A 68-year-old woman with a 10-year history of diabetes mellitus type 2 presents with orthostatic dizziness, recurrent syncope, and inability to maintain erection in her husband (she reports her partner's erectile dysfunction). On standing, her blood pressure drops from 140/85 mmHg (supine) to 110/70 mmHg (1 minute standing), and her heart rate remains unchanged at 72 bpm. Pupillary light reflex and accommodation are intact. Which autonomic fiber type is MOST likely affected in this patient?

Explanation

## Diabetic Autonomic Neuropathy: Selective Fiber Involvement **Key Point:** Diabetic autonomic neuropathy preferentially affects **small-diameter postganglionic sympathetic B fibers** (unmyelinated, slow-conducting), which are more vulnerable to hyperglycemic damage than larger myelinated fibers. ### Clinical Features of Postganglionic Sympathetic B Fiber Damage | Feature | Mechanism | Clinical Finding | |---------|-----------|------------------| | Orthostatic hypotension | Loss of α₁-mediated vasoconstriction | BP drop >20 mmHg systolic on standing | | Lack of reflex tachycardia | Impaired baroreceptor → sympathetic reflex arc | HR remains unchanged despite hypotension | | Anhidrosis | Loss of sudomotor innervation | Dry skin, impaired thermoregulation | | Erectile dysfunction | Loss of sympathetic inhibition of parasympathetic (nitric oxide pathway) | Vasogenic ED | | Pupillary light reflex intact | Parasympathetic fibers (CN II, III) preserved | Normal pupil constriction | **High-Yield:** The **intact heart rate response** (no reflex tachycardia despite hypotension) is pathognomonic for postganglionic sympathetic B fiber loss. In healthy individuals, baroreceptor reflex would trigger sympathetic discharge → tachycardia. Here, the efferent sympathetic limb is damaged. ### Mnemonic: SAND **S** — Sympathetic dysfunction (postganglionic B fibers) **A** — Anhidrosis (loss of sweat) **N** — No reflex tachycardia (baroreceptor reflex arc broken) **D** — Diabetic neuropathy (hyperglycemia → small fiber damage) ### Why Postganglionic Sympathetic Fibers Are Vulnerable 1. **Small diameter** (0.5–1.5 μm) → slower conduction → greater metabolic demand 2. **Unmyelinated** (B fibers) → exposed to glucose toxicity 3. **Long axonal length** → accumulation of advanced glycation end products (AGEs) 4. **Hyperglycemia** → polyol pathway activation → sorbitol accumulation → osmotic stress ### Clinical Pearl The **preserved pupillary light reflex and accommodation** rule out parasympathetic involvement. Parasympathetic fiber damage would cause **fixed, dilated pupils and loss of accommodation** (CN III involvement). Here, parasympathetic fibers are spared, confirming **selective postganglionic sympathetic B fiber neuropathy**. [cite:Harrison 21e Ch 377]