In this world, in this time and age, many diseases and disorders exist, with many more evolving, and others being created and discovered. Some are acquired, while others are passed down through family like traits. The latter are called genetic diseases, since inheritance plays a role in procuring the disease. There are countless of studies concerning genetic diseases. Gene mapping is used in describing how a gene could have been mutated or altered in a way for a condition to be developed.
Knowing the loci for diseases also helps in finding a treatment. Many diseases can attest to certain genes being involved in their development. Duane syndrome is one example of these genetic diseases. People who suffer from this rare condition have the limited ability to move their eyes in an abduction movement, or occasionally adduction, which is prohibited by the miswiring of certain eye muscles, along with some additional causes. Trying to move their eyes outside of the range they are capable of causes a retraction in the patients’ eyes.
This syndrome is considered a type of strabismus. The association of this hereditary ocular disease with other diseases is a plausible theory, as there have been studies which seem to show a correlation between Duane’s syndrome and other diseases having to do with hearing, sight, and skeletal malformations. Because Duane syndrome creates a hindrance in a person’s eyesight, it can be a frustrating condition. A person can develop a crooked gaze and an abnormal head posture to acquire binocular vision to help with the condition.
Although there is no cure, there are treatments available, including surgical, to help lessen the disturbance of Duane’s syndrome, and aid in bettering a person’s sight. History and Genetics Duane syndrome had been described in the late 1800’s, but it was not until a man named Alexander Duane wrote a paper on the subject that the condition was understood more (Duane, 1976). With his study of fifty-four cases, Duane was able to form theories suggesting the pathogenesis and treatment of the syndrome.
His findings lead to advancements with Duane syndrome. Eventually, a man by the name of Huber was able categorized Duane syndrome into three groups. People who are diagnosed with Duane syndrome regularly have the symptom of not being able to completely, or not at all, roll their eyes out towards the direction of their ear. This would be called abduction, and many of the Duane syndrome patients find they are not capable of adduction as well, which would be the attempt of rolling their eyes in towards their nose.
Duane syndrome patients attempting to roll their eyes horizontally may find that the eyeball will pull back into the socket, which is why this disorder is also sometimes called Duane’s Retraction Syndrome (DR or DRS), Eye Retraction Syndrome, and Congenital Retraction Syndrome. The public may as well know this disorder as Stilling-Turk-Duane syndrome, Stilling and Turk being the ophthalmologists who had studied the syndrome before Duane. Duane syndrome causes involuntary contraction of the eyes, and also inhibits voluntary contraction.
Because of the retraction, the eye opening can narrow, and the eye will roll either upwards or downwards. This condition can affect one or both eyes, depending on the type of Duane syndrome the patient may have. Though cases frequently consist of unilateral Duane syndrome, with the left eye commonly being the affected one (Baroncini, 2013). The reason for the left eye being affected more often is not entirely known. Studies show Duane syndrome tends to transpire more in females than in males. It is furthermore not known why the disease favors females.
Those who suffer from bilateral Duane syndrome have a tendency to acquire amblyopia, the reducing of vision. But researchers do not think there is a direct linkage, as the same percentage that affects Duane syndrome patients is around the same percentage that affects the general population. Though patients are born with the condition, it is not diagnosed until a later age, typically around the age of ten. No specific race is directly linked to Duane syndrome. Causes and Diagnosis The medial and lateral rectus are muscles in the eye, which are in charge of movement of the eye.
These muscles are thought to have miswiring, causing Duane syndrome. It has also been found that a number of patients who suffer from Duane syndrome do not have, or have a malfunction with, the sixth cranial nerve, also a primary component in eye movement (Vucic, 2013). This nerve is named the abducens nerve, and is specifically linked to the lateral rectus muscle. This can maybe be the reason why Type 1 occurs more frequently. MRI’s can show the absence of this cranial nerve in Duane syndrome patients.
