The flicker test measures CFF (critical flicker fusion frequency). CFF is an abbreviation for Critical Flicker Fusion frequency. As the frequency of a flickering light is gradually increased, it is perceived as continuous light at a certain frequency. This threshold is the CFF value.
It is also called central flicker value measurement or the CFF test.
A visual acuity test evaluates spatial resolution. In contrast, the flicker test evaluates temporal resolution. The two reflect different aspects of visual function.
The CFF value sensitively reflects optic nerve conduction function. Even if visual acuity is normal, the CFF value decreases if there is optic nerve damage. It has an important role as a functional test that complements visual acuity testing.
The main purpose of the CFF test is to evaluate optic nerve function. It is important to measure each eye separately and assess the difference from the healthy side.
Optic nerve diseases
Optic neuritis (retrobulbar optic neuritis): The CFF value decreases markedly. A low value often remains even after vision recovers, making it a useful indicator of recovery.
Compressive optic neuropathy: It gradually declines due to optic nerve compression from tumors or thyroid eye disease. It can precede visual acuity loss.
Multiple sclerosis: CFF value decreases due to optic nerve dysfunction caused by demyelination.
Other indications
Glaucoma: Including normal-tension glaucoma, the CFF value decreases in advanced cases2).
Central serous chorioretinopathy: It is used to assess macular function.
Preoperative evaluation for cataract surgery: It is less affected by media opacity and is useful for predicting postoperative visual acuity3).
In optic neuritis, the CFF value may remain reduced even after visual acuity recovers to 1.0. For this reason, CFF testing is also useful for judging treatment effect and follow-up.
A visual acuity test measures spatial resolution (the ability to distinguish fine details). In contrast, a flicker test measures temporal resolution (the ability to detect changes in light). If there is damage to the optic nerve, temporal resolution can decline even when visual acuity is preserved. If the CFF value is low despite good visual acuity, optic nerve dysfunction should be suspected.
The CFF testing device is small. It can be easily performed in an outpatient clinic.
The test is performed in the following steps.
It takes about 2 to 3 minutes per eye. Usually, 3 to 5 measurements are taken and the average value is used.
Normal CFF values vary by age1).
| Age group | Reference normal CFF value |
|---|---|
| 20s to 30s | 40–45 Hz |
| 40s to 50s | 35–40 Hz |
| 60 years and older | 30–38 Hz |
A decline in CFF with age is a physiological change. Age needs to be considered when interpreting the measurement.
The flicker test is completely noninvasive and causes no pain at all. You only need to look at a flashing light and press a button. It takes about 2–3 minutes per eye, or about 5–6 minutes for both eyes. No special preparation is needed.
The following criteria are used to determine abnormal CFF values.
The following shows CFF change patterns by disease.
| Disease | CFF change | Features |
|---|---|---|
| Optic neuritis | Marked decrease | Low values may persist even after visual acuity recovers |
| Compressive optic neuropathy | Gradual decrease | May precede decreased visual acuity |
| Glaucoma | Mild decrease | Pronounced in advanced cases |
| Cataract | Mild to normal | Minimal effect from ocular media opacity |
Evaluating the difference between the two eyes is especially useful for diagnosing unilateral optic nerve disease. In bilateral cases, judge based on the absolute value.
The CFF test is a subjective test. Results are affected by the examinee’s concentration and level of understanding. Reduced attention and fatigue can cause false positives. It is desirable to explain the procedure thoroughly before the test and perform practice measurements.
The CFF test is not used by itself to make a definitive diagnosis of a specific disease. It is judged together with the results of other tests such as visual acuity testing, visual field testing, OCT, and VEP. Abnormal CFF values are used as a supportive indicator suggesting optic nerve dysfunction.
Compare the main tests used to assess optic nerve function.
RAPD (relative afferent pupillary defect)
Sensitivity: Highly sensitive for detecting differences between the two eyes.
Quantifiability: Quantitative assessment is difficult.
Feature: An objective test that uses the pupillary light reflex and does not require special equipment.
VEP (visual evoked potentials)
Sensitivity: Excellent at detecting optic nerve conduction abnormalities.
Quantitativeness: Allows objective assessment of latency and amplitude.
Feature: The equipment is large, and the test takes a long time.
OCT (RNFL thickness measurement)
Sensitivity: Excellent at detecting structural changes.
Quantitativeness: Allows measurement in micrometers (μm).
Feature: There is a time lag between structural changes and functional impairment4).
CFF test
Sensitivity: Detects reduced temporal resolution.
Quantitativeness: Allows measurement in hertz (Hz).
Features: Simple, quick, and low-cost to perform.
Compared with other tests, the CFF test is especially simple. It does not require special equipment and can be completed quickly. Used together with other tests, it allows a comprehensive evaluation of optic nerve function.
There is a neurophysiological basis for a decrease in CFF values.
When demyelination or axonal damage occurs in the optic nerve, nerve conduction slows down. Because high-frequency signal transmission is impaired, the fusion frequency decreases.
Among retinal ganglion cells, M cells (magnocells) have high temporal resolution. M cells are involved in motion perception and detecting changes in light and dark. The CFF test is thought to mainly reflect the function of the M-cell pathway.
Damage to the M-cell pathway is said to occur early in glaucoma2). This is the basis for using the CFF test in glaucoma screening.
With the spread of OCT and OCT angiography, image-based tests have become the main approach for evaluating the optic nerve. The use of CFF testing has been declining relatively.
On the other hand, the following advantages of CFF testing are being re-evaluated.
In recent years, research has been progressing on simple CFF measurements using tablet devices and smartphone apps. With improvements in the screen refresh rates of digital devices, better accuracy is expected for simplified measurements.
Applications to telemedicine and home monitoring are also being considered. They may be useful for serial evaluations during the recovery process of optic neuritis.
