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Strabismus, or eye misalignment, is a condition where the eyes do not line up with each other, causing them to look in different directions. It is often caused by problems with the neuromuscular control of eye movement, which involves the brain and the six muscles that control eye movement. Accurate measurement of strabismus is crucial to guide medical and surgical treatments, which depend on the degree of eye misalignment. Various methods exist to measure strabismus, including light reflex testing, cover testing, and digitised screen tests. The choice of method depends on factors such as the patient's age, vision, and level of cooperation.
| Characteristics | Values |
|---|---|
| Definition | Strabismus, or eye misalignment, is a condition in which the eyes don't line up with one another. |
| Causes | Functional disorder of eye muscles, refractive errors of the eyes, injuries, Down syndrome, hydrocephalus, brain tumours, stroke, head injuries, neurological problems, Graves' disease, or physical trauma. |
| Symptoms | Inward turning of one or both eyes, double vision, crossed eyes. |
| Diagnosis | Light reflex testing (Hirschberg testing), cover testing, corneal reflection test, digitised tangent screen test, prism and cover testing. |
| Treatment | Surgery on the muscles of one or both eyes to correct alignment, low-cost strabismus surgery simulation models for training. |
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What You'll Learn
The Medical Research Council Manual Muscle Testing Scale (or MRC scale)
The Medical Research Council Manual Muscle Testing Scale, also known as the MRC scale or the Oxford scale, is a commonly accepted method of evaluating muscle strength. It was first introduced in 1943 and continues to be one of the most efficient means of categorising muscle strength. The MRC scale is preferred by many because it is convenient, safe, and requires minimal training.
The MRC scale involves testing key muscles from the upper and lower extremities against the examiner's resistance and grading the patient's strength on a scale of 0 to 5. Commonly tested muscles include the shoulder abductors, elbow flexors, elbow extensors, wrist extensors, finger flexors, hand intrinsics, hip flexors, knee extensors, dorsiflexors, great toe extensors, and plantar flexors. These muscle groups are chosen to assess important spinal nerve roots systematically. However, additional muscles can be tested to evaluate individual peripheral nerves. For example, testing the strength of the elbow flexors, elbow extensors, wrist extensors, finger flexors, and hand intrinsics allows for a methodical evaluation of the C5 to T1 nerve roots.
The examiner will push parts of the patient's body in specific directions, and the patient will push back against the pressure. Strength is then assessed based on the force of the muscle contraction. This test is typically performed on an examining table and does not require any specialised equipment. The MRC scale is especially useful for assessing neurologic conditions such as stroke or spinal cord injury, where there is a significant difference in strength between the two sides of the body.
While the MRC scale is widely used and accepted, it is important to note that it is a subjective measure of muscle strength. More precise methods of measurement, such as hand-grip dynamometry, provide quantifiable measurements that can be tracked over time. Dynamometry involves using a handheld device called a dynamometer to measure the force of a muscle contraction. This method requires specialised equipment and can be more costly, but it provides a more accurate assessment of muscle strength.
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Dynamometry
There are different types of dynamometers, including handheld dynamometers (HHD) and isokinetic dynamometers (IKD). HHDs are force-sensitive devices that can be comfortably held in the examiner's hand. They are equipped with a small screen that displays the recorded static force in units of kgf, lb, or N. During the assessment, the examiner provides resistive force and proximal stabilization while the subject exerts a maximum isometric force on the dynamometer, which remains stationary. HHDs have been found to be reliable and valid measures of muscle strength, particularly for smaller joints like the shoulder. However, they may not be suitable for larger joints like the knee due to the examiner's strength limitations.
Isokinetic dynamometers (IKDs) are considered the "'gold standard'" in muscle strength assessment. They measure the peak torque values in movements like hip extension. IKDs are more expensive than HHDs but offer advantages in assessing larger joints.
In the context of strabismus, or eye misalignment, dynamometry is not directly mentioned in the sources. Strabismus involves the six muscles that control eye movement failing to work together, resulting in one eye turning in a different direction from the other. While the sources do not specify how to measure the strength of these eye muscles using dynamometry, the general principles of dynamometry described above can be considered.
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Forced Duction Test (FDT)
The Forced Duction Test (FDT) is a critical pre-operative procedure that must be performed before any strabismus surgery. Strabismus, or eye misalignment, is a condition where the eyes do not line up with one another, causing them to look in different directions. This condition can lead to serious vision problems, including lazy eye or permanent poor vision in the turned eye, if left untreated.
