Introduction
Eye Muscles.jpeg
Figure 1 http://www.csdl.tamu.edu/~sam/index.htm

Duane’s Syndrome is a congenital disorder of the eyes that disrupts control of eye movement. It is more common in females then males and tends to be present in the left eye more than the right eye. Although this is present at birth, most people are not diagnosed until later in life when eye movements are more voluntary and purposeful. This page will first address the functional anatomy involved in eye movement and the neurological processing that occurs to produce normal eye movement. This will then be compared to that of a miswired eye found in Duane’s syndrome. Diagnosis, current prognosis, and treatment for someone diagnosed with Duane’s syndrome will be addressed at the end.


Symptoms and classification
The most common symptom of Duane’s syndrome is the abnormal eye position during eye tracking and sometimes while looking straight. Another symptom that might be seen is constant head turning or keeping the head fixed in a turned position in order to see objects. There are three subdivisions of Duane’s syndrome based on the direction the eye can move and which way it is inhibited. (Gutowski, 200)
Type 1: The affected eye is limited or completely incapable of abduction (movement away from the nose) but has normal ability when moving inward or adducting. This is the most common type seen.

bushy normal 1.jpg
Figure 2 http://www.strabismus.org/Duane_Syndrome.html
busy abnormal.jpg
Figure 3 http://www.strabismus.org/Duane_Syndrome.html



Figure 2(right): Duane's syndrome in left eye-note adduction
still functional
Figure 3 (left): Left eye does not abduct



Type 2: In contrast to type 1, type two presents as the inability to adduct the affected eye with normal abduction.
Type 3: The eye has the inability to abduct and adduct normally.

Eye muscle anatomy
The eye is controlled by six muscles, these are referred to as the extraocular muscles.

Medial rectus and lateral rectus: these muscles work together to control horizontal eye movements such as looking left and right. The medial rectus pulls the eye towards the nose, which is described with the term adduction. The lateral muscle pulls the eye away from the nose as if to look to your periphery.

eye muscles.png
Figure 4 http://trigem.tumblr.com/

Superior rectus: Elevates the eye and turns it medially, looking upward and toward the nose.
Inferior rectus: Eye depression and lateral rotation, looking down and away from the nose.
Superior oblique: Depresses the eye and rotates it medially as looking down and toward your nose.
Inferior oblique: Elevates the eye and rotates it laterally as in looking up and outward.
To move the eye up and down, two muscles must work together while the other two relax. This synergy is not seen in horizontal movement in which only one muscle is used to turn the eye and the other serves as an antagonist.

Normal eye circuitry
The six muscles described above are controlled by three nerves:
1) The abducens (CN 6)= controls the lateral rectus of the ipsilateral eye. This nerve produces movement of the eye away from the nose.
2) The oculomotor (CN 3)= controls four extraocular muscles, three on the ipsilateral side and one one the contralateral.
Ipsilateral: medical rectus, inferior oblique,inferior rectus
Contralateral: superior rectus
3) The trochlear (CN 4)= control the superior oblique of the contralateral eye which depresses and medially rotates.


eye nerves.png
Figure 5:View of eye muscles and nerves that innervate them http://virtual.yosemite.cc.ca.us/rdroual/Lecture%20Notes/Unit%205/cranial_nerves%20Spring%202007%20with%20figures.htm
Since Duane’s syndrome affects horizontal eye control (Strachan and Brown,1972), that will be the only type of movement addressed on this page, and the nerves of focus will be the abducens and the oculmotor. The inability of the eye to move affects all kinds of eye movements such as voluntary movement, saccades, and smooth pursuit. In order to produce a successful movement of the eye, there must be an excitation of the burst neurons to cause movement. This happens by the omnipause neurons in the dorsal raphe turning off, as their role is to keep the eye stabilized.This inhibiting of the omnipause neurons allow the burst neurons in the paramedian pontine reticular formation (PPRF) to excite the nerve. Once the burst neurons are disinhibited, the abducens sends an excitatory signal to the ipsilateral lateral rectus and also to the contralateral oculomotor nerve. The oculomotor nerve innervates the medial rectus muscle, so the opposite eye turns in the same direction as the ipsilateral eye (Ramat, Stefano,Leigh, Zee and Optican,2007). At the muscular level, a nerve impulse is sent to the muscles wherein the muscle reacts by shortening and therefore pulling the eye in the desired direction. Action potentials are sent from the upper motor neurons where they travels ispilaterally to the eye muscle or in the case of the abducens,crosses and travels down the medial longitudinal fasciculus where it also innervates the oculomotor nuclei of the contralateral eye.The nerve ends at the neuromuscular junction where the action potential is transduced from electrical stimulation to chemical which creates a mechanical force. Acetylcholine is released across the synapse where it binds to receptors that cause a calcium release in the muscles that contract the actin and myosin fibers resulting in an overall contraction of the muscle.This movement of the lateral rectus and the contralateral medial rectus allows for conjugate eye movement so that both eyes are focusing on the same visual field. In an involuntary saccade this movement is initiated by the superior colliculus and in voluntary the frontal eye fields initiate. Once in the desired position, the eye stays stable by a tonic firing of the tonic neurons in the nucleus prepositus hypoglossi.



