Introduction: Saccadic dysfunction is defined as a disturbance in saccadic movements which consist of two coordinated movements of the eyes that bring a new object of regard into fixation rapidly. Such movements are basic for vision for Patients with OPD are set for reading, scan movements and shift movements. In cases where saccades are impaired, the person is bound to struggle in his/her daily undertakings, overall vision capability will be significantly impaired hence diminution of the quality of life.
Saccadic Dysfunction
Saccades are quick movements of the eye in the same direction; they can used to look from left to right, to invert and to follow something like words on paper to writing them.
The two case indicates that if saccadic eye movements are not optimally coordinated, one may land too far afield or not far enough, which is an overshooting or undershooting of the eye movements. When the eyes stray away from the correct path a corrective small saccade begins and before completion of this corrective saccade the error movement is completed and the object of focus is seen clearly.
These types of errors may not be too serious when they are erratic in occurrence, but when saccadic dysfunction is continuous, several difficulties arise which include; problems in reading, eye strain/fatigue, words and letters moving around the page, experiencing headaches, and most worrying, some people may be misdiagnosed for dyslexia.
May as well put it this way. For instance, a child with saccadic dysfunction is learning to read, they are likely to take a lot of time to do so. The element “dog” is introduced to the child in written form. The moments when the child attempts to make close saccades to read the word, his eyes fixate beyond the letters, making him read, for instance, the first letter “g”. The correctional mini saccade then looks over to see the “d”.
This child says d-o-g differently than ‘dog’, and what he/she hears is g-o-d, which is an entirely different word and hence the meaning is different. Or, this individual will read individual words correctly, but because of saccadic dysfunction, the sentences move around.
Where in a typical manner, one would be able to comprehend a given sentence such as ‘the dog jumped over the fence and ran down the street’, the said person can only interpret ‘dog over the jumped fence down the street ran’.
Of course, the second sentence is complete gibberish. To avoid this jumbling of the words, the individual might read very slow, use a finger to help them trace down the sentence or, eventually, abstain from reading.
It might be quite challenging for persons with saccadic dysfunction to describe their condition, and this is so because; this condition is generally seen in persons who have had a brain injury, that is having concussion, or in congenital cases, the person does not know that it is an abnormal condition. Therefore, this problem may remain unnoticed for many years, before a comprehensive eye check-up is conducted.
Anatomy and Physiology of Saccades
Saccades are controlled by a sophisticated network involving several brain regions:
- Superior Colliculus: Said to reside in the midbrain and processes sensory signals while being essential for starting saccades.
- Frontal Eye Fields (FEF): Located in the frontal lobe, those areas are involved in voluntary saccade initiation and hence the decision to move the eyes.
- Parietal Eye Fields (PEF): These fields are in the parietal lobe and they are used in the selection of saccade targets due to the spatial information that is in them.
- Basal Ganglia and Cerebellum: It also tunes the saccadic movements for accuracy and precision by the above structures.
The latter structures have to be coordinated in order to make prompt and precise saccades in response to an appropriate signal.
Types of Saccadic Dysfunction
Saccadic dysfunction can manifest in various forms, including:
- Hypometric Saccades: This is where saccades are in between, normally the eyes too drift yet they are not on the target.
- Hypermetric Saccades: Saccades that cross over the intended position of fixation.
- Slow Saccades: Saccade initiation with slower velocities Slower saccades.
- Dysmetric Saccades: Saccades, which are performed with variable amplitude and/or variable direction.
Etiology of Saccadic Dysfunction
The causes of saccadic dysfunction are varied and can be categorized into neurological, vestibular, traumatic, and congenital origins:
- Neurological Disorders:
- Parkinson’s Disease: It presents with other features like slow movements (bradykinesia) and small saccades because of the disturbed basal ganglia.
- Multiple Sclerosis (MS): Impairment of the myelin layer in multiple sclerosis can deteriorate the connection of saccade related neural circuits causing poor saccadic control.
- Huntington’s Disease: Disorders voluntary saccades and results in both paresis and dysmetria of other involuntary movements.
- Stroke: May cause injuries to the regions of the brain that are responsible for the control of saccades thus leading to impaired saccades.
- Vestibular Disorders:
Again, diseases that affect the inner ear including vestibular neuritis or Meniere’s disease may cause problems of balance and coordination and hence saccadic movements.
