Near point of convergence is an eye test that measures the pursuit convergence. It is the point at which the visual axes meet under the highest effort of the confluence. NPC is assessed by bringing an object near the patient's nose and noting the point at which one of the eyes deviates out, or the patient sees double.
For a person to see an object close to their face, the eyes must converge, i.e., turn inwards towards the object. Convergence, therefore, is an individual’s ability to move or roll their eyes inward toward each other to see an object up close. It is to keep a single binocular vision when looking at a very close object. In the case of distance objects, the person’s eyes diverge or move the other direction. These two movements (divergence and convergence) are unique in that they are the only eye movements in which the eyes roll in the same direction and this is called disconjugate eye movements.
Convergence insufficiency or weak convergence is an abnormality where the eyes can’t coordinate when looking at objects up close. The disorder can cause one or both of the patient’s eyes to turn outward, creating blurred or double vision. It is most common in school-age children, young adults and people who play contact sports. It can lead to visual discomfort and vision-mediated functional difficulties like compromised attention and slowed reading, which may impair work, academic, and sports performance.
Convergence is fundamental in daily life. People depend on it for near-work activities like office work, using the smartphone, desk work at school, sports, and particularly in professions that require a good close up. Estimates indicate the disorder affects between two and thirteen percent of the population in the United States. Convergence insufficiency is the leading cause of complaints like headaches, blurred vision and eye strain.
The NPC test can be conducted at the eye clinic, hospital, or doctor’s office by:
- An orthoptist
- An optometrist
- An ophthalmologist
Patients with convergence insufficiency can have a seemingly normal vision, and it is common for the disorder to go undiagnosed. It is vital for patients to disclose to the doctor about any learning or reading difficulties. The causes of weak convergence are not yet clearly understood. They may include:
- Graves’ disease
- Myasthenia gravis
- Certain medications
- Traumatic brain injury
- Neurodegenerative diseases
It is also suspected the abnormality runs in families, and so doctors are interested in patients with a relative who suffers from convergence insufficiency. People who use the computer for long periods are also at a higher risk of it.
The signs and symptoms of the condition are different for every patient. Some patients display no symptoms. The common symptoms include:
- Vision problems
- Difficulty reading
- Squinting one eye
- Trouble concentrating
- Tired watery eyes after a near/far task
The brain may ignore or subdue one of the patient’s eyes to compensate for vision problems, also called vision suppression. It doesn’t solve the problem, but it stops the patient from seeing doubles. It may also reduce coordination, distance judgment and sports performance.
The doctor may diagnose convergence insufficiency by taking the patient’s medical history, performing a routine eye exam, assessing positive fusional vergence (PFV), and measuring the near point of convergence.
Preparation & Expectation
The NPC test doesn’t require any preparation. However, it is not appropriate for patients with heterotropia.
The patient will sit down in a well-illuminated room staring at a fixation object held before them at about 50 cm at the nose level. The doctor will observe keenly to detect any little changes in eye movements. He/she will move the fixation object smoothly and slowly close to the patient's nose and ask the patient to notify him/her when they see the fixation object in double. The patient should continue looking at the object even when it becomes blurry.
Then, the doctor will move the fixation object away from the eyes of the patient until the eyes begin to fixate on the object. The test may be repeated to ensure that the patient understands and follows the test. In the case of patients with weak convergence, the repeated tests move the NPC far from the earlier ones.
For a person with healthy eyes, the test result values are between six and ten centimeters, but the convergence recovery point can be up to 15 centimeters. An abnormal result is a situation where the NPC is over ten centimeters. It indicates convergence insufficiency. Patients with weak convergence can get diplopia and headaches after hours and intense reading or near tasks. Their convergence recovery point is above 15 centimeters.
Generally, where the patient has no symptoms, there is no need for treatment. If the patient has symptoms, the doctor can recommend various treatment options to increase eye convergence and improve or eliminate the abnormality. The most appropriate treatment depends on the patient’s preferences and age. Treatments include:
- Pencil pushups
Usually, they are the first line of treatment for weak convergence. The patient holds a pencil at arm’s length and focuses on it until they see a single image and then slowly bring it toward their nose until they see double. The exercises help to improve the patient’s convergence ability by decreasing the near point of convergence. The patient can do them at home for 15 minutes daily at least five days a week.
- In-office exercise
The therapy is done at the doctor’s office. They are visual exercises designed to improve the coordination of the patient’s eyes. It is best suited for children and young adults.
- Reading glasses
These can help to reduce double vision. It is a less effective treatment option and doesn’t correct convergence insufficiency. It only provides a temporary solution.
- Computer vision therapy
The exercises can improve the patient’s convergence ability by making their eyes focus. It is a more effective therapy option than other home exercises and can be fun for young adults and kids.
Where vision therapy is ineffective, the doctor may recommend surgery on the eye muscles. It is a rare treatment option that may lead to eye compilations such as esotropia, a disorder in which one or both of the patient’s eyes turn inward.
Risks & Complications
The Near point of convergence test has no risks or complications.