ORAL CHARACTERISTICS

2.1. Soft tissues
As stated in previous review papers, hypotonia of the orofacial muscles may result in several significant facial features. The borderline between primary and secondary orofacial pathology has been a matter of discussion as has the basis for therapeutic approaches^{16, 61} .
The upper lip is short and the angle of the mouth is pulled down with passive elevation of the upper lip. The lower lip becomes increasingly everted and fissured. An open mouth habit leads to mouth breathing, drooling, a chapped lower lip and angular cheilitis^{62, 64}.
Many different features have been attributed to the tongue of individuals with Down syndrome. Tongue protrusion during oral function and the persistence of primary lingual movements compromise fine neuromuscular motor function⁶⁵. In addition, abnormal pressure of the tongue on diastemas (unilateral) or on the teeth (bilateral) may cause the tongue to take a depressed, oval form that is circumscribed by a raised, white, scalloped border. Other characteristics, such as hypertrophy of the dorsal papillae, or a fissured, plicated or scrotal tongue, have also been described. These characteristics are probably due to developmental factors. Clinically, geographic tongue is a frequent finding. The midline junction is weak (lingual diastasis) as is the lingual frenulum^{64, 66}. Relative macroglossia is related to a small oral cavity⁶⁷ and anterior positioning, rather than to true macroglossia (originally thought to be of high incidence⁶⁸).

Hypotonia of the orbicularis, zygomatic, buccinator, masseter and temporaslis muscles, in addition to hyperlaxity of joints, results in poor masticatory function. Other oral functions such as sucking, blowing and speech are also affected^{64, 69} . Soft palate insufficiency may be present as a consequence of abnormal behaviour of the buccinator mechanism17.
The lining of the oral mucosa in early life is thin due to a reduction in salivary flow^{3, 70}. Mouth breathing may also contribute to dry the anterior oral mucosa and leads to gingival and respiratory tract infections3. Nasal mucosa may be thickened.

Enlarged tonsils and adenoids are often found in the child with DS⁷¹. This may be due to immune deficiency and an increased incidence of ear, nose and throat infections. Surgical treatment is often indicated to prevent chronic mouthbreathing and sleep apnoea⁴².
Facial appearance is not only altered by the diminutive midface, but also by short palpepral fissures, hypertelorism, and short, low ears. The nasal root is wide but the nose tends to be small and soft⁷². Obviously the degree of characterisation varies greatly between individuals, but the typical facies of Down syndrome is widely recognisable.

2.2. Hard tissues
Persons with Down syndrome tend to present brachycephaly with a flattened occiput and a flattened, short cranial base⁷³. The angle of the mandible may be increased with relative prognathism^{74, 75} , although this is not reported by all authors⁷³. The midface is hypoplastic and underdeveloped, including the ethmoid bone and frontal sinuses⁷³. The palate in people with Down syndrome has been described as being significantly reduced in length and width⁷⁶, which gives a “stair palate” appearance, often with high cleft-like folds at its base⁶⁴. Furthermore, tongue dysfunction retards functional development by reduced muscular pressure on the palate during swallowing or at rest⁷⁷. These problems tend to increase with age⁷⁸. Cleft lip and palate and bifid uvulae have been reported with a prevalence of 0.7% compared to 0.04% in the general population^{79, 80} . Nasal air passages are usually narrow and may be partially obstructed by septal deviation, and the paranasal sinuses tend to be hypoplastic⁷³. Articular hyperlaxity in persons with Down syndrome includes the temporomandibular joints and postural chronic luxation may result⁸¹.

2.3. Dental characteristics
The teeth of persons with Down syndrome may show certain characteristic defects of shape and size⁸². Taurodontism is frequent (enlarged crown and apically displaced bifurcation or trifurcation of the roots). 55.8% of subjects with Down syndrome were found to have taurodont teeth in a radiographic study by Rajic⁸³, and 36.4% were found to have taurodont lower molars on extraction by Bell et al⁸⁴. Other dental crown variants have been noted, such as shovel-shaped incisors, peg-shaped incisors, missing or reduced marginal ridges on posterior teeth or paralabial tubercles⁸⁵. Townsend found that 90% of 147 dental models taken from children and adults with Down syndrome showed one or more dental crown variants⁸⁵. The permanent teeth in persons with Down syndrome have been found to be smaller than those of controls86 and to have a decreased root to crown ratio⁸⁷. There are also reports of an increased incidence of enamel hypoplasia in the Down syndrome population, although this is thought to be due to episodes of ill health concurrent to dental development rather than to the underlying chromosomal disorder⁸⁸

Hypodontia is found in 39 to 63 % of persons with Down syndrome depending on the study^{89, 90} . Teeth most often absent are the third molars, lateral incisors and second premolars, with the incidence of tooth agenesis being greater for the maxilla in patients with an Angle’s class III occlusion⁹⁰.

