Horner Syndrome

Horner syndrome results from interruption of the oculosympathetic pathway anywhere along its course from the hypothalamus to the eye, producing the classic triad of upper eyelid ptosis, pupillary miosis, and facial anhidrosis.³ The pathway comprises three neurons in series: a first-order (central) neuron from the hypothalamus descending to the ciliospinal center of Budge (C8-T2); a second-order (preganglionic) neuron ascending through the neck near the pulmonary apex to the superior cervical ganglion; and a third-order (postganglionic) neuron traveling with the internal carotid artery into the orbit to innervate the iris dilator, Müller's muscle, and facial sweat glands. Localizing the lesion to a central, preganglionic, or postganglionic level is the central diagnostic task, since each level carries a distinct differential.

Etiology differs meaningfully between adults and children. In adults, acquired postganglionic disease is often associated with pain or headache — most frequently carotid artery dissection or cluster headache — while preganglionic disease is most often caused by tumor (classically an apical lung/Pancoast tumor) or trauma.³ Horner syndrome is also a recognized, uncommon iatrogenic complication of neck surgery, with a reported incidence around 0.03-0.48% after endoscopic thyroid surgery, from cervical sympathetic chain injury via retractor compression, thermal damage, or anatomic variation.² In children, acquired Horner syndrome carries a specific concern for neuroblastoma, with an evidence-based workup distinct from the adult approach.¹ Central lesions often have other brainstem/spinal cord signs; preganglionic lesions are more associated with neck/chest pathology; postganglionic lesions rarely produce prominent anhidrosis, since facial sweat gland innervation largely branches off before that level.³

Patients or caregivers note a droopy eyelid and/or smaller pupil on one side, sometimes with facial flushing asymmetry or reduced sweating. Acquired disease in adults, especially postganglionic, may be accompanied by ipsilateral head or neck pain, an important associated symptom given its link to carotid dissection.³ Congenital or long-standing Horner syndrome can show heterochromia (lighter iris on the affected side).¹

Key differentials: physiologic anisocoria (stable, without ptosis or dilation lag), simple aponeurotic ptosis without pupillary involvement, Adie's tonic pupil (large pupil with light-near dissociation), pharmacologic miosis, and third cranial nerve palsy (more severe ptosis, typically mydriasis rather than miosis when the pupil is involved).

Exam shows mild ptosis (1-2 mm, since only Müller's muscle, not the levator, is affected), mild elevation of the lower eyelid ("upside-down ptosis"), miosis with anisocoria greater in dim light, dilation lag, and anhidrosis or facial flushing asymmetry present in central and preganglionic lesions but often absent in postganglionic lesions. Heterochromia iridis occurs in congenital or long-standing cases. Pharmacologic testing confirms the diagnosis: apraclonidine dilates the affected pupil via denervation supersensitivity while having little effect on a normal pupil, a practical alternative to cocaine drops.³ Hydroxyamphetamine testing helps distinguish preganglionic/central from postganglionic lesions.²

Once the diagnosis is confirmed, further workup is guided by clinical context and suspected level: MRI (brain, neck, or chest as indicated) is generally recommended when a structural, vascular, or neoplastic cause is suspected.³ In children with acquired Horner syndrome, evidence-based workup for neuroblastoma and other pediatric-specific etiologies is essential and differs from the adult approach.¹ In iatrogenic Horner syndrome after neck surgery from reversible causes (hematoma, edema, inflammation compressing the cervical sympathetic chain), symptoms often resolve spontaneously or with a short course of steroids and neurotrophic therapy as the compressive process subsides.² Persistent Horner syndrome beyond about one year substantially diminishes the likelihood of spontaneous recovery, and surgical correction of residual ptosis may be considered at that point.²

• New Horner syndrome with acute head or neck pain should be treated as a possible carotid artery dissection until proven otherwise³
• Anhidrosis is a helpful localizing sign when present, but its absence does not exclude Horner syndrome, especially in postganglionic lesions³
• Apraclonidine is a practical, widely available pharmacologic confirmatory test³
• Acquired Horner syndrome in a child requires a directed workup for neuroblastoma¹
• Common pitfall: dismissing subtle, stable anisocoria as physiologic without confirming no dilation lag or ptosis
• Common pitfall: failing to consider carotid dissection in new postganglionic Horner syndrome with head/neck pain³
• Common pitfall: assuming postoperative Horner syndrome after neck surgery is necessarily permanent²