The effects of craniectomy compared to cranial vault remodeling on morphological, functional and neurological outcomes in infants with isolated non‐syndromic sysnostosis of the sagittal suture: a systematic review protocol

Review question/objective The objectives of this review are to identify the effectiveness of craniectomy on morphological, functional and neurological outcomes in isolated sagittal synostosis. More specifically, the objectives are to identify: The effectiveness of craniectomy compared with cranial vault remodeling on morphological, functional and neurological outcomes in human infants with isolated non‐syndromic sagittal synostosis. Background The human cranium consists of the bones comprising the base of skull and the calvarial vault. The latter includes the frontal, parietal, temporal, sphenoid and occipital bones. Cranial 'sutures’ are defined as “a craniofacial articulation in which contiguous margins of bone approximate each other and are united by a thin layer of fibrous tissue”.1(p11) Sutures allow the necessary overlap of the calvarial bones during the passage of the infant through the birth canal, and facilitate ongoing growth of the skull secondary to brain development. The sagittal suture lies in the center, separating left and right parietal bones. Over time, sutures differentially close through a process of bony bridging. In the human, closure of the sagittal suture begins at around 22 years of age.1 Craniosynostosis, or cranial synostosis, may occur as a primary condition either in isolation or in its syndromic form with other congenital defects. Alternatively, synostosis may be secondary to intracranial, metabolic, teratogenic or hematological causes.2 It is characterized by premature closure of one (simple) or more (compound) of the cranial sutures.3,4 This review will only consider primary non‐syndromic cases. Syndromic craniosynostosis is uncommon, comprising only 20% of cases and will not be included in this review.4 Most cases of non‐syndromic craniosynostosis are sporadic. Whilst potential risk factors have been identified, the etiology is unknown.4 Familial non‐syndromic craniosynostosis is transmitted as an autosomal dominant disorder and comprises 2‐6% of infants with sagittal synostosis.4 Premature fusion of suture lines causes characteristic changes to the morphology of the calvarium. Earlier understanding was based largely on the 19th Century writings of Virchow, whose law dictated that growth was restricted in a plane perpendicular to the affected suture.5 Over time, this simplistic view has been refined and it is now known that while growth is predominantly restricted in this plane, compensatory mechanisms take place in the unaffected sutures.6 Later authors have described the mechanisms causing the typical morphological changes. This includes compensatory bone deposition in sutures adjacent to the affected suture and asymmetric bone deposition in a direction away from the bone plate.6 This allows compensatory growth of the skull in an unrestricted direction. The sagittal suture is the most commonly affected, forming a reported 40‐60% of all cases of cranial synostoses.4 Isolated sagittal suture synostosis causing scaphocephaly is the most common presentation of craniosynostosis to craniofacial units.7 The birth prevalence of sagittal synostosis has been estimated at 190:1,000,000.8 Whilst the condition occurs in all populations, this figure relates to one human (study) sample and may vary depending on the population. Scaphocephaly is a term first introduced by Baer in 1860, and literally means 'skiff’ or ‘keel‐shaped’ head.9 Fusion of the sagittal synostosis results in limited medial‐lateral skull growth and therefore narrowing of the bi‐temporal dimension. With ongoing anterioposterior growth there may be development of frontal and occipital bossing and a ridged sagittal suture line, palpable on examination.4 This causes a narrow elongated skull ‐ the characteristic ‘boat’ appearance of the scaphocephaly ‐ with a decreased cephalic index (CI). Cephalic index is a standardized measure of head growth of the infant skull, calculated using the ratio of the short and long axis of the head (short axis / long axis x 100).10 It can be calculated using direct measurement and/or radiologically.7 Where cephalic index is greater than 70, it is unlikely there will be any noticeable deformity; while a cephalic index of less than or equal to 66 is usually noticeable.11 However, there are variations in morphology depending on differential closure of the suture, which has led to variations in practice.12 In cases in which craniosynostosis occurs, the process if commonly evident at birth. It predominantly affects male infants, with a male to female ratio of 4:1.13 It is usually diagnosed during the first few months of life as a cranial deformity. Diagnosis is based on clinical