During one of my early visits with the neurologist, he conducted a test of which I didn’t understand the significance. He’d already asked me to squeeze his fingers in my fists - “oh come on, squeeze harder” - and now he is ‘thumping’ my fingers.
What is the neurologist looking for when he is tapping my finger?
The neurologist is looking to see if there is a finger flexor response. The finger flexor response is demonstrated by a sudden flexing of the thumb and/or index finger. There are two ways to cause this response:
The doctor snaps or flicks the nail of the middle or 4th finger. A positive finger flexor response elicited in this manner is known as the Hoffmann reflex or sign.
The doctor holds the middle finger while partially flexing it between his/her finger and thumb, then taps or flicks the underside of that finger. A positive finger flexor response elicited in this manner is known as the Trömner sign.
What causes the thumb to flex?
The finger flexor response (Hoffmann relex or Trömner sign) is somewhat similar to the Babinski sign in that it is suggestive of a lesion or impingement along the corticospinal tract.
What is the corticospinal tract?
Very long nerve axons which originate in the part of the brain called the cerebral cortex travel through the brainstem, cross over at the top of the cervical spine and travel down each side of the spinal cord. This path is the corticospinal tract which is sometimes called the pyramidal tract since the area where the crossover of nerves occurs has a pyramid-like shape.
Corticospinal tract neurons are referred to as “upper motor neurons” but they do not control muscles directly. Neurons in the ventral horn that directly innervate (or stimulate) muscle are called lower motor neurons. It is damage in lower motor neurons which causes atrophy of muscle, while damage in upper motor neurons does not.
How do the Hoffmann or Trömner signs differ from the Babinski sign?
Each of these signs indicate damage in the corticospinal tract. The Babinski sign indicates damage anywhere along the corticospinal tract. However, the Hoffman and Trömner signs are a bit more specific in that they indicate a lesion or damage above the C5 or C6 level of the cervical spine.
If I have a positive Hoffmann reflex, what symptoms might I notice?
The finger flexor response is often associated with noticeable weakening of the grip in the hands.
Is MS the only disease which can cause a positive Hoffmann reflex?
No. The finger flexion response is an indicator of a number of neurological conditions such as multiple sclerosis and ALS, or diseases which cause spinal cord compression (myelopathy) such as cervical spondylitis, tumors, or degenerative arthritis. A Hoffmann reflex can also happen in the presence of generalized hyperreflexia (overactive reflexes) as seen with anxiety or hyperthyroidism.
Does every patient with MS demonstrate the Hoffmann reflex?
No. Only patients who have damage along the corticospinal track will show the sign. On the other hand, researchers have found that patients without any damage may also show a false positive sign.
Researchers at the Medical University of South Carolina concluded that “the Hoffmann test is not a reliable screening tool for predicting the presence of cervical spinal cord compression.” However, the same researchers suggest that it is possible the Hoffmann test was more sensitive for finding early spinal cord dysfunction than MRI imaging conducted during the study. Perhaps it is that sensitivity which makes the Hoffmann test useful to the neurologist when MS or lesions in the cervical spine are suspected.
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