The finger flexor reflex is an intrinsic reflex of the finger flexors that is triggered by a blow to the palmar aspect of the middle finger distal phalanges. An exaggeration of reflex flexion is considered an uncertain pyramidal tract sign or a sign of autonomic dystonia. Definitive workup involves imaging and cerebrospinal fluid (CSF) diagnosis.
What is the finger flexor reflex?
The finger flexor reflex is an intrinsic reflex of the finger flexors that is triggered by a blow to the palmar aspect of the middle finger distal phalanges. The hand has several flexor muscles. Two of these flexors are the flexor digitorium profundus muscle and the flexor digitorium superficialis muscle. These are the deep and superficial finger flexors. The muscle reflex of these finger flexors is called the finger flexor reflex. The reflex flexion movement is triggered by a blow to the palmar side of the middle finger phalanx and corresponds to finger flexion. The monosynaptic intrinsic reflex was discovered in the 20th century by the German neurologist Ernest L. O. Trömner. The nerves involved are spinal cord segments C7 and C8, as well as the median and ulnar nerves. The finger flexor reflex is, in itself, an intrinsically physiological reflex. In contrast, when the reflex is exaggerated or merely unilateral, it is referred to as a Trömner sign with pathological value. Unlike the Trömner reflex, the Trömner sign can be evaluated as an indication of lesions of the pyramidal tracts (albeit an uncertain one) and thus corresponds to a weak pyramidal tract sign. The pyramidal tracts are the spinal cord tracts between the upper and lower central motor neurons and are an important circuit for all voluntary and reflex motor activity.
Function and purpose
Muscle intrinsic reflexes are monosynaptically connected protective reflexes that protect skeletal muscles from overextension. They are triggered by beating the tendons in which the muscle spindles of the respective muscles are located. Muscle spindles are extroceptive receptors. They detect strains and convert these mechanical stimuli into bioelectrical information. The non-contractile center of a muscle spindle fiber is surrounded by afferent sensory nerve fibers. These fibers are known as Ia fibers and conduct excitations to the central nervous system. When a muscle is stretched, stretching of the muscle spindle occurs at the same time. The Ia fibers conduct this stimulus in the form of an action potential across the spinal nerves in the posterior horn of the spinal cord and transmit the excitation through a synapse in the anterior horn of the spinal cord to the so-called α-motoneurons. These motoneurons transmit the information back to the skeletal muscle fibers along efferent pathways, thus stimulating the stretched muscle to contract. The flexor digitorum superficialis muscle and the flexor digitorum profundus muscle are involved in the finger flexor reflex according to this principle. The flexor digitorium superficialis muscle forms the middle flexor layer of the forearm. It is divided into four tendons in the carpal tunnel, which divide into two distinct reins shortly after their insertion. The muscle consists of the caput humeroulnare and caput radiale muscle heads. The flexor digitorum profundus muscle, on the other hand, forms the deep flexor layer on the forearm and, like the flexor digitorum superficialis muscle, divides into four different end tendons in the carpal tunnel. Both flexors are supplied by the median and ulnar nerves. The median nerve is a mixed arm nerve with motor and sensory parts. It originates from the medial and lateral fasciculus at the brachial plexus and has fibrous parts of segments C6 to Th1. Via the medial ulna, the median nerve runs to the forearm, where it descends between the flexor digitorum profundus and superficialis muscles to the wrist. Motorically, in addition to the ulnar portion, the median nerve innervates the flexor digitorum profundus muscle and many other flexors of the forearm. On the palmar hand, the sensitive portion of the nerve also supplies the skin above the ball of the thumb and skin surfaces of the index, ring and middle fingers. The mixed ulnar nerve also contains fibrous parts from C8 and Th1. It motor innervates the ulnar portions of the finger flexor.
Diseases and complaints
Pyramidal tract signs can give the neurologist an initial suspicion of pyramidal damage during the standardized reflex examination, if the clinic matches.This initial suspicion may prompt him to order an MRI with contrast administration. However, an exaggerated finger flexor reflex is merely an extremely weak pyramidal tract sign and need not actually indicate a lesion of the pyramidal tracts. Stronger pyramidal tract signs include Babinski group reflexes, which are far more important for an appropriate tentative diagnosis. In the past, the Trömner sign was interpreted as an undoubted indication of spastic lesions of the pyramidal tracts. Meanwhile, the neurological abnormality is more likely to represent vegetative dystonia as long as no other pyramidal tract signs are present and the patient’s clinical picture also does not fit a pyramidal lesion. Vegetative dystonia is present when there is hyperexcitability of the central nervous system. Nervousness, restlessness, insomnia and irritability or dizziness characterize the picture. In this phenomenon, the unconscious autonomic body function regulation of the autonomic nervous system is disturbed. The sympathetic nervous system and its antagonist, the parasympathetic nervous system, no longer work in harmony. Strain, hectic or stress can promote this phenomenon. Living contrary to natural rhythms such as the day-night rhythm can also favor vegetative dystonia. If the Trömner sign is after all associated with pyramidal lesions after a single examination, it may be accompanied by spastic or flaccid paralysis, muscle weakness or similar complaints. Depending on the exact location of the pyramidal lesion, neurological diseases such as MS or ALS, cerebral infarctions, spinal cord infarctions, space-occupying lesions or trauma to the structures involved may be possible causes. Final information is often provided by CSF diagnostics in addition to an MRI of the brain and spine.
