## 1 Introduction  
It is well established that craniofacial growth is a complex phenomenon determined by genetic and environmental factors. Identifying and eliminating variables that may determine skeletal deformities by altering normal facial growth are considered some of the primary goals of the orthodontisti [1]. According to [PERSON] Functional Matrix hypothesis, functional needs and soft tissues are important determinants of craniofacial morphology [2]. Breathing, swallowing, breastfeeding, and digit or pacifier-sucking habits play an important role in the growth of the craniofacial skeleton [3; 4; 5].  
The effect of prolonged mouth breathing on craniofacial growth continues to be a debated issue in the orthodontic literature. Some authors sustain that a prolonged mouth-breathing habit can be considered a risk factor for malocclusion as it can interfere with normal facial growth [4; 6]. Mouth breathing in children can lead to adaptations in the stomatognathic system that include changes in the position of the tongue and lips (lowered tongue position and incompetent lip seal at rest), posteroinferior rotation of the mandible,elongated face, mentalis muscle hypertrophy, and other dental and skeletal alterations [7]. The normal function of the nose can also be impaired by some anatomical and pathological conditions. A child affected by these conditions may be constrained to breathe through the mouth in response to obstructed nasal passage.  
Respiratory allergies have become a major public health problem in the last decades. According to the World Health Organization (WHO), more than 20% of the world's population suffers from IgE-mediated allergic diseases and these percentages are likely to increase in the future. Allergic rhinitis (AR), which is an inflammation of the nasal mucosa, is mediated by immunoglobulin E induced by exposure to environmental allergens [8]. In response to allergen exposure, IgE bound to the mast cells present in the nasal mucosa triggers an excessive immune response characterized by a release of neuroactive and vasoactive substances such as histamine, prostaglandandins, and leukotrienes. An inflammatory process is initiated in the mucosa and, should the stimuli persist, the inflammation can become chronic [8]. AR can be seasonal or perennial, depending on the timing of the allergen exposure (if it occurs at certain times of the year or is present year-round) [9] and it is characterized by symptoms such as rhinorrhea, itching, sneezing, nasal obstruction, concomitant conjunctivitis, etc. Subjects affected by AR have a significantly higher risk of asthma, decreased sleep quality, and other conditions such as chronic sinusitis and otitis media [10].  
While a number of studies have shown that altered breathing patterns due to AR and asthma are associated with reduced inter-canine and intermal distances, a higher prevalence of posterior crossbite and anterior open-bite, and increased palatal height [11; 12; 13; 14], none have examined the effect of these pathologies on the long-term stability of transverse expansion treatments. This retrospective study aimed to evaluate the prevalence of AR and/or asthma in a large group of children affected by posterior crossbite and/or ogival palate with respect to that in a control group. It also aimed to assess the influence of AR and/or asthma on the stability of transverse expansion treatments in children between 5-12 over a 2-year follow-up period.