Each time I meet a new patient, I inquire whether they are right-handed, left-handed, or ambidextrous. Most patient’s answer without hesitation, however some patients are inquisitive why this would matter to me.
Whether dealing with a brain or spine problem, handedness is crucial in evaluation, and then in determining a treatment plan.
If the patient presents with a brain or spine problem, weakness may result. In determining the degree of weakness it is important to know the hand/ leg dominance. Paradoxically, right-handed people typically have about a 12.7% strength difference between their right and left sides, yet left-handed people have nearly equivalent grip strength.1 Therefore if a right-handed patient has slight left sided weakness, I may conclude this is due to normal asymmetry and the left being nondominant. However if a left-handed patient has slight right sided weakness, it is potentially more likely due to a problem related to the brain or spine.
Surgery on the nervous system inevitably involves manipulating normal neural structures. Even though low risk, new neurologic deficits, including weakness, may result from the surgery. Therefore the decision to pursue surgery, or the type and extent of surgery, takes into consideration the patient’s dominant side. Any surgery that could weaken the dominant side requires more scrutiny. At the same time, surgery may be considered more urgent on the dominant side to prevent permanent problems since, left untreated, this could be more debilitating.
Handedness, historically, was thought to be determined by which brain hemisphere was “dominant”. We all have two major brain (cerebral) hemispheres, right and left. Within each hemisphere, there are specialized primary motor and sensory areas, which control movement and sensation on the opposite side of the body. However, despite having a primary motor area on both the right and left hemispheres, we all tend to favor, have more strength, and be more coordinated in the right or left hand/ leg. About 85% of us are right handed.
Since motor movement in a human starts in the primary motor cortex and then travels through the spine to the extremities, it seems intuitive that handedness is determined by the brain. However newer research may question this assumption. Embryos at 8 weeks already tend to move their right arms more often than their left arms.2 At this stage of development, the connections are not yet solidified between the brain and spinal cord, and signals to the arms only come from the spinal cord. (The motor pathways from the brain to spinal cord pathways do not form until about 15 weeks). Furthermore, it is not until a few weeks later that left-right differences become visible in the shape and size of the premature brain.
Additionally, genetic analysis has shown that the left and right spinal cord develop at different paces. The left side of the spinal cord matures slightly faster than the right.
Therefore the ontogenesis of handiness likely resides in the spinal cord. Multiple genes have been linked to handedness, however a number of studies suggest that only ¼ of the variance in handedness is attributed to genetic variation, while ¾ is explained by environmental factors.3 These crucial genes may be altered by intrauterine stressors during pregnancy.
Petersen P. Grip strength and hand dominance: challenging the 10% rule. Am J Occup Ther. 43(7):444-447, 1989.
De Kovel C. Left-right asymmetry of maturation rates in human embryonic neural development. Biologica psychiatry. 82(3):204-212, 2017
Ocklenburg S. Epigenetic regulation of lateralized fetal spinal gene expression underlies hemispheric asymmetries. 3Life. 2017