- Traumatic Brain Injury (TBI): Injury to the brain can physical disorganize some areas that look after the saccadic movements.
- Congenital Disorders:
- Congenital Nystagmus: Thus, they are non-purposive, continuous, and arrhythmic ocular motions that may disrupt saccadic performance.
- Ocular Motor Apraxia: Inability to produce voluntary saccade often seen in children with difficulty in initiating the voluntary eye movement.
- Clinical Manifestations Symptoms of saccadic dysfunction can vary widely but often include:
- Difficulty Reading: They may jump from a line and go to another, or get confused in a text or maybe do a lot of page swapping.
- Visual Fatigue: Experiencing difficulty when focusing with the eyes particularly when changing the focal point from one point to the other, for instance, while reading or carrying out a computer work.
- Dizziness and Disorientation: Especially if the object of regards is a moving one or if constantly shifting one’s attention from one object to the other.
- Poor Coordination: Coordination issues, especially with fine motor skills which involve the eyes and hands, performing, for instance, sports or driving.
- Compensatory Head Movements: Saccadic movements would be substituted by gross head movements in order to change vision on the field since they are not as effective as they should be.
- Diagnosis: Diagnosing saccadic dysfunction requires a comprehensive evaluation:
- Patient History: Report establishment of patient’s symptoms, their nature, severity and any other ailment they are suffering from.
- Clinical Examination: Various tasks are assigned to patients in order to determine how their saccadic function is during the performance of these tasks.
- Specialized Tests for Saccadic Dysfunction:
- Electrooculography (EOG): Can track eye movement patterns and is sensitive to disturbance in saccadic movement.
- Video-oculography (VOG): Engages video technology to make and monitor eye movements on the human face.
- Saccadometry: Assists in giving a numerical analysis of the saccadic characteristics like latency which is the time taken to make a saccade, velocity which is the rate at which the saccade occurs or the degree of precision made by the subject.
- Neuroimaging: Structural brain abnormalities are known to have effects on saccadic movements, and these may be spotted from MRIs or CT scans of the patient.
These basic tests are done initially in any case by the eye doctor to help elicit the problem to see if it is actually a saccadic problem or more of an information processing problem optically.
Some of the commonly used tests for saccadic dysfunction include; NSUCO- oculomotor test, DEM test and the King- Devick test.
The NSUCO oculomotor test is for the assessment of both horizontal saccades as well as an apparently distinctive form of ocular movement, namely pursuits.
To assess saccades the patient is requested to take a standing position. The examiner is positioned approximately at a distance of 40 cm from the patient and in his / her hands, two targets are used most of the time being a small back steel wand and a small gold steel wand and are placed 20 cm apart. The patient is then instructed to stare at one target and then the other target and the eyes movement are observed for accuracy.
Another assessment that is written is the Developmental Eye Movement test which can be used to distinguish between a saccadic deficit and information processing speed.
An individual is provided a card containing a column of vertically arranged letters and is timed on reading the letters. The same is then done with letters aligned horizontally; the letters are merged, whilst the designer adjusts other aspects such as the spaces between the letters and strokes.
The examiner monitors any mistakes or letters omitted and documents the mistakes and the time it took to get to that letter and then divide them to get a ratio that is compared with a certain age equivalent.
While we won’t get into the specifics of the ratio here, here is a basic understanding of the results:
Normal vertical response, but abnormal horizontal response points to saccadic dysfunction.
If the subject has an abnormal vertical response but a normal horizontal response the test is said to be of low reliability.
An abnormal vertical and abnormal horizontal response tells about the either of the following interpretations: There is a problem with random automaticity of naming as the ratio is within normal limits; there is problem with both random automaticity of naming and saccadic dysfunction other the interpretation of the ratio which is outside the normal limits.
The King-Devick is a word-recognition test of the eyes in which a letter set is demonstrated to the person, horizontally printed on a card. He/she has to read off the letters in as fast a manner as possible.
The examiner observes the patient’s eye movements and then tests the patient’s ability to identify letters which are on the card with a view of estimating saccadic dysfunction.
Treatment of saccadic dysfunction
Treatment of saccadic dysfunction focuses on addressing the underlying cause and may include:
- Medical Management:
- Medications: For other related medical illnesses or diseases that may be causing your problem, but are not directly linked to the accident. For instance, medications used to stimulate dopamine neurotransmission in Parkinson’s disease or drugs that alter the immune response in multiple sclerosis.
