A 4 base-out prism test is an eye diagnosis that an examiner performs on a patient to establish a small central scotoma's presence or absence. Scotoma refers to a blind spot that appears in the eye's macula. It may manifest as a gray or black spot, or a blurry or distorted view.
Since it affects the part of the eye that enables reading and clear vision, its presence can significantly affect one's capacity to carry out daily activities.
Central scotoma can result from retinal or optic nerve disorders, eye infections, strokes, brain injuries, and tumors. It may also be suspected in patients with:
- Macular diseases that affect the foveal area
- A slight decrease in visual sharpness in one eye
- A slight reduction in binocular vision
- Monofixation syndrome that results from a small strabismus
- Decreased stereopsis
A 4 base-out prism test happens typically in a room with enough lighting, and it involves using a 4 dioptre prism and a fixation target. It is referred to as a distance method since an examiner places the fixation target a short distance away from the patient.
The test is objective, easy to perform, and includes both adults and cooperative children.
On the other hand, the disadvantages are:
- It can sometimes lead to a misdiagnosis
- It requires accurate observation and cooperation from patients
Also Known As
- Four prism dioptre reflex test
- 4 prism dioptre base-out test
- Irvine prism test
Before the Procedure
The patient needs to have a brief session with the doctor to understand what the test entails, the procedure's safety, and the expected outcomes. During this session, the patient may ask questions for clarity, and it is only after consent that the doctor should proceed with the test.
The eye specialist may carry out an eye exam to assess:
- The ability to direct one's gaze to a distant target
- Eye alignment
- Visual sharpness and amblyopia to determine the preferred eye for fixation
- Absence of nystagmus
- The degree of stereo acuity
The test usually happens in an eye hospital or clinic, and it entails the following steps:
- The examiner will first ensure that the patient is relaxed and comfortably positioned in a chair.
- S/he will then ask the patient to direct their gaze to a specific target and then place a 4 base-out prism over one eye. The prism displaces an image towards its base.
- After placing the prism, the examiner will observe the eye movements and record the results before proceeding to do the same for the other eye.
The results of the test would either be positive or negative. A positive outcome indicates the presence of a central scotoma, whereas a negative outcome indicates its absence. Patients may sometimes exhibit some characteristics that could affect a doctor's ability to make an inference. The characteristics include:
- An uncooperative behavior
- Weak vision
- Presence of nystagmus
- Inadequate focusing on the fixation target
For a normal response, both eyes will move away from the prism and then shortly after, the eye without a prism will make an adduction movement to avoid diplopia. The same trend should occur when the examiner places the prism over the other eye.
If the prism is placed over an eye with a foveal suppression in monofixation patients, no movement in either eye will happen. When it is placed over the other eye, both eyes will move away from the prism, but there will be no adduction movement due to the other eye's suppression.
Risks & Complications
The test is generally a safe procedure. However, there might be the risk of incorrectly interpreting the eye movements, especially if the patient fails to cooperate.