A study was conducted in where it was found that all Type 1 patients did not have the abducens nerve. Type 2 patients did indeed have it, but there was some abnormality to it. Most of Type 3 patients did not have the nerve. Researchers still do not understand what causes the malfunctions with the eye muscles and nerve. Though it is theorized the damages could have been caused during the embryonic development (Gupta, 2014). This is reasoned through recognizing that the eye muscles and cranial nerves are developed during weeks three through eight of a pregnancy.
This would make sense since Duane syndrome is occasionally associated with diseases that are developed when there are disturbances during the embryonic stage. Though researchers are not entirely sure of the causes for Duane syndrome, they do believe many influences contribute to the development of the condition, including genetic and environmental factors. Duane syndrome is usually noticed in patients who lack the ability to move their eye in abduction or adduction. These patients will most likely have a retraction in the eye when trying to move their eyes inward, and there will be a narrowing of the palpebral fissure.
Abnormal head posture will every so often be seen as well. Certain postures are adapted to help keep their eyes straight. It compensates for the inability to roll their eyes horizontally. There are three types of Duane syndrome. Type 1 is the most frequently seen, with about 78% of recorded cases of Duane syndrome. Patients with Type 1 are, or are almost, capable of adduction of their eyes, but incapable of abduction (Baroncini, 2013). Retraction in patients occurs when trying to look inwards towards the nose. A head turn to the side of the eve affected may be seen.
Eyes can look a bit turned inwards. Type 2 is seen in patients who do not have the ability to look in towards their nose, but can look out towards the direction of their ears. Patients with Type 2 will have a retraction when looking towards their nose. This type is the rarest of Duane syndrome, with it only being about 7% of recorded cases, Patients with Type 3 of Duane syndrome cannot move their eyes out or in. Their eyes also pull into the socket if there is an attempt at adduction. Type 3 happens in 15% of Duane syndrome patients (Baroncini, 2013).
There are three subgroups which characterize the direction of the affected eyes when attempting to look straight forward. Sub-group details affected eyes which appear esotropic, meaning they turn in towards the nose. Sub-group B is for eye which turn towards the ear, or exotropia. In sub-group C, patients’ eyes look straight forward, with no rolling of the eye in or out. Inheritance and Genes Duane syndrome has been found to be both dominant and recessive. When the disease is dominant, it can at times skip a generation. The location for a gene pertaining to Duane syndrome is a mystery.
The condition is considered a heterogeneous disease, as researchers have found evidence through cytogenetic testing showcasing multiple genes being involved with Duane syndrome. This means that different genes can be the cause of Duane syndrome. One gene in particular, is gene CHN1, located on the chromosome 2q31. This gene has the instructions to make proteins needed for the formation of nerves dealing with ocular muscles (Arya, 2011). Mutations of this gene can cause the inability to move the eyes in certain directions.
When patients acquire Duane syndrome through mutation of the gene CHN1, the disease shows an autosomal dominant pattern of inheritance (Kim, 2014). There have been cases where the children in a family develop Duane syndrome, yet their parents do not have it. This suggests that Duane syndrome can also be an autosomal recessive trait. In studies, Duane syndrome seems to be connected to 8 anomalies. These include 8912 duplication, 8913 deletion, and 8q13. 2 translocation (Amouroux, 2012). With findings like these, the locus for Duane syndrome has been identified as 8913 (Calabrese, 1998).
Deletions on chromosome 4 and markers on chromosome 22 are likewise related to Duane syndrome. Chromosomes 20 has also been linked to Duane syndrome, specifically when Duane syndrome is associated with Okihiro syndrome, or Duane Radial Ray Syndrome. When these two syndromes are connected, there seems to be a mutation of the SALL4 gene (20q13. 2), linking them. Evidence of all these genes show Duane syndrome is hereditary. Yet, in only 10% of recorded cases does Duane syndrome show a familial pattern. These cases habitually concern both eyes being affected.