eye schema.png
Figure 6: eye circuitry schematic http://www.sciencedirect.com/science/article/pii/S0028393210002617%20
What is different in Duane’s syndrome?

It is known that Duane’s syndrome is a problem with the nerves rather than the muscles. The exact cause is still not known but some logical postulations are being researched. First of all because there are different types and manisfestiation, there may be various problems or various nerves affected. In type 1 the eye is unable to abduct which is the role of the lateral rectus innervated by the abducens nerve. Since contralateral medial movement is still in tact and the abducens projects to the opposite eye, the lesion or dysfunction must be beyond the nuclei. There could either be not enough motor neuron development or a dysfunction at the neuromuscular junction that does not allow the nerve impulse to translate to the muscle. It may even be possible that the nerve did not develop fully during pregnancy. The most research has been on the abducens in relation to Duane's syndrome because type 1 is the most common. Based on the knowledge of the function of eye control one can make some hypotheses about the cause of the other two types. Type 2 presents as a problem with the medial rectus muscle which traces back to the oculomotor nerve. This inactivation of the muscle could be caused by any of the reasons stated above but must be affecting the oculomotor nerve rather than the abducens.The third type of Duane’s is the inability to move the affected eye in or outward. This would have to involve both the abduces and oculomotor nerves on the ipsilateral side.








Diagnosis
Duane’s syndrome is diagnosed based on clinical observations.
newborn duane's
Duane's syndrome at birth (personal image)
Affected individuals will have a strabismus, which is when the eyes are not in the same line of view. Many times patients will present with more head movement in order to view objects while preventing diplopia. This may also lead to a constant head tilt. This is more commonly found a year after birth because babies tend to have rapid eye movements and it is not uncommon for newborns to have trouble controlling their eyes as they are still learning to control their body. As eye movements become more purposeful, the irregularity of movement is observed. Most people observe the unusual eye position in pictures of the child and will often think that the “good” eye is actually the deviating one, as the affected eye remains straight. The picture to the left depicts a newborn baby that was later diagnosed with Duane's syndrome in the left eye and shows how the wrong eye may thought of as the "bad" one. This baby appears to be looking straight so the right eye looks like it is deviating medially.


Treatment and Prognosis
After diagnosis, regular ophthalmologist appointments are recommended for the best success and reduction of symptoms. There is a nonsurgical treatment that works well for mild cases, this includes patching the “good” eye so that the individual has to turn their head in order to see objects in their periphery. Since this involves nerves, it cannot be fixed with the training but it will prevent the symptoms from occurring by proactively turning the head
eye patch.jpg
Eye patch used in treatment of Duane's syndrome (personal image)
out of habit. This solution is not a cure for the problem, but allows the individual to prevent seeing two images at once and makes their eyes cosmetically appealing.

Surgery is an option but once again does not fix the problem. This is used in very severe cases. It can reduce eye misalignment or permanent turned head posture that developed from always correcting vision. The horizontal eye muscle is cut and sutured back in a new position that will align the eyes. Another type of surgery, transposition surgery, is used on some patients. This is when the superior rectus and/or the inferior rectus are cut and moved toward the weakened side enabling more movement of the eye (Ozkan and Can, 1997).

