Normal Age-Related Changes in the Nervous System
The phenomenon of deep tendon reflexes (DTRs) decreasing with age is part of the natural biological process of aging. The nervous system, like all other organ systems, undergoes structural and functional changes over a person's lifespan. These changes collectively contribute to a slower and less robust reflex response. For many reflexes, this change is not dramatic, and reflexes remain present, though they may be less brisk than in younger years.
One of the most significant changes is a reduction in the speed of nerve conduction. The protective myelin sheath surrounding nerve fibers can begin to deteriorate over time, much like insulation on an old electrical wire. This slows the transmission of signals from the sensory receptors to the spinal cord and back to the muscles, delaying the reflex response. As a result, older adults may experience slightly slower reaction times overall, affecting activities that require quick responses.
Another factor is the natural loss of muscle mass and muscle fiber over time, a condition known as sarcopenia. This loss can lead to weaker muscle contractions, which contributes to a less forceful reflex response. Furthermore, changes in motor neurons in the spinal cord, specifically a loss of anterior horn cells, mean that fewer neurons are available to transmit the signal back to the muscle.
The Effect on Different Reflexes
While the decrease in DTRs can affect various parts of the body, it is most pronounced in the lower extremities, particularly the Achilles (ankle) reflex. Several studies have quantified this effect. The patellar (knee) reflex is generally more stable and less affected by normal aging. It's also worth noting that many other DTRs, such as those in the upper limbs, tend to be preserved.
In addition to the decrease in some reflexes, older individuals may also exhibit so-called "pathological" or primitive reflexes that are not considered abnormal in a healthy older adult. These include the glabellar, palmomental, and snout reflexes. While these are typically a sign of disease in younger individuals, their presence in isolation in the elderly may simply be a result of the aging process. However, the presence of multiple primitive reflexes, or the presence of a Babinski reflex, can still signal an underlying neurological issue.
Normal Aging vs. Pathological Hyporeflexia
It is important for clinicians to distinguish between the symmetrical, gradual decline in reflexes caused by normal aging and the absent or asymmetric reflexes that signal an underlying disease. A decrease in reflexes, known as hyporeflexia, can result from damage to the reflex arc.
Causes of Hyporeflexia
- Peripheral Neuropathy: A common cause of hyporeflexia in older adults, often linked with diabetes, kidney disease, or vitamin deficiencies.
- Lower Motor Neuron Lesions: Damage to the motor neurons in the spinal cord or peripheral nerves can cause decreased reflexes.
- Endocrine Disorders: Conditions like hypothyroidism can lead to muscle weakness and slower nerve conduction, manifesting as hyporeflexia.
- Electrolyte Imbalances: Abnormal levels of electrolytes, such as high magnesium, can affect muscle and nerve function.
- Spinal Cord Issues: Spinal cord compression or injury can interrupt the reflex arc.
Distinguishing Normal Aging from Disease
| Characteristic | Normal Age-Related Change | Pathological Hyporeflexia |
|---|---|---|
| Symmetry | Typically bilateral and symmetrical. | Often unilateral or asymmetric. |
| Distribution | Primarily affects the ankle reflexes; others usually preserved. | Can affect reflexes across multiple regions, especially if associated with widespread neuropathy. |
| Reinforcement | Reflex can still be elicited or improved with maneuvers like the Jendrassik maneuver. | May remain absent even with reinforcement. |
| Progression | Gradual and slow over decades. | Can be sudden or progress more rapidly. |
| Accompanying Symptoms | May have mild changes in sensation or balance, but no severe motor weakness. | Often accompanied by other neurological symptoms like significant muscle weakness, numbness, or loss of coordination. |
The Clinical Importance
While a diminished Achilles reflex in an older person might not be a cause for concern on its own, a complete absence of DTRs, particularly after reinforcement, is often considered a pathological finding that warrants further investigation. The assessment of reflexes is a key part of the neurological examination and helps the clinician build a full picture of a patient's nervous system health. Monitoring these changes over time can help determine if a new or worsening neurological issue is developing.
For example, if an elderly patient's reflexes are consistently symmetric but diminished, and they have no other concerning neurological symptoms, this may be considered a normal part of aging. Conversely, a patient who develops an asymmetric reflex response or loses reflexes completely may need to be evaluated for a condition like diabetic neuropathy or a spinal cord issue. Regular physical activity can help mitigate some age-related decline in motor function, but it cannot stop the underlying physiological changes.
Conclusion
The answer to the question, do deep tendon reflexes decrease with age, is a qualified "yes." A slow, symmetric decline in certain reflexes, particularly the ankles, is a normal and expected part of aging due to nervous system changes, including slower nerve conduction and muscle alterations. However, significant asymmetry, complete absence, or the presence of other neurological symptoms may point to an underlying medical condition. The reflex examination remains an essential diagnostic tool for healthcare professionals to differentiate between the physiological effects of aging and potential neurological disease, ensuring appropriate and timely treatment.
Here is an excellent resource on the importance of neurological exams in the geriatric population.
