Harlequin syndrome is characterized by the sudden onset of unilateral facial 
flushing and sweating, often preceded by exercise, excessive heat, or, rarely, 
regional anesthesia. Although the exact mechanism remains unclear, it is often 
referred to as transient or permanent interruption of the sympathetic nervous 
system. We present a case of Harlequin syndrome without Horner syndrome in a 
patient with unilateral right-sided facial flushing that started shortly after a 
left-sided thoracic paravertebral nerve block for a mastectomy. We discuss the 
interruption of the sympathetic and parasympathetic nervous system and the 
levels of spinal nerve block associated with a thoracic paravertebral nerve 
block.