Horner Syndrome

Overview

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 Because the sympathetic pathway is a three-neuron chain spanning the central nervous system, the neck, and the orbit, accurately localizing where along that chain the lesion has occurred is the central diagnostic task — different levels carry very different, sometimes urgent, underlying causes.

Epidemiology & Risk Factors

Etiology differs meaningfully between adults and children. In adults, acquired Horner syndrome is often postganglionic and associated with pain or headache, most frequently as a consequence of carotid artery dissection or during cluster headache; preganglionic disease is most often caused by a tumor (classically an apical lung/Pancoast tumor) or trauma.³ Horner syndrome is also a recognized, though uncommon, iatrogenic complication of neck surgery — for instance, endoscopic thyroid surgery, with a reported incidence around 0.03-0.48% in that specific surgical context, resulting from intraoperative cervical sympathetic chain injury via retractor compression, thermal energy device damage, or anatomic variation.² In children, etiology diverges further from adults: acquired pediatric Horner syndrome carries a specific and important concern for neuroblastoma, and the evidence-based diagnostic workup in infants and children differs from that used in adults.¹

Pathophysiology

The oculosympathetic pathway comprises three neurons in series: a first-order (central) neuron from the hypothalamus descending through the brainstem and cervical spinal cord to the ciliospinal center of Budge (C8-T2); a second-order (preganglionic) neuron exiting the spinal cord, ascending through the neck near the pulmonary apex and carotid sheath, and synapsing at the superior cervical ganglion; and a third-order (postganglionic) neuron traveling with the internal carotid artery into the cavernous sinus and orbit to innervate the iris dilator muscle, Müller's muscle of the upper eyelid, and facial sweat glands. A lesion anywhere along this chain produces the syndrome, but the specific combination of associated findings differs by level: central lesions often have other brainstem/spinal cord signs, preganglionic lesions are more associated with neck/chest pathology (including tumors), and postganglionic lesions are most often associated with head/neck pain and rarely produce prominent anhidrosis, since sweat gland innervation to the face largely branches off before the postganglionic level.³

Clinical Presentation

Patients (or, in the case of infants, their caregivers or examining clinicians) note a droopy eyelid and/or a smaller pupil on one side, sometimes accompanied by facial flushing asymmetry or reduced sweating on the affected side. Acquired disease in adults, especially postganglionic, may be accompanied by ipsilateral head or neck pain, which is itself a diagnostically important associated symptom given its association with carotid dissection.³ In congenital or long-standing Horner syndrome, heterochromia (a lighter iris color on the affected side) can develop, since sympathetic innervation influences iris melanocyte pigmentation during early development.¹

Exam Findings

• Mild ptosis of the upper eyelid (typically 1-2 mm, since only Müller's muscle — not the levator — is affected, unlike the more severe ptosis seen in a CN III palsy)
• Mild "upside-down ptosis" or elevation of the lower eyelid may also be seen, from loss of sympathetic tone to the inferior tarsal muscle
• Miosis, with the anisocoria greater in dim illumination (since the affected pupil fails to dilate normally in the dark)
• Dilation lag: the affected pupil dilates more slowly than the normal pupil when the lights are dimmed
• Anhidrosis or facial flushing asymmetry, present in central and preganglionic lesions but often absent or minimal in postganglionic lesions
• Heterochromia iridis in congenital or long-standing cases

Classification

Horner syndrome is classified by the level of the sympathetic pathway affected:

• Central (first-order): lesion from hypothalamus to the cervical spinal cord — usually accompanied by other central nervous system signs (e.g., brainstem stroke with crossed sensory/motor findings)
• Preganglionic (second-order): lesion from spinal cord exit to the superior cervical ganglion — associated with neck or chest pathology, including apical lung tumors, thyroid surgery, or trauma
• Postganglionic (third-order): lesion from the superior cervical ganglion to the eye — associated with carotid artery dissection, cluster headache, or cavernous sinus pathology; anhidrosis is typically absent or minimal at this level³

Differential Diagnosis

• Physiologic anisocoria (a small, stable, non-progressive pupil size difference present in a substantial minority of the normal population, without ptosis or dilation lag)
• Simple/aponeurotic ptosis without pupillary involvement (see the dedicated entry on blepharoptosis)
• Adie's tonic pupil (a large, not small, pupil, with light-near dissociation, distinct from Horner syndrome)
• Pharmacologic miosis
• Third cranial nerve palsy (produces ptosis, but typically more severe, often with an enlarged rather than constricted pupil when the pupil is involved)

Management

Confirmation and localization proceed in steps. Pharmacologic testing helps confirm the diagnosis and, in some cases, assists with localization: apraclonidine (which causes pupil dilation in the affected Horner eye due to denervation supersensitivity, while having little effect on a normal pupil) has become a good, widely available alternative to cocaine drops for confirming the diagnosis.³ Hydroxyamphetamine testing can help distinguish preganglionic/central from postganglionic lesions, based on whether the postganglionic neuron itself is intact.²

Once the diagnosis is confirmed, further workup is guided by the clinical context and suspected level: MRI (of the brain, neck, or chest as indicated, with special sequences as needed) is generally recommended in the evaluation, particularly when a structural, vascular, or neoplastic cause is suspected.³ In children with acquired Horner syndrome, evidence-based workup for neuroblastoma and other etiologies specific to the pediatric population is essential and differs from the adult approach — this is a key etiologic difference between the populations that should shape the diagnostic pathway from the outset.¹ In cases of iatrogenic Horner syndrome after neck surgery, particularly when related to reversible causes like hematoma, edema, or inflammation compressing the cervical sympathetic chain in a confined surgical space, 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 the residual ptosis may be considered at that point.²

Complications

• Underlying missed diagnosis if the cause of Horner syndrome (e.g., carotid dissection, apical lung tumor, neuroblastoma) is not identified promptly
• Persistent cosmetic ptosis and anisocoria if the underlying lesion does not resolve
• In carotid dissection specifically, risk of stroke if the underlying vascular pathology is not recognized and managed

Clinical Pearls

• New Horner syndrome with acute head or neck pain should be treated as a possible carotid artery dissection until proven otherwise, particularly in the postganglionic pattern.³
• Anhidrosis is a helpful localizing sign when present, but its absence does not exclude Horner syndrome — it is often minimal or absent in postganglionic lesions specifically.³
• Apraclonidine is now a practical, widely available pharmacologic confirmatory test for Horner syndrome, an alternative to cocaine drops.³
• Acquired Horner syndrome in a child requires a directed workup for neuroblastoma — this is a distinct and important consideration not paralleled in typical adult acquired disease.¹
• Iatrogenic Horner syndrome after neck surgery (e.g., endoscopic thyroid surgery) often results from reversible compressive causes (hematoma, edema, inflammation) and can resolve spontaneously or with brief steroid treatment as these subside.²

Common Pitfalls

• Dismissing a subtle, stable anisocoria without ptosis as physiologic without confirming there is truly no dilation lag or ptosis on careful exam.
• Failing to consider carotid dissection in a patient with new postganglionic Horner syndrome and head/neck pain, given the time-sensitive stroke risk.³
• Applying the adult Horner syndrome differential (carotid dissection, apical lung tumor) to a pediatric patient without considering the pediatric-specific concern for neuroblastoma.¹
• Assuming postoperative Horner syndrome after neck surgery is necessarily permanent, when many cases from reversible compressive causes resolve spontaneously or with brief treatment.²
• Confusing Horner syndrome's mild ptosis and miotic pupil with a third cranial nerve palsy's typically more severe ptosis and, when the pupil is involved, mydriasis rather than miosis.