assessment, physical examination and radiological studies, including plain x‐ray and/or computed tomography (CT). Plain radiography may be sufficient in diagnosis of single suture craniosynostosis.4 It is important to differentiate between a cranial deformity that is due to synostosis as the underlying mechanism and one which is secondary to position, e.g. in the case of positional plagiocephaly. Other appropriate investigations at the time of diagnosis may include genetic screening and neurodevelopmental analysis.14 Sagittal synostosis has traditionally been considered a surgical problem with surgical solutions. Like surgical technique, indications for surgery have evolved over time.12 Earliest reports suggested that surgical treatment was needed to prevent brain injury due to restricted growth. More traditionally, the main indicator for operative intervention was based on cosmesis, i.e. correcting the physical deformity in early childhood to prevent stigmatization later in life. It has previously been demonstrated that in general, children who do not undergo surgery for management of scaphocephaly have greater psychosocial sequelae compared with those who did, and head shape certainly played a significant role.11 Eighty percent of brain growth occurs in the first two years of life and most of that occurs in the first 12 months. Therefore the other rationale for earlier intervention is that rapid brain growth in early infancy could reconfigure the skull towards a more normal morphology after correction of the synostosis.12 Surgery should therefore be undertaken within the first 12 months of life.12 Somewhat similarly, other authors have reported that surgery should be within three to nine months, ideally in the first six months, with the intention of preventing further progression of the deformity and the potential for complications associated with raised intracranial pressure (ICP).14 All craniosynostoses can cause an increase in ICP; even if it is only one suture that is involved. Raised ICP may itself be an indicator for surgery, with or without neurological and developmental impairment. It has been proposed that raised ICP is the cause of the detrimental neurological effects observed in synostosis, as opposed to the synostosis itself.12 This acts as a potential confounding factor in this area of research as it may be difficult to separate neurological complications as a result of synostosis and/or raised ICP. Compounding this is the fact that there is no true ‘normal’ range of ICP in pediatric patients. Techniques used in the surgical management of non‐syndromic sagittal synostosis have varied and evolved over time and differ between centers of care. The earliest and most conservative technique of treating synostosis involves excision of the diseased suture; this is known as strip craniectomy or suturectomy. Over time various adjuncts have been trialed, including interposition of the cut edges with synthetic material. In recent times, endoscopy has been used to the same effect. Alternatively, surgery may involve remodeling of the cranial vault through various methods, designed not only to excise the diseased suture, but also to definitively treat the associated cranial abnormalities, e.g. frontal bossing.4,7 The exact method of cranial vault remodeling may vary between centers. However, all are based on the principle of not only resection of the affected suture, but removal and re‐contouring of the cranial bones before replacing them, in an attempt to create a favorable head shape. Other recent advances include spring‐mediated cranioplasty, where springs are utilized in remodeling the growing skull. This technique will not be discussed within the confines of this review, as there is little consistency in its application between centers and this heterogeneity makes it difficult to draw comparison. Surgery in all areas continues to evolve as new techniques and new technology come to light. In the area of synostosis surgery it has come full circle; from limited craniectomy and excision of the diseased suture, to remodeling of the cranial vault to varying extents, and now back to limited procedures with various adjuncts such as helmet therapy. However lack of longitudinal comparison between these procedures limits the surgeon's ability to select the appropriate procedure for a particular patient and advise one procedure over another in terms of outcomes and complications.12 As such this systematic review will investigate the difference in postoperative outcomes between those patients who undergo limited craniectomy for primary non‐syndromic sagittal synostosis and those who are managed with cranial vault remodeling. A preliminary search of the Joanna Briggs Library of Systematic Reviews, Cochrane Library, PubMed, CINAHL, DARE and PROSPERO have shown there is currently no systematic review (or one underway) on this topic.