Prognosis is good if this syndrome is detected early. The main concerns are trying to prevent secondary complications such as amblyopia or loss of binocular vision due to ignoring the affected eye (Sloper and Collins, 1999). Cosmetically, the early the action, the more likely it is to instill a habit such as turning the head that prevents the eyes from crossing.



Conclusion
Duane's syndrome has been understood on the basis of the neurological processing of eye movement. Each muscle of the eye is innervated by one nerve which allows for isolation of the problem. Duane's syndrome directly affects the quality of horizontal eye movement by having an underdevelopment or dysfunction with the abducens nerve which control the lateral rectus muscle (type 1). Patients will present will the lack of motion in the affected eye in one or both directions. There are no cures for the disease but prognosis is good as there are solutions that help dull the side effects and new surgical procedures may even allow the patient to gain a little more movement.



Glossary
Antagonist-muscle that opposes another one
Abduction- movement away from the body
Adduction- movement towards the body
Amblyopia-lazy eye
Binocular vision- vision when both eyes are used together
Contralateral- on the opposite side
Congenital- present since birth
Dorsal Raphe-located in the brainstem
Extorsion-rotation of the bottom of the eye towards the midline or nose
Intorsion-rotation of the top of the eye towards the midline or nose
Ipsilateral- on the same side
neuromuscular junction- connection of the nervous system with the muscular system
Omnipause neurons- inhibit the burst neurons so their net effect is no movement
Strabismus- Eyes are not aligned with each other
Superior Colliculus-part of the midbrain below the thalamus
Transduced- change form one energy from to another
Upper motor neurons-neurons that originate in the motor cortex



Quiz

True/False
1) T/F. Duane’s syndrome is more common in girls than boys.
2) T/F. Surgery is a complete cure to Duane's syndrome.

Multiple choice
3) Which of the following nerves innervates the lateral rectus?
a. Trochlear Nerve
b. Abducens Nerve
c. Oculomotor Nerve

4) The Superior Colliculus initiates eye movement in this situation
a. voluntary
b. preprogrammed
c. involuntary
d. all of the above

5) In which type must the lesion be beyond the abducens nuclei (meaning it goes through the nuclei successfully)?
a. Type 1
b. Type 2
c. Type 3
d. Type 2 and 3

6) Essay: Describe the symptoms associated with each type of Duane's syndrome.
7) Essay: How do the two types of surgery discussed help Duane's syndrome if this is not a muscle problem?
8) Essay: Describe what transduction means and how that is used in the neurological and muscular system of the eye?

Answers
1) True
2) False
3) b
4) c
5) a




Suggested Readings
*Video- A detailed commentary on a case study of Duane's syndrome and the surgery performed including outcomes


* Possibility of Duane's syndrome being hereditary

*Extraocular testing procedures

*AAPOS website with general information on Duane's syndrome



Dedication: This page is for my older sister who will now understand the details of why her left eye cannot abduct!


REFERENCES

1)Gutowski, N.J. (2000). Duane’s Syndrome. European Journal of Neurology, 7:
145-149.
2)Ozkan, S.B, Can, D et al. (1997). The Results of surgical treatment in Duane’s
reaction syndrome. Strabismus, 5(1): 5-11.
3) Ramat, Stefano, R. John Leigh, David S. Zee and Lance M. Optican. (2007).What
clinical disorders tell us about the neural control of saccadic eye
movements. Brain. 130: 10-35. doi:10.1093/brain/awl309
4) Sloper, John J. and Collins, Alan D. (1999). Effects of Duane’s retraction
syndrome on sensory visual development. Strabismus. 7(1): 25-36.
5) Strachan, I.M and Brown,B.H. (1972). Electromyography of extraocular muscles
in Duane’s Syndrome. Brit. j. Ophthal. 56: 594-599.
6) Knierim J. (1997). Oculomotor Control. Neuroscience Online [Online]. The University of Texas Health Science Center at Houston (UT Health).
http://neuroscience.uth.tmc.edu/s3/chapter08.html [10, Dec 2012]


Websites:
http://www.genome.gov/11508984
http://www.aapos.org/terms/conditions/46
http://www.childrenshospital.org/az/Site3103/mainpageS3103P1.html
http://www.ncbi.nlm.nih.gov/books/NBK1190/