Is the brachial plexus involved in Horner syndrome?

Brachial Plexus Involvement in Horner Syndrome

Yes, the brachial plexus can be involved in Horner syndrome, particularly when the syndrome occurs due to malignancies like superior sulcus (Pancoast) tumors that directly invade the lower trunk of the brachial plexus. 1

Pathophysiological Relationship

Horner syndrome is characterized by the classic triad of:

• Ptosis (drooping eyelid)
• Miosis (constricted pupil)
• Anhidrosis (decreased sweating)

The relationship between Horner syndrome and the brachial plexus varies based on etiology:

Malignant Causes

• Superior sulcus (Pancoast) tumors: These lung tumors often directly invade the lower trunk of the brachial plexus and are frequently associated with Horner syndrome and pain along the ulnar nerve distribution 1
• Neurolymphomatosis: Horner syndrome can be the initial manifestation of brachial plexus neurolymphomatosis, even without bulky adenopathy 2

Iatrogenic Causes

• Brachial plexus blocks: Horner syndrome occurs in 100% of interscalene blocks and can also occur with other types of supraclavicular blocks 3
• Infraclavicular blocks: Can cause Horner syndrome as a side effect due to local anesthetic spread 4

Anatomical Considerations

• In adults, Horner syndrome typically indicates avulsion of C8 and T1 ventral roots
• In infants with obstetric brachial plexus palsy, the pathophysiology differs: some sympathetic preganglionic neurons in T1 innervate the superior cervical ganglion via the C7 ventral root, meaning Horner syndrome can occur with C7 root avulsion alone 5

Clinical Significance

1. Diagnostic value: The presence of Horner syndrome in brachial plexopathy of indeterminate etiology should raise clinical suspicion of malignant involvement, even without obvious adenopathy 2

2. Imaging implications:

   • MRI of the brachial plexus is the gold standard for evaluation with high sensitivity (84%) and specificity (91%) 6
   • FDG-PET/CT can be beneficial to differentiate radiation plexitis from tumor recurrence 6

3. Prognostic implications:

   • In adults with traumatic brachial plexus injury, Horner syndrome typically indicates a more severe injury involving avulsion of lower roots
   • In children, the implications may differ due to the anatomical variations mentioned above 5

Management Considerations

When Horner syndrome is observed in association with brachial plexopathy:

• Thorough imaging is essential to rule out malignant causes, particularly Pancoast tumors 1
• Reassurance is important when Horner syndrome occurs as a side effect of brachial plexus blocks, as it typically resolves within hours without sequelae 3, 4
• Close monitoring is recommended during resolution 4, 7

Common Pitfalls

1. Misinterpreting the significance: In adults, assuming Horner syndrome always indicates C8-T1 avulsion, when other causes should be considered

2. Age-related differences: Failing to recognize that the pathophysiological basis differs between adults and infants 5

3. Delayed onset: Horner syndrome can appear with delay after brachial plexus blocks, potentially causing unnecessary concern if not anticipated 7

4. Overlooking malignancy: Failing to consider neurolymphomatosis or other malignancies when Horner syndrome appears with brachial plexopathy 2

The presence of Horner syndrome should always prompt thorough evaluation of the brachial plexus, particularly when the etiology is unclear or when malignancy is suspected.