A 38-year-old woman presents with weakness of thumb opposition and abduction, along with sensory loss over the lateral three and a half fingers. Nerve conduction studies confirm median nerve compression at the wrist. All of the following clinical findings would be expected in this patient EXCEPT:

Question

Accepted Answer

A. Atrophy of the hypothenar eminence with preserved thenar eminence. ## Carpal Tunnel Syndrome — Clinical Presentation

### Case Analysis

This is a **classic presentation of carpal tunnel syndrome (CTS)** — median nerve compression at the wrist (within the carpal tunnel). The patient has:
- Motor loss: thumb opposition weakness (recurrent motor branch / thenar muscles)
- Sensory loss: lateral 3.5 digits (median nerve digital branches)

### Expected Findings in CTS (Options 0, 1, 3)

**High-Yield:** These three findings are hallmarks of carpal tunnel syndrome and would all be present.

| Clinical Test / Finding | Pathophysiology |
| --- | --- |
| **Phalen's test (positive)** | Wrist flexion increases pressure in carpal tunnel, compressing median nerve and reproducing paresthesias |
| **Weakness of FPL and FDP (digits 1–2)** | These muscles are innervated by the anterior interosseous nerve (AIN), a branch of the median nerve proximal to the carpal tunnel; compression at the wrist does NOT affect AIN |
| **Tinel's sign (positive)** | Percussion over the compressed median nerve at the wrist elicits tingling in its sensory distribution (lateral 3.5 digits) |

### Why Option 2 Is Wrong

**Key Point:** Carpal tunnel syndrome causes **thenar eminence atrophy** (not hypothenar), because the recurrent motor branch of the median nerve innervates the thenar muscles (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis).

**Clinical Pearl:** The hypothenar eminence is innervated by the **ulnar nerve** (deep branch). In CTS, the hypothenar eminence is preserved; the thenar eminence atrophies.

**Warning:** Confusion arises because both are "eminences" at the palm. The thenar eminence (lateral, thumb side) is median; the hypothenar eminence (medial, little finger side) is ulnar.

### Anatomical Basis of Motor Loss in CTS

```mermaid
flowchart TD
  A["Median Nerve at Wrist"]:::outcome --> B["Carpal Tunnel Compression"]:::urgent
  B --> C["Recurrent Motor Branch (Thenar)"]:::action
  C --> D["Abductor Pollicis Brevis"]:::action
  C --> E["Flexor Pollicis Brevis (Superficial head)"]:::action
  C --> F["Opponens Pollicis"]:::action
  D --> G["Thenar Eminence Atrophy"]:::outcome
  H["Anterior Interosseous Nerve"]:::outcome
  H -->|"Proximal to compression"| I["FPL, FDP (digits 1–2) NOT affected"]:::action
  J["Ulnar Nerve"]:::outcome
  J -->|"Separate nerve"| K["Hypothenar Eminence Preserved"]:::action
```

### Sensory Loss Pattern in CTS

**Mnemonic:** **LOAF** — Lateral (radial) 3.5 digits in median nerve distribution:
- Thumb (digit 1)
- Index finger (digit 2)
- Middle finger (digit 3)
- Lateral half of ring finger (digit 4, radial half)

[cite:Clinically Oriented Anatomy 8e Ch 6; Harrison 21e Ch 379]

Answer

B. Positive Tinel's sign (percussion over the median nerve at the wrist elicits tingling in the median nerve distribution)

Answer

C. Positive Phalen's test (wrist flexion for 60 seconds reproduces paresthesias)

Answer

D. Weakness of flexor pollicis longus and flexor digitorum profundus to the index finger

A 38-year-old woman presents with weakness of thumb opposition and abduction, along with sensory loss over the lateral three and a half fingers. Nerve conduction studies confirm median nerve compression at the wrist. All of the following clinical findings would be expected in this patient EXCEPT:

Explanation

Carpal Tunnel Syndrome — Clinical Presentation

Case Analysis

Expected Findings in CTS (Options 0, 1, 3)

Why Option 2 Is Wrong

Anatomical Basis of Motor Loss in CTS

Sensory Loss Pattern in CTS

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Clinical Test / Finding	Pathophysiology
Phalen's test (positive)	Wrist flexion increases pressure in carpal tunnel, compressing median nerve and reproducing paresthesias
Weakness of FPL and FDP (digits 1–2)	These muscles are innervated by the anterior interosseous nerve (AIN), a branch of the median nerve proximal to the carpal tunnel; compression at the wrist does NOT affect AIN
Tinel's sign (positive)	Percussion over the compressed median nerve at the wrist elicits tingling in its sensory distribution (lateral 3.5 digits)