What is the most likely location of the nerve lesion in a 52-year-old man with a 3-month history of loss of dexterity in his left hand, characterized by difficulty pinching fingers, inability to abduct or adduct the index, middle, ring, and small fingers, and absent sensation over the medial aspect of the left ring and small fingers, with Hyperesthesia (increased sensitivity) not being the primary complaint but rather loss of motor function and sensation?

Differential Diagnosis

• Single most likely diagnosis
  • Ulnar nerve at the elbow: The patient's inability to abduct or adduct the fingers, difficulty pinching, and sensory loss over the medial aspect of the hand are consistent with an ulnar nerve lesion. The ulnar nerve controls the interosseous muscles, which are responsible for finger abduction and adduction, and also provides sensation to the medial aspect of the hand.
• Other Likely diagnoses
  • Median nerve in the proximal forearm: A lesion here could affect the thenar muscles, leading to difficulty with thumb opposition and pinching. However, the sensory loss described is more consistent with an ulnar nerve lesion.
  • Ulnar nerve at the wrist: While an ulnar nerve lesion at the wrist could cause similar symptoms, the proximal location (at the elbow) is more likely given the patient's specific pattern of weakness and sensory loss.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • None specifically identified in this case, as the symptoms are relatively localized to the hand and forearm. However, it's essential to consider other potential causes of hand weakness, such as:
    • Cervical radiculopathy (e.g., C8 radiculopathy could cause similar symptoms)
    • Brachial plexopathy
    • Peripheral neuropathy (e.g., diabetic neuropathy, although the patient takes no medications and has no history of serious illness)
• Rare diagnoses
  • Anterior interosseous nerve lesion: This nerve is a branch of the median nerve and primarily controls flexion of the thumb and index finger. While it could contribute to some of the patient's symptoms, it's less likely to be the primary cause.
  • Radial nerve in the spiral groove of the humerus: A lesion here would more likely cause wrist drop or difficulty extending the wrist and fingers, which is not described in the patient's symptoms.