Notes
Skull Development & Developmental Anomalies (eg Craniosynostosis)
Sections
Skull Development & developmental anomalies
Introduction
Skull Malformations
- Macrocephaly, Megalencephaly, and Microcephaly.
Craniosynostosis
Premature closure of the cranial sutures.
- Disorders of a single suture: Trigonocephaly, Plagiocephaly, Scaphocephaly, and Brachycephaly.
- Disorders that involve multiple suture synostoses.
skull malformations
Macrocephaly
- Cranial enlargement (to > 98% of normal range). Although it can be due to enlargement of any of the 3 brain compartments: brain tissue, CSF, or blood, it's most commonly due to obstructive hydrocephalus – enlargement of the CSF ventricles. It typically necessitates ventriculostomy or shunting.
Megalencephaly
- Generalized cranial enlargement of the gray and white matter of the brain due to either anatomic abnormalities (eg, neurocutaneous disorders) or metabolic abnormalities (eg, lysosomal storage disorders or leukodystrophies).
- Megalencephaly is technically a form of macrocephaly, since, indeed it involves enlargement of the cranium – but, again, the majority of cases of macrocephaly are NOT generalized brain matter enlargement but rather due to hydrocephalus.
Microcephaly
- A generalized abnormally small cranium (to < 98% of normal range), either due to a primary genetic cause (ie, a chromosomal or metabolic abnormality (eg, phenylketonuria) or an acquired condition (eg, perinatal infection).
normal skull anatomy and development
Neurocranium
Divides into the following:
- Cranial vault, which provides a roof for the brain.
- Skull base, which provides a floor for the brain.
Viscerocranium
- Comprises the facial bones.
skull development
The cranial vault and viscerocranium develop via intramembranous ossification (again, which has no intermediate cartilaginous model).
The skull base develops via endochondral ossification, which develops via a cartilaginous matrix.
Cranial Vault
- Frontal bone, Parietal bone, upper portion of the Occipital bone, and Squamous portion of the Temporal bone – all of these develop via intramembranous ossification.
Skull Base
- The lower portion of the Occipital bone, the Petrous portion of the Temporal bone, and the Sphenoid bone – all of these bones develop via endochondral ossification.
- Key viscerocranial bones:
The Zygomatic bone and Maxilla and Mandible – they develop via intramembranous ossification. - The bones of the face derive from embryonic cells from the pharyngeal arches (from neural crest cells (other than the laryngeal cartilages, which derive from mesoderm)).
Skull Sutures
At birth, the skull has openings (sutures) to accommodate brain growth, because the cranial vault ossifies early via intramembranous ossification. These sutures allow the fetal skull to ossify quickly prior to delivery [via intramembranous ossification] (so the brain doesn't get squashed) and yet still accommodate skull distortion during birth (called, molding) and permit rapid brain growth during the first two years of life when the brain quadruples in size to 75 percent of its adult volume!
Sutures
- Metopic - forms between midline aspects of the left/right aspects of the frontal bone.
- Coronal - lies between the frontal and parietal bones.
- Sagittal - lies between the bilateral parietal bones.
- Lambdoid - lies between the parietal bones and occipital bone.
- There are three additional minor sutures: frontonasal, temporosquamosal, and frontosphenoidal.
fontanelles
Fontanelles are the large openings that exist between bones in the newborn calvarium.
Key bones
- The frontal bone covers the majority of the anterior frontal lobes.
- The parietal bones cover the remainder and the parietal lobes.
- The occipital bone covers the occiput.
Anterior fontanelle
- Forms at the junction of the sagittal, coronal, and metopic sutures at the anterior of the skull – it's palpable in midline, just behind the forehead; it closes at 1.5 to 2 years of age.
Posterior fontanelle
- Forms from the intersection of the sagittal and lambdoid sutures; it closes at 3 - 6 months of age.
Craniosynostoses
Craniosynostosis is premature cranial suture closure (synostosis).
Scaphocephaly
- The most common type of synostosis; accounts for half of the incidences of synostosis each year.
- There is synostosis of the sagittal suture – the skull is shaped like the narrow hull of a boat (the derivation of its name).
- In accordance with Virchow's law, the interruption of brain growth is in perpendicular to the plane of the synostosis – thus the abnormal brain growth is in parallel to the synostosis. The skull elongates (in parallel to the synotic suture) – this results in an elongated, narrow skull.
- The term dolichocephaly (elongated head) is either used synonymously scaphocephaly or as a broader catch-all for elongated head.
Brachycephaly
- Results from bicoronal synostosis.
- The skull cannot develop normally along the sagittal plane and we show, instead, that it manifests with a wide, short skull – again, in accordance with Virchow's law, the skull develops in parallel to the plane of the synostosis.
- In unilateral coronal synostosis (as opposed to bilateral), also results in plagiocephaly because, as we can imagine, it results in a twisting/oblique appearance (but here of the frontal calvarium).
Trigonocephaly
- Secondary to metopic synostosis, which results in a failure of frontal outward development.
- It manifests with a pointed forehead… the eyebrows may appear "pinched."
- To help link the name to the shape, we show that trigonocephaly results in a triangular shaped head, when viewed from above.
Lambdoid synostosis
- Posterior plagiocephaly (a twisted skull) due to an inability of a side of the occiput to grow outward, thus there is an oblique oblique deformity of the posterior calvarial vault.
Descriptive Terminology for Multiple Suture Synostoses
Cloverleaf Deformity (aka Kleeblattschädel)
- Multiple sutures fuse prematurely – unfortunately, the brain actually can grow through the anterior fontanelle, can be a finding of certain genetic syndromes as described below.
SYNDROMES
FGFR2 gene (Fibroblast Abnormality)
Apert Syndrome
- Multiple suture synostoses w/possible cognitive delays
- Syndactly vs Polydactly
- Possible hearing loss, hyperhidrosis, spine fusion, oro-palatal malformations
- Genetics: Autosomal dominant, FGFR2 gene abnormality, which is important in fibroblast embryonic development of bone.
Crouzon Syndrome
- Multiple suture synostoses w/o cognitive delays
- Possible hearing loss and oro-palatal malformations
- Genetics: Autosomal dominant, FGFR2 gene abnormality, which is important in fibroblast embryonic development of bone.
Pfeiffer Syndrome
- Multiple suture synostoses w/anywhere from no neurologic complications to significant complications.
- Syndactly vs Brachydactly
- Possible Anykylosis (bone fusion at the joints)
- Genetics: Autosomal dominant, FGFR2 gene abnormality, which is important in fibroblast embryonic development of bone.
RAB23 or MEGF8 gene (vesicle transport)
Carpenter syndrome
- Multiple suture synostoses w/possible cognitive delays
- Brachydactly vs Polydactly vs Syndactly
- Cryptorchidism
- Kyphoscoliosis
- Genetics: Autosomal recessive, RAB23 or MEGF8 gene mutations, which is important in vesicle transport.
Genetic Information from: https://ghr.nlm.nih.gov