The Finger-to-Nose Test (FNT) is a common, non-invasive procedure used in a neurological examination to quickly screen a person’s coordination and motor control. Clinicians use this simple bedside test to assess the ability to perform smooth, accurate, and goal-directed movements of the upper limbs. The FNT checks the functionality of the nervous system pathways responsible for balance and the precise execution of voluntary actions. This information helps medical professionals localize potential issues within the brain that might be affecting motor skills.
The Mechanics of the Test
The procedure begins with the patient sitting or standing comfortably, facing the examiner. The patient is asked to fully extend one arm out to the side, then touch the tip of their own nose with their index finger. They then reach out and touch the examiner’s index finger, held at a distance requiring full elbow extension.
This sequence is repeated several times using the same hand. The examiner often moves their finger to different locations to ensure the patient accurately targets a new endpoint. Variables include the speed of the movement and whether the patient performs the task with their eyes open or closed.
The test is repeated with the opposite hand to compare performance between the two sides. The arm should not be held against the side of the body, as this can mask a tremor or incoordination. The clinician observes the entire trajectory of the finger as it moves between the two targets, looking for smoothness and precision.
The Primary Target: Cerebellar Function
The Finger-to-Nose Test primarily measures the function of the cerebellum, a structure located at the back of the brain, under the cerebrum. Although it accounts for only about 10% of the brain’s volume, the cerebellum contains over half of the brain’s total neurons and is a major center for motor control. It does not initiate movement but acts as a coordinator and regulator of voluntary actions.
The cerebellum integrates sensory input about the position of the body and limbs with motor commands from the cerebrum. This continuous feedback loop allows for the fine-tuning of movements, ensuring they are fluid and accurate. It coordinates the timing and force of different muscle groups to produce smooth limb movements.
During the FNT, the cerebellum ensures the motor plan is executed precisely. It manages the interplay between agonist and antagonist muscles to control the trajectory, speed, and stopping point of the limb. Disruption to the cerebellum or its connecting pathways results in a noticeable loss of this smooth, regulated control.
Abnormal Findings and Related Conditions
Difficulty performing the Finger-to-Nose Test suggests a disruption in the cerebellar circuitry. The most common finding is dysmetria, an error in judging the distance or range required for a movement. This inability to control the force and extent of the action results in the finger consistently overshooting or undershooting the intended target.
Dysmetria
Dysmetria is categorized into two types. When the finger consistently travels past the target, it is called hypermetria. Undershooting the target is known as hypometria. Dysmetria is caused by a deficit in the brain’s ability to control the complex interactions of forces across multiple joints during movement. This loss of accuracy results in a clumsy or unsteady movement pattern.
Intention Tremor
Another characteristic finding is an intention tremor, which manifests as rhythmic, involuntary oscillations of the limb. This tremor becomes noticeable or worsens as the finger gets closer to the target. Unlike a tremor at rest, this oscillation only occurs during a purposeful, goal-directed action. This reflects the cerebellum’s struggle to correct the movement in the final moments before contact. The combination of dysmetria and intention tremor is a hallmark of cerebellar dysfunction.
These findings are part of ataxia, a general term for a lack of voluntary coordination of muscle movements. Conditions causing cerebellar ataxia detectable by an abnormal FNT include acute events like a stroke or hemorrhage affecting the cerebellar tissue. Neurodegenerative disorders, such as Multiple Sclerosis, spinocerebellar ataxias, or a tumor, may also damage the cerebellum or its pathways. Temporary cerebellar dysfunction can also be induced by certain medications or intoxication.