Miller syndrome, also known as postaxial acrofacial dysostosis, is a rare genetic disorder characterized by craniofacial malformations occurring along with abnormalities of the arms, hands and/or feet. Craniofacial abnormalities include underdevelopment of the cheekbones (malar hypoplasia); an abnormally small lower jaw (micrognathia); incomplete closure of the roof of the mouth (cleft palate); small, protruding, “cup-shaped” ears; and/or absence of tissue (colobomas) from the lower eyelids. Limb abnormalities may include incomplete development, webbing (syndactyly), and/or closure or absence of certain fingers and/or toes; and/or improper development and/or abnormal fusion of bones in the forearms (radioulnar synostosis), causing the forearms to appear unusually short. Additional physical abnormalities can occur in some cases. Intelligence is not affected. Miller syndrome is inherited as an autosomal recessive trait caused by mutations in the DHODH gene.Introduction
Miller syndrome was first described in the medical field between 1969 and 1979 through several independent reports. The disorder is refereed by several names derived from some of the physicians who first reported the disorder including M. Miller, H.R. Wiedemann, and E. Genee. Some researchers believe that Miller syndrome represents a distinct entity under the broader term of “postaxial acrofacial dysostosis,” which would include other disorders with similar and overlapping craniofacial and limb abnormalities, but ultimately distinct symptoms.
The specific symptoms associated with Miller syndrome may vary from one person to another. Affected individuals may develop a variety of craniofacial and limb abnormalities. Most abnormalities are noticeable at birth.
Common craniofacial abnormalities include underdevelopment of the cheekbones (malar hypoplasia); an abnormally small lower jaw (micrognathia); incomplete closure of the roof of the mouth (cleft palate); narrowing of the back of the nasal cavity (choanal atresia); small, protruding, “cup-shaped” ears; and/or absence of tissue (colobomas) from the lower eyelids. Additional craniofacial findings can include a gap of missing tissue in the upper lip (cleft lip), a broad bridge of the nose, downward-slanting palpebral fissures (which means that the opening between the eyelids slants downward), partial or total absence of the eyelashes of the lower eyelids, and lower eyelids that are turned (inverted) outwards so that the inner surface is exposed (ectropion).
Micrognathia and choanal atresia may contribute to difficulty breathing and/or feeding difficulties. Some affected individuals develop hearing loss due to improper conduction of sound from the outer or middle ear to the inner ear (conductive hearing loss). Hearing impairment may cause speech development to be delayed when not treated.
Individuals with Miller syndrome also have various abnormalities affecting the arms and legs including absence or abnormality of either the fifth or both the fourth and fifth fingers of the hands and toes of the feet and, in some cases, the ulna and/or the fibula may be underdeveloped. The ulna is the forearm bone on the pinky side of the arm; the fibula is the outer and narrower long bone of the lower legs, thus the medical term “postaxial” which refers to fourth and fifth fingers or toes; as well as the ulna and fibula. This can cause the forearms and/or the lower legs to appear short. Additional symptoms can occur including webbing (syndactyly) of the fingers and toes, underdevelopment of the thumbs, and the formation of an abnormal bone or soft tissue connection between the ulna and the radius, two main bones of the forearm (radioulnar synostosis).
In some cases, additional symptoms develop including growth deficiency after birth (postnatal growth deficiency), a sunken breastbone so that the chest appears ‘scooped out’ (pectus excavatum), rib defects, and extra (accessory) nipples. Affected infants may have hips that dislocate easily and may potentially be dislocated at birth. Affected male infants may have undescended testicles (cryptorchidism).
Some individuals have kidney, gastrointestinal or heart abnormalities. Kidney abnormalities may include the backflow of urine from the bladder to the kidneys (kidney reflux). Gastrointestinal abnormalities can include abnormal positioning of the intestines (midgut malrotation) and narrowing of the opening that connects the stomach to the duodenum, the first part of the small intestine (pyloric stenosis). Heart defects can include ventricular septal defects (VSDs), which are also known as a ‘hole in the heart’. VSDs are characterized by an abnormal opening in the wall (septum) that separates the lower chambers of the heart (ventricles).
Miller syndrome is caused by mutations in the dihydroorotate dehydrogenase (DHODH) gene. Genes provide instructions for creating proteins that play a critical role in many functions of the body. When a mutation of a gene occurs, the protein product may be faulty, inefficient, or absent. Depending upon the functions of the particular protein, this can affect many organ systems of the body.
Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother. Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%. The risk is the same for males and females.
Investigators have determined that the DHODH gene is located on the long arm (q) of chromosome 16 (16q22.2). Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated “p” and a long arm designated “q”. Chromosomes are further sub-divided into many bands that are numbered. For example, “chromosome 16q22.2” refers to band 22.2 on the long arm of chromosome 16.
The DHODH gene contains instructions for creating (encoding) the enzyme dihydroorotate dehydrogenase. Mutations in this gene result in a deficiency of functional dihydroorotate dehydrogenase. This enzyme plays a role in the production (biosynthesis) of pyrimidine, which are compounds found in deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and certain molecules within the body. The exact underlying manner that decreased levels of functional dihydroorotate dehydrogenase contributes to the specific symptoms associated with Miller syndrome is not fully understand. Research into the underlying causes and genetic aspects of this disorder is ongoing.
Miller syndrome is a rare disorder with an estimated prevalence of approximately 1 case per 1 million newborns. Because cases may go undiagnosed or misdiagnosed, determining the true frequency in the general population is difficult. Fewer than 75 cases have been described in the medical literature. Males and females are affected in equal numbers.
A diagnosis of Miller syndrome is based upon a thorough clinical evaluation, a detailed patient history, and identification of characteristic physical findings. Many of the associated abnormalities are present at birth (congenital).
Specialized x-ray studies will confirm the presence and/or extent of certain observed craniofacial abnormalities. For example, such imaging tests show the abnormal smallness of the jaw (micrognathia) due to underdevelopment of the lower jaw bone (mandibular hypoplasia).
Molecular genetic testing can confirm a diagnosis of Miller syndrome. Molecular genetic testing can detect a mutation the DHODH gene, but is available only as a diagnostic service at specialized laboratories.
The treatment of Miller syndrome is directed toward the specific symptoms that are apparent in each individual. Treatment may require the coordinated efforts of a team of specialists. Pediatricians, oral surgeons, plastic surgeons, pediatric ear, nose and throat specialists (pediatric otolaryngologists), specialists in diagnosing and treating eye disorders (ophthalmologists), specialists in diagnosing and treating ear disorders (otologists), specialists in treating hearing loss (audiologists), psychologists, and other healthcare professionals may need to systematically and comprehensively plan an affect child’s treatment. Affected individuals may benefit from referral to a craniofacial center where a team of experienced specialists will coordinate care.
Specific treatment may consist of surgery to create a small opening in the throat, through which a small tube is inserted to assist with breathing. Surgery may also be necessary to create a small opening in the stomach to allow the insertion of a feeding tube in infants experiencing difficulty eating in order to maintain proper nutrition.
Surgery may be required to correct abnormalities of the jaws, limbs, and eyes. Surgery and/or speech therapy may be necessary when cleft palate or cleft lip is present. Congenital heart defects often require surgical intervention.
Early intervention with appropriate physical, occupational, and speech therapy services is important in ensuring that affected children reach their full potential. Physical and occupational therapy may be necessary to aid in walking and using one’s hands. Speech therapy may be of benefit for individuals with speech development delays due to hearing loss. Hearing loss may require the use of a hearing aid.
Genetic counseling may be of benefit for affected individuals and their families. Psychosocial support for the entire family is essential as well.
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