Diabetic Neuropathy

Overview

• Adams and Victor's Principles of Neurology, p. 2367
• Robbins & Kumar Basic Pathology, p. 461

Classification of Diabetic Neuropathies

• Harrison's Principles of Internal Medicine 22E, p. 587
• Adams and Victor's, p. 2369

1. Distal Symmetric Sensorimotor Polyneuropathy (DSPN)

• Paresthesias, numbness, tingling, burning pain beginning in the toes
• Gradual ascent up the legs in a "stocking" distribution; eventually involves fingers/arms ("glove" distribution)
• Severe cases: sensory loss on the trunk (first midline anteriorly, then laterally)
• Deep aching pains may be present
• Loss of ankle reflexes is typical
• Large-fiber loss: reduced vibration, proprioception; small-fiber loss: reduced pain/temperature

• Axonal degeneration (dying-back centripetal pattern)
• Segmental demyelination and remyelination
• Endothelial hyperplasia and microvascular changes in endoneurial capillaries
• Thickened/duplicated basement membranes of intraneural capillaries
• Painless form affects large fibers; painful form shows depletion of small myelinated and unmyelinated fibers with active regeneration generating neuropathic pain

• Reduced amplitudes
• Mild to moderate slowing of conduction velocities
• Proximodistal gradient
• Bradley and Daroff's Neurology, p. 961
• Harrison's 22E, p. 589

2. Autonomic Neuropathy

• Cardiovascular: Resting tachycardia, orthostatic hypotension, exercise intolerance, loss of heart rate variability
• Sudomotor: Anhidrosis, heat intolerance, hyperhidrosis
• GI: Gastroparesis (nausea, vomiting, early satiety, bloating), nocturnal diarrhea alternating with constipation
• Genitourinary: Cystopathy (urinary retention), erectile dysfunction, retrograde ejaculation, female sexual dysfunction
• Pupillary: Reduced pupillary response to darkness
• Dry eyes and mouth

• Harrison's 22E, p. 591

3. Diabetic Amyotrophy (Radiculoplexopathy / Bruns-Garland Syndrome)

• Presents with severe pain in the low back, hip, and thigh - often unilateral
• Weakness and atrophy of proximal and distal muscles develop within days to weeks
• Often accompanied by dramatic weight loss
• Is the presenting feature of DM in ~1/3 of patients
• CSF protein is usually elevated; ESR is often raised
• EMG/NCS: active denervation in proximal/distal limb muscles and paraspinal muscles
• Nerve biopsy may show axonal degeneration and perivascular inflammation
• Gradual spontaneous recovery is typical but residual deficits are common; high-dose glucocorticoids are sometimes used in the acute phase (evidence is limited)
• Harrison's 22E, p. 596

4. Cranial and Peripheral Mononeuropathies

• Third nerve palsy: Most common cranial mononeuropathy in diabetics; typically pupil-sparing (distinguishes it from compressive CN III palsy which affects the pupil)
• Sixth nerve palsy: Next most common
• Seventh nerve palsy: Relatively common but may have non-diabetic etiologies
• Peripheral: Median nerve at wrist, ulnar at elbow, peroneal at fibular head, lateral femoral cutaneous nerve (meralgia paresthetica)
• Mechanism: ischemic infarction of nerve fascicles from vasa nervorum occlusion

Pathogenesis

• Bradley and Daroff's Neurology, p. 968-974
• Adams and Victor's, p. 2427-2434

Diagnosis

• Clinical examination: Loss of vibration and position sense (posterior column), reduced ankle jerks, reduced monofilament sensation
• Nerve conduction studies (NCS) / EMG: Reduced amplitudes, slowed velocities; denervation in amyotrophy
• Quantitative sensory testing (QST)
• Skin punch biopsy: Intraepidermal nerve fiber density - most sensitive for small-fiber neuropathy
• Autonomic testing: Heart rate variability, Valsalva ratio, tilt-table testing, QSART
• Lab: HbA1c, B12 levels (metformin reduces B12 absorption), folate, TSH (to exclude other causes)

