What is the most likely diagnosis in this case?

Academic Discussion: Harlequin Syndrome is a rare disorder of the autonomic sympathetic nervous system characterized by episodic flushing and sweating of one half of the face or body with pallor and dryness of the contralateral side — The pathophysiological mechanism is a functional defect in vasomotor and secretomotor sympathetic fibers with asymmetric innervation — The most common cause is injury or surgery in the neck or mediastinum affecting the sympathetic chain — The history of prior cervical surgery in this case makes this diagnosis the most likely | Differential Diagnosis: Horner Syndrome includes ptosis | miosis | and anhidrosis which are absent here — Carcinoid syndrome causes generalized flushing with diarrhea and systemic symptoms which are absent — Frey Syndrome follows parotid surgery and causes sweating and flushing triggered by chewing not by heat — Migraine does not produce this symmetric pattern of flushing and contralateral pallor | Distinguishing Features: Strict hemifacial distribution with contralateral pallor is the key distinguishing feature — Triggered by heat or exertion rather than chewing or odors — Absence of systemic symptoms | Work-up Logic: MRI of neck and chest to exclude compressive lesions — Starch-iodine test to confirm sweating asymmetry — CT chest to exclude apical lung lesion — No need for carcinoid workup in the absence of systemic symptoms | Results Interpretation: Normal imaging excludes serious structural causes — Sweat test confirms asymmetry and supports the diagnosis | Initial Management: Reassure the patient that the condition is benign in most cases — Avoid heat triggers and intense exertion when necessary — No specific pharmacological treatment required in mild cases | When to Observe: Mild cases not affecting quality of life require only observation and reassurance | When to Start Medical Therapy: In distressing cases beta-blockers or clonidine may be tried to reduce episode frequency | When to Admit: No admission required in uncomplicated cases | When it is an Emergency: Not an emergency — however urgent evaluation is warranted if accompanied by neurological weakness | severe pain | or signs of spinal cord compression | When to Escalate Airway / ICU: Not required in isolated cases | When Surgery is Indicated: No surgery required in primary cases — intervention may be discussed if a surgically correctable compressive lesion is identified | Complications: Psychosocial impact is the most common complication — thermoregulatory disturbances are rare | Red Flags: Development of ptosis or miosis requires exclusion of Horner Syndrome and spinal cord lesions — Any associated neurological deficit requires urgent evaluation | Clinical Pitfalls: Common error is misdiagnosis as skin allergy | migraine | or Horner Syndrome — Overlooking prior cervical surgery as a potential cause — Failure to perform confirmatory sweat testing | Follow-up and Referral: Referral to neurology for autonomic nervous system evaluation — Periodic follow-up to monitor symptom progression — Psychiatric or psychosocial referral if psychological impact is significant | Consultant Pearl: In any patient following cervical surgery who presents with hemifacial flushing and sweating with contralateral pallor Harlequin Syndrome must be considered immediately before pursuing dermatological or allergic etiologies — The starch-iodine test is the simplest and most readily available confirmatory tool