- Neuro-rehabilitation: Special tasks designed to address the patients’ gross motor control and saccade generation.
- Vision Therapy: Exact and specific activities and approaches developed to facilitate visual abilities and saccadic precision. This might include:
- Saccadic Training Exercises: Activities that involve the patient to look from target to another with ease.
- Perceptual Learning Exercises: Skills that help the brain to understand visual materials or information received through the optic nerve.
- Assistive Devices: Prism glasses are useful in assisting patients with reading and mobility because they modify the input to the patient’s brain to compensate for saccadic dysfunction.
- Surgical Interventions: In very special and extreme occasions, other structural impairments that could be causing saccadic dysfunction could compel the doctor to recommend surgery.
Saccadic dysfunction is treated by vision therapy in a bid to attempt to reduce the manifestations to better the reading time as well as the understanding resulting there from. Vision therapy will entail focusing on the patient’s ability to shift his/her eyes towards different objects while keeping the head steady.
Some of the common exercises that could be done for vision therapy include the following ; Hart Chart Saccades, Saccade Sticks, work sheets especially the Visual Tracking Worksheet series or the Letter/Word Tracking Worksheet series, computer programs especially the VTS system and also enlisting children’s games but in modified form.
Hart Chart Saccades is a chart with different easy letters or numbers like 4, C, 1, J, 5 written in different directions. The patient is then asked to read a lower case alpha in the left side of the chart and then jump up to read another letter on the right side of the chart.
The procedure is carried out severally in order to focus on refining the control of the saccadic function. To increase the level of complexity the individual may be required to read letters, which are placed closer together for instance the two letters presented in the middle part of the chart. This is due to the fact that smaller movements are more challenging in terms of the entire muscle system successfully finishing the action.
As with Hart Chart Saccades, Saccade sticks are basically the same but two large sticks with letters and pictures are used. The patient is asked to identify the letter or the picture on the left stick and on the right stick the patient is expected to shift the eyes towards the right stick to read the indicated letter or picture.
The herein presented saccade sticks can be useful as the distance between the objects (i. e. the size of the saccades) can be changed by moving the sticks closer or further.
It was found that worksheets have an application to assist in amelioration of saccadic dysfunction and are useful in designing a ‘real-life’ situation to enhance the patient’s rate and precision of reading. As it has been mentioned earlier worksheets are quite different and they are created with intention to train the saccadic system. These are good for home therapy and depict the progress in the form of the increase in the speed and number of saccades per time.
It’s a computer program containing a number of exercises to address the various oculomotor dysfunctions: the VTS system. It entails the use of specialized computers and technology and is, therefore, mostly applied during office-based therapy.
Since vision therapy is more common in children, the vision therapist or the doctor can involve some of the games the child loves to play into vision therapy in order to make the child less scared.
The games that the therapist will use will be those in which the child is required to look for an object or draw a comparison like the Spot It, Matching, Hidden Object Diagrams, etc The patient will be warned not to move his head but just use his eyes.
For all therapy activities, it is always understood that the paradigms will be adaptable and will be based on the patient and his or her capabilities. A plan will be developed by the doctor and the patient on which course of treatment will have to be undergone according to the need of the patient and the set goal and later on it is still changeable.
Outcome
The outcomes of saccadic dysfunction depend on the primary cause of the condition and the applied treatment. Conditions that are associated with neurological diseases may worsen over time, on the other hand, conditions that can be treated through rehabilitation most probably will show immense improvement over time. It should be noted that diagnosis at an early stage and timely initiation of treatment significantly increases the likelihood of recovery and positively affects the quality of life of the affected person.
Conclusion
Saccadic dysfunction is a broad and diverse disorder which, perhaps due to its variability, can greatly affect a person’s quality of life and their ability to perform tasks. Thus, it is critical to evaluate the various causes of the disease, symptoms, and approaches to their treatment. This disease remains both difficult to manage and devastating for patients, but improvements are being made in terms of diagnostics leading towards improving therapy. Future studies are envisioning to uncover more details about saccadic deficit causation and the creation of more accurate therapies, which will promote the chance of rehabilitation and monitoring.
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