Down syndrome, a genetic condition resulting from an extra copy of chromosome 21, presents a range of characteristics, including distinct features of the skull and face. These craniofacial traits are a direct result of developmental differences that begin early in gestation. This article covers the typical skull and facial anatomy associated with Down syndrome, its developmental origins, methods of identification, and related health considerations.
Common Skull and Facial Traits in Down Syndrome
The skull in individuals with Down syndrome often exhibits brachycephaly, which describes a head that is shorter and broader than average. This is accompanied by a flattened occiput, meaning the back of the skull is less rounded. The overall size of the skull may approach that of a typical adult, but the proportions are different.
The facial structure is characterized by midface hypoplasia, where the central part of the face, including the upper jaw and cheekbones, is underdeveloped. This leads to a retruded, or set back, midface and a smaller maxilla (upper jaw bone). This underdevelopment also contributes to a flattened nasal bridge, giving the nose a small appearance.
Other common features involve the eyes, ears, and mouth. Many individuals have upslanting palpebral fissures, an upward slant of the eyes. Small skin folds on the inner corner of the eyes, known as epicanthal folds, are also frequently observed. The ears may be smaller and set lower on the head, while a small oral cavity combined with a relatively large tongue can result in the tongue protruding.
The mandible, or lower jaw, is also smaller, which contributes to a reduction in the lower anterior face height. Despite these reductions in bone size, dental characteristics can include proclined, or forward-tilting, upper and lower incisors. This dental positioning can lead to an anterior open bite, where the front teeth do not meet when the jaw is closed.
Genetic and Developmental Origins of Skull Differences
The craniofacial structure in Down syndrome originates from the presence of an extra copy of chromosome 21, known as trisomy 21. This means every cell in the body has three copies of this chromosome instead of two. This “gene dosage” imbalance disrupts embryonic development, as genes on chromosome 21 influence bone and cartilage growth.
Research has pinpointed specific genes on chromosome 21 that contribute to these developmental changes, such as DYRK1A. Studies using mouse models that mimic trisomy 21 have shown that a third copy of DYRK1A leads to a reduced number of cells in the bones of the face and front of the skull. This alteration hinders the growth of neural crest cells, which are foundational for forming these bones.
This genetic disruption causes significant hypoplasia, or underdevelopment, in many craniofacial features. This widespread impact explains why so many different features are affected, from the cranial base to the maxilla and mandible. The altered genetic instructions also lead to changes like the premature fusion of cartilaginous joints at the base of the skull, altering its final shape.
Identifying Craniofacial Markers
The features of the skull and face in Down syndrome are often identified by healthcare professionals through clinical examination after birth. A combination of traits like a flattened facial profile, small nose, and brachycephaly are indicators that prompt genetic testing to confirm the diagnosis. The presentation of these features can vary, and no single trait is sufficient for a diagnosis on its own.
Prenatal identification of these markers is also possible through ultrasound imaging. During prenatal screenings, an increased nuchal translucency, a fluid-filled space at the back of the fetal neck, is a well-known early marker. Other sonographic findings can include a flattened facial profile, a hypoplastic (underdeveloped) or absent nasal bone, and evidence of brachycephaly.
Cephalometric analysis, a technique involving measurements from radiographic images of the head, can be used to precisely quantify these differences. Such analyses have confirmed findings like reduced anterior and posterior face heights and a smaller maxilla and mandible in individuals with Down syndrome compared to the general population.
Health Implications and Supportive Care
The craniofacial anatomy in Down syndrome can lead to several health challenges. The underdevelopment of the midface and a smaller oral cavity can create narrower airways, increasing the risk of obstructive sleep apnea. This condition involves repeated pauses in breathing during sleep and may require intervention, such as a continuous positive airway pressure (CPAP) machine or surgical procedures.
Feeding difficulties can be present in infancy due to a combination of factors, including a small mouth, a relatively large tongue, and lower muscle tone. These challenges may require support from feeding specialists or the use of specialized bottles or techniques. The structure of the head and Eustachian tubes also leads to an increased frequency of ear infections (otitis media), making regular audiology assessments important to monitor hearing.
Dental issues are also common and directly related to the jaw and skull structure. Malocclusion, or a poor bite, is prevalent, often presenting as a Class III malocclusion where the lower jaw is positioned forward relative to the upper jaw. Other common dental findings include delayed eruption of teeth, congenitally missing teeth (hypodontia), and teeth that are smaller than average (microdontia). Early and consistent dental and orthodontic care is beneficial for managing these conditions and planning for any necessary treatments.