Apert Syndrome: Comprehensive Imaging Guide and Clinical Overview
Apert syndrome, also known as acrocephalosyndactyly type I, is a rare genetic disorder characterized by premature fusion of cranial sutures (craniosynostosis) and syndactyly of the hands and feet. This condition typically presents at birth with distinctive craniofacial and limb anomalies, prompting early imaging evaluation. The genetic basis involves mutations in the FGFR2 gene, leading to abnormal bone development. Radiologists play a crucial role in characterizing the extent of skeletal involvement and identifying potential complications.
Patients are often imaged due to the visible physical deformities, particularly the characteristic skull shape and fused digits. Imaging is essential for surgical planning, monitoring disease progression, and detecting associated intracranial abnormalities. The typical age at presentation is infancy, with genetic inheritance patterns being autosomal dominant, though most cases arise from de novo mutations.
Key Imaging Features
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CT of the skull demonstrates bicoronal synostosis, leading to a tall, pointed skull (acrocephaly) with a flattened posterior vault and a broad forehead.
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CT reveals midface hypoplasia, characterized by a retruded maxilla, shallow orbits, and a depressed nasal bridge, often described as a “beak-like” nose.
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CT or X-ray of the hands shows severe syndactyly, most commonly involving the middle three digits (index, long, ring) and the fourth webspace, with varying degrees of bony fusion (synostosis).
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X-rays or CT of the feet demonstrate syndactyly, typically less severe than in the hands, often affecting the second to fifth toes.
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MRI can reveal intracranial complications such as hydrocephalus, Chiari malformations, or venous sinus abnormalities secondary to craniosynostosis, with abnormal dural venous sinus patterns noted in up to 30% of cases.
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Early imaging (infancy) may show a large anterior fontanelle with a midline calvarial defect that coalesces over time without suture formation.
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Follow-up CT scans are crucial to assess the effectiveness of surgical interventions for craniosynostosis, such as fronto-orbital advancement or posterior vault distraction, and to monitor for recurrence.
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MRI sequences like DWI may be used to evaluate for potential ischemic insults if neurological compromise is suspected, though this is not a primary feature of the syndrome itself.
Pathophysiology
The underlying pathophysiology of Apert syndrome stems from mutations in the FGFR2 gene, which encodes a receptor tyrosine kinase critical for skeletal development. These mutations lead to constitutive activation of the receptor, promoting premature differentiation and fusion of osteoblasts. This aberrant signaling disrupts the normal process of suture separation and bone growth during fetal development.
The premature fusion of cranial sutures, particularly the coronal sutures, restricts brain growth in specific directions, leading to the characteristic brachycephalic or turricephalic skull shape. Compensatory growth at other sutures and the skull base can result in increased intracranial pressure. The midface hypoplasia is a direct consequence of impaired growth of the first branchial arch derivatives. Similarly, the syndactyly of the digits arises from a failure of programmed cell death (apoptosis) in the interdigital mesenchyme during limb development.
Differential Diagnosis
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Pfeiffer syndrome: Similar craniosynostosis and midface hypoplasia, but typically has less severe syndactyly (often affecting only the first and second digits or first webspace) and may have more prominent thumb abnormalities (broad thumbs, clinodactyly).
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Crouzon syndrome: Characterized by craniosynostosis and midface hypoplasia, but lacks the significant limb anomalies (syndactyly) seen in Apert syndrome.
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Saethre-Chotzen syndrome: Features craniosynostosis, often unilateral coronal, with ptosis and characteristic facial asymmetry; syndactyly is typically mild or absent.
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Carpenter syndrome: Presents with craniosynostosis (often trigonocephaly or cloverleaf skull), short limbs, and syndactyly, but typically has distinct facial features like hypertelorism and a short philtrum, and often associated polydactyly.
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Isolated syndactyly: While syndactyly is a hallmark of Apert syndrome, isolated syndactyly without craniosynostosis or other craniofacial anomalies is a distinct entity.
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Muenke syndrome: Characterized by craniosynostosis (often coronal) and hearing loss, with variable limb anomalies that are generally less severe than in Apert syndrome.
Imaging Protocols and Techniques
The initial imaging modality of choice for evaluating the bony structures in suspected Apert syndrome is typically a non-contrast CT scan of the head and orbits. This provides excellent detail of the cranial vault, sutures, and facial bones, essential for surgical planning.
For assessing intracranial complications such as hydrocephalus or venous sinus abnormalities, MRI of the brain with and without contrast is recommended. Specific sequences like FLAIR and DWI are useful for evaluating brain parenchyma. X-rays of the hands and feet can be used for initial assessment of syndactyly but CT offers superior detail for bony fusion.
A critical technical pitfall is inadequate slice thickness or field-of-view on CT, which can obscure subtle bony fusions or cranial base abnormalities. For follow-up imaging after surgical correction of craniosynostosis, serial CT scans are performed at intervals determined by the surgical team, typically to assess bone healing, cranial vault remodeling, and to monitor for signs of re-ossification or increased intracranial pressure.
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