What is the most likely cause of left scapular paresthesia, forearm fatigue with typing, and new left-hand weakness despite a normal cervical spine MRI?

Brachial Plexopathy: Most Likely Diagnosis Requiring MRI Brachial Plexus

Given left scapular paresthesia, forearm fatigue with typing, and new left-hand weakness despite a normal cervical spine MRI, the most likely diagnosis is nontraumatic brachial plexopathy, and you should order an MRI of the brachial plexus with and without IV contrast as the next diagnostic step. 1

Why Cervical MRI Was Insufficient

• Cervical spine MRI is inferior to brachial plexus MRI for evaluating plexopathy because it does not directly visualize the brachial plexus lateral to the neural foramina. 1
• The clinical overlap between radiculopathy and plexopathy is substantial, which is why cervical MRI is often performed first due to the higher prevalence of degenerative spine disease, but a normal cervical MRI does not exclude plexopathy. 1
• Your patient's symptoms—scapular paresthesia, typing-related forearm fatigue, and hand weakness—suggest involvement beyond a single nerve root, pointing toward plexus pathology. 1

Recommended Next Step: MRI Brachial Plexus

• Order MRI brachial plexus with and without IV contrast, which has 81% sensitivity, 91% specificity, and 88% overall accuracy for detecting plexopathy when compared to surgical findings and clinical follow-up. 1
• Brachial plexus MRI provides additional diagnostic information beyond clinical evaluation and electrodiagnostic studies in 45% of patients. 1
• The contrast enhancement is useful for detecting and characterizing various etiologies of nontraumatic brachial plexopathy including inflammatory, compressive, and neoplastic causes. 1

Differential Diagnosis to Consider

Thoracic Outlet Syndrome (TOS)

• TOS should be high on your differential, as it presents with upper limb pain, numbness, tingling, and weakness exacerbated by shoulder or neck movement—matching your patient's typing-related symptoms. 2, 3
• TOS is caused by dynamic neurovascular compression between the clavicle and first rib, often from anatomical variations, hypertrophied neck musculature, abnormal posture, or repeated overhead motions. 2
• MRI can demonstrate deviation or distortion of the brachial plexus in 79% of symptomatic cases and can identify cervical ribs, fibrous bands, or aberrant muscles (like subclavius posticus) causing dynamic compression. 4, 5

Other Plexopathy Etiologies

• Entrapment neuropathies from anatomical variants or muscle hypertrophy. 1
• Inflammatory or autoimmune processes affecting the plexus diffusely. 1
• Neoplastic lesions (schwannomas, neurofibromas, or metastatic disease) that can focally involve the plexus—contrast helps characterize these. 1
• Scapular instability with secondary brachial plexus irritation, which can cause diffuse upper extremity symptoms and may be related to proximal nerve irritation at the thoracic outlet. 6

Complementary Diagnostic Testing

• Electrodiagnostic studies (EMG/NCS) should be performed alongside imaging, as brachial plexus MRI is most useful when electrodiagnostic and physical findings are nonspecific. 1
• Look for reduced sensory nerve action potentials from the fourth and fifth digits, prolonged F-responses, or evidence of C8/T1 muscle denervation. 4
• Ultrasound of the neck may serve as a supplemental test in selected centers for assessing nerve enlargement, though it is not recommended as the primary imaging modality. 1

Critical Pitfalls to Avoid

• Do not assume the normal cervical MRI excludes all neurological pathology—the brachial plexus extends well beyond the neural foramina and requires dedicated imaging. 1
• Do not rely on plain radiographs alone, as they have no value in predicting brachial plexus distortion unless they demonstrate a cervical rib. 4
• Avoid ordering FDG-PET/CT or CT cervical spine as initial tests, as there is no supporting literature for their use in nontraumatic brachial plexopathy without known malignancy. 1
• Correlate imaging findings with clinical examination—MRI abnormalities do not always correspond to symptom location, so integration with physical findings is essential. 7, 8

Physical Examination Specifics to Document

• Test for thoracic outlet compression using provocative maneuvers (Adson's, Wright's, Roos test) to assess for dynamic plexus compression. 3
• Assess scapular stability and look for local myofascial trigger points, as scapular instability can contribute to brachial plexus irritation. 6
• Perform upper limb tension testing and evaluate pectoralis minor extensibility, as tightness can contribute to proximal nerve irritation. 6
• Document specific motor weakness patterns, particularly in C8/T1 innervated muscles (intrinsic hand muscles), and sensory deficits in the medial forearm and hand. 4