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Anatomy

Upper Limb

Question 39 of 180

A 54 year old man is admitted to the ED having been involved in a road traffic collision. He has sustained multiple fractures of the humerus. On examination, you identify weakness of flexion of the arm at the shoulder. Flexion and supination of the forearm is also weak and he complains of loss of sensation over the lateral forearm. Which of the following nerves has most likely been injured:

Answer:

The musculocutaneous nerve innervates the anterior arm muscles and skin over the lateral forearm. Injury to the nerve results in weakness of the arm, weakness of flexion and supination of the forearm, and loss of sensation over the lateral forearm.

Musculocutaneous Nerve Injury

Anatomy / Upper Limb / Innervation of Upper Limb

Last Updated: 29th January 2019

Table: Clinical Features of Musculocutaneous Nerve Injury

Nerve Musculocutaneous
Mechanism of injury Stab wound in axilla
Motor loss Weakness of flexion and supination of the forearm, weakness of arm flexion
Sensory loss Lateral aspect of forearm

Injury to the musculocutaneous nerve is rare, as it is relatively protected in the axilla.

The most common cause of injury is a stab wound in the axilla.

Flexion of the arm and flexion and supination of the forearm are weakened but not lost entirely due to the actions of the pectoralis major and deltoid, the brachioradialis and the supinator muscles respectively.

There is loss of sensation over the lateral aspect of the forearm.

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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