Diminished Deep Tendon Reflexes in a 30-Year-Old Male
Diminished deep tendon reflexes in a 30-year-old male indicate lower motor neuron pathology affecting any point of the reflex arc from peripheral nerve through nerve root to anterior horn cell, and the most common causes in this age group include peripheral neuropathy, radiculopathy, plexopathy, Guillain-Barré syndrome, and myopathies. 1
Primary Pathophysiologic Framework
Lower motor neuron lesions cause reduced or absent reflexes by disrupting the reflex arc, while upper motor neuron lesions cause increased reflexes and spasticity—this fundamental distinction guides the entire diagnostic approach. 1
The reflex arc can be interrupted at multiple levels:
- Peripheral nerve (polyneuropathy or mononeuropathy)
- Nerve root (radiculopathy)
- Plexus (plexopathy)
- Anterior horn cell (motor neuronopathy)
- Muscle (myopathy) 1
Common Causes by Anatomic Location
Peripheral Neuropathy
Length-dependent polyneuropathies affect the Achilles tendon reflex (S1-mediated) earlier and more severely than the patellar reflex (L4-mediated), creating an asymmetric pattern of reflex loss. 1
Key causes include:
- Diabetic neuropathy: Loss of ankle reflexes correlates with inability to detect the 5.07 (10 g) monofilament, indicating loss of protective sensation 1
- Toxic neuropathies: Bortezomib causes suppression or reduction of deep tendon reflexes proportional to sensory loss 1; thalidomide causes bilateral symmetric sensory disorders with diminished reflexes in 70% of patients treated for 12 months 1
- Nutritional deficiencies: Vitamin B6 deficiency causes numbness/paresthesia progressing to loss of distal sensation, motor ataxia, weakness, and loss of deep tendon reflexes 1
Radiculopathy
L2-L4 radiculopathy affects the patellar reflex, while L5-S1 radiculopathy affects the ankle jerk reflex—this anatomic specificity makes reflex examination highly localizing. 1
Needle EMG has 90% sensitivity for lumbosacral radiculopathy, while nerve conduction studies alone have low diagnostic value. 1 Electrodiagnostic studies should confirm clinical diagnosis and differentiate radiculopathy from plexopathy. 1
Plexopathy
Complete plexopathy causes flaccid loss of tendon reflexes in regions innervated by the affected nerve distribution (C5-T1 for brachial, L1-S3 for lumbosacral). 2, 1
Plexopathy manifests as neuropathic pain, dysesthesia, and/or burning sensation occurring in more than one peripheral nerve distribution. 2 The clinical diagnosis requires electrodiagnostic confirmation. 2
Diabetic lumbosacral radiculoplexus neuropathy (diabetic amyotrophy) 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
Guillain-Barré Syndrome
Absent or decreased tendon reflexes in affected limbs is a required diagnostic feature of GBS, typically developing over days to 4 weeks. 2
Supporting features include:
- Progressive bilateral weakness of arms and legs
- Relative symmetry of symptoms
- Relatively mild sensory symptoms
- Bilateral facial palsy
- Increased CSF protein (though normal protein does not rule out diagnosis) 2
Electrodiagnostic studies reveal sensorimotor polyradiculoneuropathy with reduced conduction velocities, reduced amplitudes, and typical "sural sparing pattern" where sural sensory nerve action potential is normal while median and ulnar sensory potentials are abnormal. 2 However, electrophysiology may be normal when performed within 1 week of symptom onset. 2
Myopathies
Myotonic dystrophy shows absent or weak tendon reflexes in almost all cases, while cutaneous reflexes remain normal. 3 The selective atrophy of Type 1 muscle fibres, which are involved in deep reflex responses, is responsible for early disappearance of tendon reflexes. 3
Congenital myopathies present with hypotonia and preserved or diminished reflexes with normal creatine kinase, distinguishing them from muscular dystrophies which show significantly elevated CK. 4, 5
Critical Diagnostic Considerations
Pattern Recognition
Assess for asymmetry: Left-right differences, differences between upper and lower extremities, and overall balance of limb reflexes determine whether diminished reflexes are pathological. 6
Timing matters: In the Achilles reflex, the latency pattern helps differentiate causes—prolonged latencies occur in both peripheral neuropathy and L2-L4 radiculopathy, but the adductor reflex shows prolonged latency specifically in L2-L4 radiculopathy. 7
Red Flags That Suggest Alternative Diagnoses
Features that cast doubt on lower motor neuron pathology:
- Hyperreflexia or clonus suggests upper motor neuron lesion 2
- Extensor plantar responses indicate corticospinal tract involvement 2
- Sharp sensory level indicates spinal cord injury 2
- Marked persistent asymmetry suggests focal structural lesion 2
- Bladder or bowel dysfunction at onset suggests cauda equina syndrome or spinal cord pathology 2
Examination Technique Pitfalls
Initial preservation of deep tendon reflexes in the presence of profound muscle weakness can occur in early stages of anterior horn cell disease before cell destruction is complete, and may actually indicate better prognosis. 8
The adductor reflex can be useful in differential diagnosis of lower motor neuron disorders presenting with patellar reflex disturbance—it is preserved in 95.5% of patients with L2-L4 radiculopathy but absent in diabetic lumbosacral radiculoplexus neuropathy. 7
Recommended Diagnostic Approach
Evaluate deep tendon reflexes, vibratory sense, and position sense to assess for peripheral neuropathy. 1 Touch sensation is best evaluated using monofilaments, with inability to detect the 5.07 (10 g) monofilament indicating loss of protective sensation. 1
Check for focal neurologic findings: Muscle weakness, abnormal deep tendon reflexes, and severe abnormalities in muscle tone may require brain or spine MRI. 2
Order electrodiagnostic studies when clinical localization is unclear or to confirm diagnosis, recognizing that studies may be normal early in disease course. 2, 1
Consider MR neurography for plexopathy evaluation, using high-resolution T2-weighted sequences through oblique planes of the plexus with T1-weighted, T2-weighted, and fat-saturated sequences. 2