What is the cause of left hand weakness in digits 4 and 5, accompanied by intermittent paresthesia on the inner left scapula and arm numbness in certain positions, persisting for over 4 months?

Ulnar Nerve Compression at the Cervical Spine (C8-T1 Radiculopathy) or Thoracic Outlet Syndrome

Your presentation of weakness in digits 4 and 5 (ulnar distribution), scapular paresthesia, and positional arm numbness when lying supine strongly suggests either cervical radiculopathy at C8-T1 or neurogenic thoracic outlet syndrome, both requiring urgent MRI of the cervical spine to evaluate for cord compression or nerve root impingement. 1, 2, 3

Clinical Localization

Your symptom pattern points to a specific anatomical problem:

• Digits 4 and 5 weakness indicates ulnar nerve distribution involvement, which receives innervation from C8-T1 nerve roots 2, 3
• Inner scapular paresthesia suggests C8 or T1 nerve root irritation, as these roots supply sensation to the medial scapular region 1
• Positional arm numbness when supine with arm elevated (reading in bed) is pathognomonic for either cervical radiculopathy exacerbated by neck position or thoracic outlet syndrome from sustained arm abduction 1
• Unilateral symptoms (left side only) make systemic causes like diabetes or B12 deficiency less likely 1

Most Likely Diagnoses

Cervical Myelopathy or Radiculopathy (Primary Consideration)

The combination of hand weakness in ulnar digits with positional symptoms raises concern for cervical cord or nerve root compression 2, 3:

• "Myelopathy hand" classically presents with weakness and wasting of the ulnar fingers, loss of power in adduction and extension of digits 4-5, and inability to grip and release rapidly with these fingers 3
• The "amyotrophic type of myelopathy hand" specifically shows localized wasting and weakness of hand muscles without sensory loss or spastic quadriparesis, seen with cervical spondylosis at C5-6 and C6-7 levels affecting the C7-T1 spinal cord segments 2
• Critical red flag: 4 months duration with progressive weakness warrants urgent evaluation, as cervical myelopathy can cause irreversible cord damage if left untreated 4, 2

Thoracic Outlet Syndrome (Secondary Consideration)

Your positional symptoms strongly suggest TOS 1:

• Repetitive overhead arm positioning or sustained arm abduction (holding a book while lying supine) narrows the thoracic outlet, causing neurogenic TOS with chronic arm and hand paresthesia, numbness, or weakness 1
• The fact that symptoms occur specifically when lying on your back with arms elevated is highly characteristic of TOS 1
• Scapular pain/paresthesia can occur with TOS due to compression of the lower trunk of the brachial plexus 1

Immediate Diagnostic Workup

Order MRI of the cervical spine immediately to evaluate for:

• Cervical cord compression or signal changes indicating myelopathy 4, 2
• Nerve root compression at C7-T1 levels 1
• Anteroposterior canal diameter (normal >13mm; narrowing suggests myelopathy risk) 2
• Multisegmental spondylosis, particularly at C5-6 and C6-7 disc levels 2

Perform clinical examination tests for TOS 1:

• Adson's test (arm symptoms with neck rotation and deep inspiration)
• Wright's test (arm symptoms with arm abduction to 90 degrees)
• Roos test (arm fatigue/paresthesia with arms elevated and repetitive hand opening/closing for 3 minutes)

If TOS is suspected after clinical testing, consider dynamic CTA or MRA with imaging in neutral and stressed positions (arm abduction) to confirm neurovascular compression 1

Critical Red Flags Requiring Urgent Neurosurgical Consultation

Watch for these warning signs that indicate progressive myelopathy 4, 1:

• Rapidly progressive bilateral weakness with paresthesias
• Development of leg weakness, gait instability, or balance problems
• Bowel or bladder dysfunction
• Areflexia or hyporeflexia in affected limbs
• Ascending pattern of symptoms

Management Based on Diagnosis

If Cervical Myelopathy/Radiculopathy Confirmed

Surgical decompression should be considered within 48-96 hours if cord compression with myelopathy is present 4:

• Early surgery (within 96 hours) shows better outcomes than delayed intervention for traumatic or compressive myelopathy 4
• Posterior decompression (laminoplasty or laminectomy) with or without fusion is the typical approach for multilevel compression 4, 2
• Six of seven patients with "amyotrophic myelopathy hand" who underwent surgery showed recovery from muscle wasting and weakness 2

Conservative management may be attempted first if radiculopathy without myelopathy:

• Cervical collar for positional relief
• Physical therapy focusing on neck posture and nerve gliding exercises
• NSAIDs for inflammation
• However, if symptoms persist beyond 6-8 weeks or worsen, surgical consultation is warranted 2

If Thoracic Outlet Syndrome Confirmed

Conservative management is first-line 1:

• Physical therapy emphasizing postural correction, shoulder girdle strengthening, and scalene muscle stretching
• Avoid provocative positions (overhead activities, sustained arm abduction)
• Modify sleeping position to avoid arm elevation
• NSAIDs for inflammation

Surgical decompression (first rib resection, scalenectomy) is reserved for:

• Failure of 3-6 months of conservative therapy
• Progressive neurological deficits
• Severe functional impairment

Common Pitfalls to Avoid

• Do not assume carpal tunnel syndrome based solely on hand paresthesia; CTS typically affects digits 1-3 (median nerve distribution), not digits 4-5 5
• Do not delay imaging in the setting of 4-month duration with weakness; this timeframe suggests structural pathology requiring visualization 2, 3
• Do not attribute symptoms to "sleeping wrong" when they occur consistently in the same position; this indicates anatomical compression 1
• Do not miss bilateral symptoms that might develop later, as this would suggest systemic causes requiring different workup 1

Prognosis

• Cervical myelopathy outcomes are time-dependent; earlier surgical intervention correlates with better neurological recovery 4, 2
• "Myelopathy hand" can improve with appropriate surgical decompression if performed before irreversible cord damage occurs 2, 3
• TOS managed conservatively shows improvement in 50-90% of patients with appropriate physical therapy 1