Research from the University of Cruzeiro do Sul in São Paulo, Brazil may contribute to an earlier diagnosis of diabetic neuropathy, a condition characterized by damage to peripheral nerves with symptoms such as pain and paresthesia (stinging, burning, and numbness) mainly in the legs and feet.
In the study, a group led by Professor Paulo Barbosa de Freitas Júnior measured grip strength in diabetics while they were holding and touching objects. The results were compared with data from healthy volunteers and patients with other neurological diseases such as multiple sclerosis, Parkinson’s and carpal tunnel syndrome (pain, numbness, and tingling in the hand and arm due to a pinched nerve in the wrist).
Freitas and his group tested volunteers to measure the grip strength of diabetics with and without a diagnosis of neuropathy, as well as healthy volunteers, and developed a methodology to manufacture devices for use in clinical practice. The innovation is intended to help doctors diagnose the disease quickly and easily in the future, not long after the first symptoms of neuropathy appear in diabetics.
The analysis focused on the gripping force used to hold and manipulate objects and the relative safety margin, i.e. the gripping force normalized by the coefficient of friction between the skin in contact with the object and the surface of the manipulated material. The central nervous system “calculates” the force it takes to hold an object and learns to do it better over time. “Every object has a contact surface with which our fingers create friction when they are held. The smoother the surface, the more gripping force is required to hold it. When it is rough, we can use less grip force thanks to the friction, ”says Freitas.
Given the gripping force and safety margin, people with neurological conditions like multiple sclerosis and Parkinson’s tend to grip objects more strongly than healthy people. The force required to manipulate an object is moderately greater than the minimum force required to hold it in the desired position. “In the case of people with neurological changes, we assume that as a conservative strategy they grip objects more strongly,” says Freitas. “The nervous system recognizes a neurological deficiency and sends the hand the order to use more force when grasping the objects. This process is of course completely unconscious.”
The test results showed that healthy subjects used between 100 and 120% of the minimum force required to hold an object, while the force used in participants with neurological changes was two and a half to three times as high.
Then Freitas and his team examined the performance of diabetics, who typically suffer from neuropathies as the disease progresses. “There was no previous research on diabetics with the type of experiment we used in our study,” he said.
The hypothesis was that diabetics would grip objects more firmly, as is the case with people with carpal tunnel syndrome, multiple sclerosis, or Parkinson’s disease. “We found the opposite, however: diabetics used half as much force to hold an object as controls did on the simplest task, the static test, in which a person only has to hold an object without manipulating it,” said controls Freitastas.
An article published in the journal Human Movement Science reports the key results of the study, which was funded by FAPESP through a Regular Research Grant and a scientific initiation grant.
Calculation of the gripping force
Three types of tests were performed on 36 volunteers, including 24 diabetics who were divided into two groups: 12 who had developed neuropathy and 12 who were neither diagnosed with neuropathy nor had clinical signs of the disease. The other 12 participants were healthy and formed the control group. Before the tests began, the researchers measured the skin sensitivity of each participant, as the sense of touch is a key factor in conveying the information to the central nervous system that the brain needs to calculate the force it takes to hold and manipulate objects.
The volunteers were each asked to perform three tasks with the same type of object that was instrumented to measure force. In the static hold test, they had to hold the object with their dominant hand as if they were holding a glass of water. After ten seconds, there was a beep and they were asked to slowly open their fingers to release the object while a researcher measured the friction between the fingers and the object. The second task was to pick up an object from a table, lift it about 2 inches, hold it for 10 seconds, and place it back on the table. In the third task, the so-called oscillation, the test subjects had to grab the object, hold it in front of the navel and move it up and down about 20 cm for 15 seconds.
For the second and third tasks, the results in diabetics and diabetics with neuropathy were similar to the controls. The surprise arose with the simplest of all tasks, the static hold, since diabetics and diabetics with neuropathy used half the force exerted by the controls.
The explanation is not exactly a loss of sensitivity in the fingers of diabetics, explained the researchers, but poor tactile information sent from their fingertips to the central nervous system. There is not enough high quality information for the brain to make the necessary calculations and for the hand to apply the correct force. “There are also studies that show that certain areas of the spinal cord and cerebral cortex that receive and process this sensory information are smaller in diabetics than in healthy people,” said Freitas.
The study suggests that diabetes not only affects the periphery of the body, resulting in, for example, loss of sensitivity in the toes and fingers, but also affects the central nervous system. “This happens early in diabetes. People tend to think that these complications don’t appear until a certain age or after a person has had diabetes for some time, but in fact, patients have the problem before neuropathy is diagnosed,” Freitas said.
New device for quick early detection
Scientists do not fully understand what causes neuropathy in diabetics. One hypothesis has to do with death of neurons or loss of function due to changes in blood vessels and metabolism. “Because of metabolic changes, the blood does not reach the nerve endings in the soles of the feet, palms, toes and fingers, which leads to the death of neurons in these peripheral regions,” suggested Freitas. As the disease progresses, neurons in other regions closer to the trunk, knees, etc., also become damaged and can die.
Given this lack of knowledge, prevention is the best option, and the study discussed here will help, according to Freitas. “Our research provides a foundation upon which a simple device for use in the doctor’s office could be developed with quick results,” he said. Grip and loading strength can serve as behavioral biomarkers to detect neurological changes before the patient shows clear symptoms of neuropathy. “The idea is to have a device that allows doctors to measure strength in a simple test and see if the patient shows signs of the onset of neurological changes.”
The diagnosis of neuropathy is currently made through a painful invasive examination with a so-called electroneuromyography, in which an electrical current is passed through small electrodes in the form of needles into the patient’s muscles and the reaction time is measured in order to assess the nerve conduction speed. Freitas suggests a procedure that can be followed during regular visits to the doctor. The patient would be holding an instrument that is instrumented to measure gripping force. “After 10 to 15 seconds of this test, the doctor would have the result: the object was held with this or that force level and the measured value is below or above the value that is considered safe for holding the object, neurological changes due to diabetes prove, “he said.
The next steps Freitas envisaged include developing the instrumented object for use in tests that might be simpler than those used in research. To do this, he has to determine the best combination between the weight of the object and the surface smoothness or roughness to prove the difference between diabetics and non-diabetics. “We need different combinations of smoothness and roughness and lighter and heavier weights to measure the differences between the combinations and choose the best ones for future testing,” he said. He partners with hospitals and companies interested in developing a solution, as well as volunteers for upcoming studies. He can be reached at Paulo.deFreitas@cruzeirodosul.edu.br or firstname.lastname@example.org.
Research Foundation São Paulo (FAPESP)
Lima, KCA, et al. (2021) Control and coordination of grip and load force in people with diabetes in various manipulation tasks. Human movement science. doi.org/10.1016/j.humov.2021.102793.