During the FDT, the limbus (the edge of the cornea and sclera) is carefully gripped with forceps, and the eyeball is gently moved into multiple positions to stretch the rectus muscles and superior oblique, which are responsible for eye movement. This test provides valuable information about the tone and integrity of the extraocular muscles, which control eye movement and positioning.
The FDT is particularly important because it helps surgeons identify any restrictions or abnormalities in the extraocular muscles that could interfere with the success of strabismus surgery. By performing this test, surgeons can make more informed decisions about the surgical approach and technique, ensuring optimal outcomes for patients.
Additionally, the FDT can be influenced by certain medications, such as succinylcholine, which can cause sustained contractions of the extraocular muscles, interfering with accurate test interpretations for up to 20 minutes. Therefore, the use of muscle relaxants like pancuronium, which do not alter the FDT, may be considered to ensure accurate assessments of the extraocular muscles' function and health.
In summary, the Forced Duction Test is an essential tool in the evaluation and treatment planning for strabismus surgery. By assessing the integrity and mobility of the extraocular muscles, surgeons can tailor their surgical approach to achieve the best possible alignment and visual outcomes for patients suffering from eye misalignment.
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Muscle strength grading
The most commonly accepted method of evaluating muscle strength is the Medical Research Council (MRC) Manual Muscle Testing scale. This involves testing key muscles from the upper and lower extremities against the examiner's resistance and grading the patient's strength on a scale of 0 to 5:
- Grade 0: No contraction observed or felt.
- Grade 1: Visible or palpable muscle contraction, but no movement produced.
- Grade 2: Movement produced with gravity eliminated (e.g. in a horizontal position).
- Grade 3: Movement produced against gravity but not against resistance.
- Grade 4: Movement produced against gravity and some resistance, but not full resistance.
- Grade 5: Normal muscle strength with movement produced against full resistance.
The muscle strength evaluation is typically performed by the examiner applying resistance while the patient performs a specific movement. This assessment helps in diagnosing muscle weakness or paralysis, which can indicate neuromuscular disorders or nerve injuries. It is especially useful for patients with neurological issues such as stroke, brain injury, spinal cord injury, neuropathy, or amyotrophic lateral sclerosis.
Another approach to muscle strength testing involves testing functional movements instead of quantifiable strength. Examples of functional tests include squatting or rising from a chair. These tests provide information about whether the patient can perform essential daily activities, but they do not provide a grade or numeric quantity to track improvement over time. Functional assessments complement muscle strength grading in comprehensive neurological evaluations, providing insights into a patient's ability to perform daily activities, participate in sports, or return to work.
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The Oxford Scale
While the Oxford Scale provides a quantitative approach to muscle strength evaluation, it is important to note that it is primarily designed for clinical use and may not capture all aspects of muscle function. Other tools and assessments may be used in conjunction with the Oxford Scale to obtain a comprehensive understanding of an individual's muscle health and function.
In the context of strabismus, or eye misalignment, there are separate measurement scales and systems used to evaluate the condition. These scales focus on assessing the level of control and the direction of eye deviation in individuals with strabismus. While the Oxford Scale is not directly applied to measuring strabismus, the underlying principle of quantitative assessment may be shared by some of these measurement systems.
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Frequently asked questions
Strabismus is a condition in which your eyes don't line up with one another. In other words, one eye is turned in a different direction from the other. This can be caused by a problem with the neuromuscular control of eye movement, or less commonly, with the actual eye muscle.
Strabismus can be measured by several different methods, including light reflex testing (called Hirschberg or Krimsky testing), cover testing, and prism and cover testing. The age, vision, and level of cooperation of the patient determine which method is most accurate and feasible.
Light reflex testing involves directing a patient to look at a point of light held about three feet from their face. If the light reflexes are located in the same spot in each pupil, the reflexes are symmetric and the eyes are straight. If the light reflexes fall asymmetrically in the pupils, strabismus may be present.
Cover testing involves instructing the patient to look at a target. While the patient is fixating on the target, one eye is covered. The uncovered eye is then observed. If the uncovered eye is well aligned, it will not shift position when the other eye is covered. If it is misaligned, its position will change as the eye shifts to look at the target.