Tooth eruption has been found to be delayed in children with Down syndrome compared to that of controls⁹¹. This is particularly true for those teeth that erupt last in the sequence (premolars, canines and second molars)⁹². The sequence of tooth eruption does not seem to be greatly affected. In some cases, tooth eruption may be blocked due to crowding and underdevelopment of the maxilla. 15% of individuals with Down syndrome were found in one study to have canine impaction and a further 15% to have maxillary canine/first premolar transposition⁹³.

2.4. Dental caries
Caries rates are generally considered to be lower in children and young adults with Down syndrome than in the population as a whole or than in other groups of persons with intellectual disability^{94, 96} . Morinushi et al⁹⁷ suggested the possibility that serum antibodies against Streptococcus mutans might be protective and responsible for the reduced caries rate seen in the primary dentition of children with Down syndrome. 84% of young subjects with Down syndrome were found to be free from caries in one study by Stabholz et al⁹⁴. These results are likely to be due to lack of autonomy in food selection in the young Down syndrome population, which may limit sugar intake. Young adults with Down syndrome are also often under nutritional restriction to prevent obesity, at least in Western countries, and this may limit the intake of sweet foods. The results may also be skewed due to the frequent incidence of hypodontia and later eruption of the teeth. Unfortunately, very little data exists regarding the incidence of caries in the adult population with Down syndrome and the clinical impression is that the caries rate increases with age and autonomy^{22, 98} . Adult persons with Down syndrome are more likely to have easy access to sweet foods throughout the day than youngsters or institutionalised individuals. They may also generally choose to eat softer, more cariogenic foods due to reduced masticatory function and missing teeth⁹⁹. “Self-cleansing” or the removal of food debris by the tongue and perioral musculature is reduced due to hypotonia and unsupervised toothbrushing is not effective in those persons with reduced psychomotor function⁹⁹. Food stagnation is thus likely to create a favourable milieu for cariogenic bacteria and to increase the overall incidence of caries.

It is important to note that despite a generally low caries incidence in youngsters with Down syndrome, nursing caries is a real problem for some infants. Behavioural difficulties and/or feeding problems may mean that a bottle with sweetened drink is left to the child continuously as a pacifier or to supplement dietary intake. Infants with Down syndrome are generally weaned later than their peers and thus the dangers of bottle-feeding are prolonged. In one Australian study it was found that infants with Down syndrome were more likely to take a bottle to bed than controls (40% compared to 12%) and were more likely to be weaned at over 18 months of age (56% compared to 24% of controls)¹⁰⁰.

2.5. Periodontal disease
Periodontal disease is a major oral health problem for persons with Down syndrome as they are prone to an early-onset, aggressive form of the disease¹⁰¹. Periodontal inflammation is noted from a very early age^{102, 103} and in one study 94% of subjects with Down syndrome aged 16 to 20 years already had periodontal disease¹⁰⁴. The disease type is similar to that seen in juvenile periodontitis^{105, 106} and the bacterium Actinobacillus actinomycetemcomitans is also implicated. This organism has been detected in 35% of subjects with Down syndrome compared to only 5% of controls¹⁰⁷. Immunological deficiency with anomalies in the specific and non-specific host mechanisms^{27, 108-113} leads to aggressive inflammation that is far greater than would be expected for the quantity of plaque detected^{103, 114} . Poor oral hygiene, reduced masticatory function and lack of “self-cleansing” aggravate the accumulation of plaque²². Another important factor in the anterior region of the mouth is the frequent incidence of mouthbreathing that reduces the protective function of the saliva and leads to mucosal inflammation. Without treatment, progression of the disease is rapid and leads to tooth mobility and premature tooth loss. However, in a 5-year follow up study by Saxén et al^{115, 116} , periodontal bone loss was recorded and compared to a control group of persons with intellectual disability that lived in the same institution. The disease progressed more rapidly in the control group for this study.

2.6. Occlusal characteristics
Jaw relationships. The underdeveloped maxilla and a protruding mandible often results in mandibular overjet which should be carefully assessed in order to determine whether a malocclusion exists or if it is a postural position. A longitudinal cephalometric study of twelve individuals with Down syndrome suggested differences in growth tendencies both in the maxilla and the mandible between these patients and control subjects^{117, 118} .

Previous radiographic studies had shown that the combination of a more obtuse mandibular angle¹¹⁹ and the underdevelopment of the facial midthird leads to this abnormal jaw relationship. These results vary from those obtained by other authors^{73, 120-121} who concluded that growth anomalies of the cranial base are mainly responsible for the abnormal relationship, as it also influences mandibular growth. Gonial angle was reported normal according to these authors’ findings.
The discrepancy between the maxillary and mandibular arches in untreated adults with Down syndrome leads to a situation where interdental contact may either be absent or unstable. The patient unconsciously tries to find a stable mandibular rest position, either by grinding the teeth or by adopting extreme postural positions, often with luxation of the temporomandibular joints. The tongue may be positioned interdentally to stabilise the mandible and create an anterior seal. Parafunctional movements result and care should be taken that dyskinesia is not assumed to be related to the degree of intellectual deficiency alone81.
Malalignment. Malalignment is frequent in the dentitions of people with Down syndrome and is usually related to crowding in the upper jaw¹²². Crowding is due to the underdevelopment of the palate and facial midthird. In some cases, this lack of space may be compensated by a high prevalence of tooth agenesis, specially the upper lateral incisors¹²³.

