis is confirmed by ALPL sequencing. A multidisciplinary team of experts is essential for the effective management. Calcium restriction is recommended in infants/children to manage hypercalcaemia. A targeted enzyme replacement therapy for HPP has become available and correct diagnosis is crucial to allow early treatment.

Tissue-Nonspecific Alkaline Phosphatase-A Gatekeeper of Physiological Conditions in Health and a Modulator of Biological Environments in Disease

Tissue-nonspecific alkaline phosphatase (TNAP) is a ubiquitously expressed enzyme that is best known for its role during mineralization processes in bones and skeleton. The enzyme metabolizes phosphate compounds like inorganic pyrophosphate and pyridoxal-5′-phosphate to provide, among others, inorganic phosphate for the mineralization and transportable vitamin B6 molecules. Patients with inherited loss of function mutations in the ALPL gene and consequently altered TNAP activity are suffering from the rare metabolic disease hypophosphatasia (HPP). This systemic disease is mainly characterized by impaired bone and dental mineralization but may also be accompanied by neurological symptoms, like anxiety disorders, seizures, and depression. HPP characteristically affects all ages and shows a wide range of clinical symptoms and disease severity, which results in the classification into different clinical subtypes. This review describes the molecular function of TNAP during the mineralization of bones and teeth, further discusses the current knowledge on the enzyme’s role in the nervous system and in sensory perception. An additional focus is set on the molecular role of TNAP in health and on functional observations reported in common laboratory vertebrate disease models, like rodents and zebrafish.

Investigation of alpl expression and Tnap-activity in zebrafish implies conserved functions during skeletal and neuronal development

Hypophosphatasia (HPP) is a rare genetic disease with diverse symptoms and a heterogeneous severity of onset with underlying mutations in the ALPL gene encoding the ectoenzyme Tissue-nonspecific alkaline phosphatase (TNAP). Considering the establishment of zebrafish (Danio rerio) as a new model organism for HPP, the aim of the study was the spatial and temporal analysis of alpl expression in embryos and adult brains. Additionally, we determined functional consequences of Tnap inhibition on neural and skeletal development in zebrafish. We show that expression of alpl is present during embryonic stages and in adult neuronal tissues. Analyses of enzyme function reveal zones of pronounced Tnap-activity within the telencephalon and the mesencephalon. Treatment of zebrafish embryos with chemical Tnap inhibitors followed by axonal and cartilage/mineralized tissue staining imply functional consequences of Tnap deficiency on neuronal and skeletal development. Based on the results from neuronal and skeletal tissue analyses, which demonstrate an evolutionary conserved role of this enzyme, we consider zebrafish as a promising species for modeling HPP in order to discover new potential therapy strategies in the long-term.

Craniosynostosis and metabolic bone disorder. A review

INTRODUCTION

Some metabolic bone disorders may result in the premature closure of one or more calvarial sutures during childhood, potentially leading to a cranioencephalic disproportion. The aim of this paper is to review the characteristics and consequences of craniosynostosis associated with metabolic disorder.

MATERIAL AND METHODS

A review of the literature on metabolic forms of craniosynostosis was performed.

RESULTS

The most common forms of craniosynostosis associated with metabolic bone disorder were isolated sagittal suture fusion with or without scaphocephaly, and sagittal suture fusion associated with coronal suture fusion (oxycephaly) or also with lambdoid suture fusion (pansynostosis). Synostosis may be well-tolerated, but in some subjects results in neurodevelopmental and functional impairment that is sometimes severe.

CONCLUSION

The impact of metabolic synostosis is very variable, depending on the specific underlying metabolic disease, with a large spectrum of morphological and functional consequences. Diagnosis should be early and management should be carried out by a multidisciplinary team with expertise in both rare skeletal disorders and craniosynostosis. The impact of emergent medical therapies recently developed for some of these diseases will be assessed by systematic coherent follow-up of international registries.