What causes diminished deep tendon reflexes in a patient, particularly in older adults with a history of diabetes?

Diminished Deep Tendon Reflexes: Causes and Clinical Significance

Diminished deep tendon reflexes indicate lower motor neuron pathology affecting the reflex arc at any point from the peripheral nerve, through the nerve root, to the anterior horn cell of the spinal cord. 1

Primary Pathophysiologic Mechanisms

Diminished or absent deep tendon reflexes result from disruption of the monosynaptic reflex arc, which requires intact sensory afferents, spinal cord connections, motor efferents, and functional muscle spindles. 1 The key distinction is that lower motor neuron lesions cause reduced or absent reflexes, while upper motor neuron lesions cause increased reflexes and spasticity. 1

Common Causes in Older Adults with Diabetes

Diabetic Peripheral Neuropathy

• Distal symmetric diabetic polyneuropathy is the most common cause of diminished reflexes in diabetic patients, typically presenting with prolonged latencies of ankle and patellar reflexes. 2
• Loss of protective sensation, assessed by inability to detect the 5.07 (10 g) monofilament, correlates with diminished deep tendon reflexes and indicates significant peripheral neuropathy. 3
• The Achilles tendon reflex (S1-mediated) is typically affected earlier and more severely than the patellar reflex (L4-mediated) in length-dependent neuropathies. 4

Diabetic Lumbosacral Radiculoplexus Neuropathy (Diabetic Amyotrophy)

• This condition presents with weakness, wasting, and diminished reflexes in the proximal lower limbs, representing a lower motor neuron disorder affecting both peripheral nerves and anterior horn cells. 1
• The adductor reflex is notably absent in diabetic amyotrophy (recorded in only 1 of 8 patients), which helps differentiate it from L2-L4 radiculopathy where the adductor reflex remains preserved in 95.5% of cases. 2

Radiculopathy

• L2-L4 radiculopathy affects the patellar reflex, while L5-S1 radiculopathy affects the ankle jerk reflex. 4
• Complete plexopathy causes flaccid loss of tendon reflexes in regions innervated by the affected nerve distribution (C5-T1 for brachial, L1-S3 for lumbosacral). 3

Other Important Causes

Drug-Induced Peripheral Neuropathy

• Bortezomib-induced peripheral neuropathy presents with suppression or reduction of deep tendon reflexes proportional to sensory loss, affecting predominantly sensory rather than motor function. 3
• Thalidomide-induced neuropathy develops in 70% of patients treated for 12 months, causing bilateral symmetric sensory disorders with diminished reflexes. 3

Vitamin B6 Deficiency (Pyridoxine Deficiency)

• Neurological manifestations include numbness/paresthesia progressing to loss of distal sensation, motor ataxia, weakness, and loss of deep tendon reflexes. 3
• The pathophysiology involves axonal degeneration similar to Wallerian degeneration, with motor neurons feeding distal muscles most affected, leading to denervation of muscle fibers. 3

Hepatic Encephalopathy

• In noncomatose patients with hepatic encephalopathy, deep tendon reflexes may diminish and even disappear, although pyramidal signs can still be observed—an unusual combination. 3
• This contrasts with the more typical presentation of hypertonia and hyperreflexia in hepatic encephalopathy. 3

Clinical Assessment Considerations

Examination Technique

• Evaluation should include checking deep tendon reflexes, vibratory sense, and position sense to assess for peripheral neuropathy. 3
• Touch sensation is best evaluated using monofilaments, with inability to detect the 5.07 (10 g) monofilament indicating loss of protective sensation. 3

Diagnostic Pitfalls

• Peak tap forces used by clinicians fall into three ranges: 0-20 Newtons for hyperreflexia, 21-50 Newtons for normoreflexia, and >50 Newtons for hyporeflexia. 5
• To determine whether diminished DTR is pathological, assess left-right differences, differences between upper and lower extremities, and overall limb balance. 6
• The Taylor hammer has a ceiling effect in the hyporeflexic range due to its small mass and short handle. 5

Confirmatory Testing

• Needle EMG has 90% sensitivity for lumbosacral radiculopathy, while nerve conduction studies alone have low diagnostic value. 4
• Electrodiagnostic studies should confirm clinical diagnosis and differentiate radiculopathy from plexopathy. 4
• The Hoffmann reflex (H reflex) can help distinguish muscle spindle dysfunction from other causes when tendon reflexes are absent but H reflexes remain normal. 7

Key Clinical Correlations in Diabetic Patients

In older diabetic patients, diminished reflexes most commonly indicate distal symmetric polyneuropathy when affecting ankle reflexes bilaterally, or diabetic amyotrophy when affecting proximal lower limb reflexes with associated weakness and wasting. 1, 2 The pattern of reflex loss—distal versus proximal, symmetric versus asymmetric—combined with sensory findings and muscle strength testing guides the specific diagnosis.