Malocclusion. Despite different criteria used to classify malocclusion in patients with Down syndrome, most authors suggest that there is a high prevalence of mesial molar occlusion, anterior and posterior cross bite, anterior open bite, mandibular over jet and sagittal malocclusion^{124, 127} .

A wide variety of percentages have been given for each diagnosis, depending on the origin or type of report. There are many factors that play an important role and should be taken into account when assessing malocclusion: mouth breathing, orofacial musculature dysfunction, jaw relationships, temporomandibular joint dysfunction, delayed eruption and congenital tooth agenesis are the main examples3. Ethnic or racial features¹²⁸, as well as cultural and environmental factors⁶, have been reported to be responsible for differences in analysis between Latin American and European children with Down syndrome. Oreland et al¹²⁵ suggest that the most severe malocclusion is found in those with greater intellectual deficiency.

2.7. Functional alterations and parafunctions
Respiration. An underdeveloped facial midthird, reduced nasal passages and large tonsils often lead to mouth breathing²², with an open mouth posture and anterior tongue position. The primary function of nasal respiration must be in place if the subsequent development of suckling, swallowing, feeding and speech functions is to be normal.
Feeding. The development of feeding patterns evolves from suckling until full masticatory ability is reached. This process is a sequence where every step has a strong influence on the development of the next step. As previously stated, infants with Down syndrome present generalised hypotonia at birth. Breast feeding may be threatened by that hypotonia and mothers should be encouraged to persist with feeding by means of helpful strategies and exercises applied to the orofacial musculature¹²⁹. Breast-feeding has been largely promoted by the WHO for its positive effect on general health and on the development of the oral structures.
Swallowing patterns are strongly influenced by tongue movements. Creation of intraoral pressure is inefficient in Down syndrome because of a poor anterior seal, and depression of the tongue is difficult to achieve due to poor tonicity. The bolus tends to be moved backward in vertical movements that increase tongue thrust and mimic the primary swallow reflex^{130, 131} . Such difficulties are observed to increase in adulthood, compromising not only oral but general health¹³². Drooling is a common consequence of swallowing difficulties¹²⁶ and silent aspiration¹³¹may be related to frequent chest infections.
There are two components that lead to abnormal chewing function: perceptual alteration and muscular dysfunction. Several authors have reported a failure to progress through the normal sequence of food textures and, particularly, the refusal of hard textures^{133, 137} . In one study, 38% of persons with Down syndrome over 8 years of age were reported as being unable to eat all types of food and 24% spilt food on eating¹³⁷. Dysfunctional buccinator mechanism results in a poor intraoral pressure and food remains between the cheeks and the vestibular surfaces of the teeth. In addition, the tongue is inefficient in lateralising food for chewing in the molar region^{138, 139} , therefore, mastication is impaired with excessive vertical opening. Muscular control of the mandible is insufficient to manage small vertical movements and food may be lost due to lack of lip closure¹³⁶. Co-ordination between the tongue, lips and muscles of mastication is poor and sequencing is arrhythmic¹³⁶.
A third factor limiting masticatory function is related to orofacial features, such as a small oral cavity, abnormal development of the jaws, tooth agenesis and temporomandibular joint dysfunction⁸¹. These anatomical considerations may create a difficult setting for normal development of masticatory function. Reduced masticatory function leads to a feeding pattern that includes soft textured and cariogenic food¹³³.
Speech. Abnormal development of oral function in people with Down syndrome is directly related to structural features of the chromosomal aberration. The underlying pathology described in the tongue, palate and vault, facial muscles, jaw relationships, tooth position, shape and number, usually compromise normal articulation of speech^{3, 17}. Persistence of primary tongue movements over 5 years of age has been observed in 100% of 22 individuals with Down syndrome compared to a gender and age matched control group⁶⁵. Primary movements, when still present in the older infant, have been reported to affect the articulation of dento-lingual and lingual-palatal sounds which require tongue vibration⁶⁵.
Intellectual deficiency, aphasia, poor oral closure, hearing problems and orofacial characteristics are suggested causes for the delay of expressive language. Nevertheless, children with Down syndrome have been found to have a particular way of talking and building sentences that suggests specific neurological alteration in those central areas where complex movements are produced for expressive language¹⁴⁰.
Bruxism. Bruxism can be defined as an oral parafunction associated with diurnal or nocturnal grinding or clenching of teeth of multifactorial aetiology. An increased incidence of this parafunction has been observed in individuals with Down syndrome as many of the aetiological factors are present in this population (anxiety, dental malocclusion, neurological hyperactivity and the action of certain neurotransmitters)¹⁴¹. It is suggested that this habit may develop in an attempt to find a position of comfort and to eliminate occlusal interference due to abnormal development of the jaws⁸¹. This situation is facilitated due to the laxity of temporomandibular joint ligaments²². Other parafunctional facial movements (dyskinesias) are also encountered in both children and adults with Down syndrome⁸¹.