Treatment

Glycemic Control (Primary Prevention)

Lifestyle Modifications

• Exercise and diet have evidence for benefit in type 2 DM with DSPN
• Avoid neurotoxins: alcohol, smoking
• Correct vitamin deficiencies (B12, folate)

Pain Management in Painful DSPN

• Harrison's 22E, p. 1673-1674

Autonomic Neuropathy Management

• Orthostatic hypotension: Salt and fluid intake, compression stockings, avoid dehydration and diuretics, physical activity; pharmacologic: midodrine, droxidopa (FDA-approved)
• Resting tachycardia: Beta-blockers (with caution if hypoglycemia unawareness)
• Gastroparesis: Dietary modification (small frequent meals), metoclopramide, erythromycin (short-term)
• Erectile dysfunction: PDE-5 inhibitors, referral to urology
• GI symptoms in type 1 DM: Evaluate for associated celiac disease

Foot Care (Critical)

• Daily foot inspection for cracks, fissures, ulcers, or inflammation
• Avoid walking barefoot; wear protective footwear at all times
• Avoid heating pads on feet (thermal injury risk)
• Podiatry referral for deformities, calluses, nonhealing ulcers
• Charcot foot deformity results from undetected repeated fractures from loss of protective sensation
• Goldman-Cecil Medicine, p. 1425-1435

Risk Factors for Development

• Long duration of diabetes (strongest predictor)
• Poor glycemic control
• Presence of retinopathy and nephropathy
• Age > 50 years
• Type 1 and type 2 both susceptible (rates similar)
• Metabolic syndrome components: hypertension, hypertriglyceridemia, obesity
• Smoking (promotes microvascular disease)

Recent Literature

• Zhu et al. (2023) - "Diabetic peripheral neuropathy: pathogenetic mechanisms and treatment" [PMID: 38264279]
• Strand et al. (2024) - "Diabetic Neuropathy: Pathophysiology Review" [PMID: 38558164] - Curr Pain Headache Rep
• Yang et al. (2025) - "Diabetic neuropathy: cutting-edge research and future directions" [PMID: 40274830] - Signal Transduct Target Ther

1. DSPN is the most common form - stocking-glove sensory loss with neuropathic pain
2. Glycemic control is the only disease-modifying treatment; symptomatic pain treatment only manages symptoms
3. Multiple mechanisms converge on endoneurial ischemia and metabolic nerve injury
4. Foot care and daily foot inspection are non-negotiable to prevent ulceration and amputation
5. Autonomic neuropathy carries significant morbidity - cardiac, GI, and genitourinary involvement must all be screened for

Distal Symmetric Sensorimotor Polyneuropathy (DSPN) - Deep Dive

What It Is

• Washington Manual of Medical Therapeutics, p. 4953

Clinical Presentation

Symptom Progression

Toes → Feet → Ankles → Lower legs → Fingers → Hands → Forearms
                    (classic "stocking-glove" distribution)

Advanced: sensory loss on trunk (midline anteriorly first, then laterally)

Positive (Painful) Symptoms

• Burning pain, especially in feet
• Tingling and paresthesias
• Electric-shock-like sensations
• Hyperesthesia (even light touch is painful)
• Dysesthesias (unpleasant, abnormal sensations to touch)
• Pain is typically worse at night and at rest

Negative (Loss) Symptoms

• Numbness and sensory loss
• Loss of pain and temperature sensation (small fiber loss)
• Loss of vibration and proprioception (large fiber loss)
• Difficulty with balance (due to proprioceptive loss)
• Loss of ankle deep-tendon reflexes (early and reliable sign)

Motor Involvement (Later)

• Distal muscle wasting and weakness
• Foot drop (peroneal involvement)
• Foot deformities (hammer toes, high arch) from intrinsic muscle imbalance

Key Clinical Course Note

• Harrison's 22E, p. 1647-1648

Histopathology

• Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 2454
• Bradley and Daroff's Neurology, p. 961

Large Fiber vs. Small Fiber Involvement

• Painless DSPN predominantly affects large fibers
• Painful DSPN shows marked depletion of small myelinated and unmyelinated fibers, with active regeneration generating ectopic discharges

Electrodiagnostics (NCS/EMG)

• Reduced sural nerve amplitude (sensory) - earliest and most sensitive change
• Reduced amplitude in peroneal and tibial motor nerves
• Mild-to-moderate slowing of conduction velocities (reflects secondary demyelination, not primarily a demyelinating process)
• Proximodistal gradient - abnormalities worse distally
• EMG may show mild denervation in intrinsic foot muscles in advanced cases

Diagnosis Screening Protocol

• Type 1 DM: Begin 5 years after diagnosis
• Type 2 DM: Begin at time of diagnosis (neuropathy may precede clinical DM diagnosis)

Screening Tools

• Harrison's 22E, p. 1647-1648; Washington Manual, p. 4958

Pathogenesis (DSPN-Specific)

Hyperglycemia
     │
     ├──► Polyol pathway ──► Sorbitol/fructose accumulation
     │         └──► Myoinositol depletion ──► ↓ Na+/K+-ATPase ──► Axonal dysfunction
     │
     ├──► AGE formation ──► Protein crosslinking + vascular damage
     │
     ├──► PKC activation ──► Endoneurial vasoconstriction ──► Ischemia
     │
     ├──► Oxidative stress (ROS) ──► Direct axonal/Schwann cell injury
     │
     └──► Endoneurial capillary changes:
              - Endothelial hyperplasia
              - Basement membrane reduplication
              - Luminal narrowing
                    └──► Nerve hypoxia ──► Axonal atrophy + ↓ NCV

Plus: ↓ NGF, VEGF, erythropoietin (neurotrophic factor deficiency)

• Bradley and Daroff's Neurology, p. 968-974

Treatment (Detailed)

1. Disease Modification

2. Lifestyle

• Exercise and dietary modification: evidence for benefit in type 2 DM
• Avoid alcohol and smoking (neurotoxins + worsen microvascular disease)
• Check and replace B12 (metformin depletes B12; pernicious anemia more common in type 1 DM)
• Check folate

3. Symptomatic Pain Treatment

• Tapentadol (centrally acting opioid): FDA-approved but modest efficacy + addiction risk
• Opioids: Generally avoid - limited evidence, high addiction risk

• α-Lipoic acid 600 mg TID: antioxidant; some evidence in early DPN
• High-dose thiamine (benfotiamine) 50-100 mg TID: tested in early DPN
• Washington Manual, p. 4965; Harrison's 22E, p. 1673-1674

4. Foot Care (Non-Negotiable)

• Aggressive debridement of necrotic tissue
• Culture-guided systemic antibiotics if infected
• Pressure off-loading (total contact casting, special footwear)
• Revascularization if peripheral arterial disease is present

• Daily self-inspection of feet
• Never walk barefoot (even indoors)
• Avoid heating pads on feet
• Podiatry referral for deformities, extensive callus, or nonhealing ulcers

Key Points to Remember

1. Up to 50% of DSPN patients are asymptomatic - screening is essential
2. Pain paradoxically diminishes as neuropathy worsens (fibers are destroyed)
3. NCS can be normal in pure small-fiber neuropathy - biopsy is gold standard
4. Glycemic control is the only disease-modifying treatment - everything else is symptomatic
5. LOPS (failure to feel 10-g monofilament) is the critical threshold for foot ulceration risk
6. Annual screening: type 1 DM at 5 years, type 2 DM at diagnosis
7. The histologic hallmark is axonal loss with endoneurial microvascular disease - NOT primarily demyelination