Anatomy · Head & Neck

The Cervical Sympathetic Chain: A Drooping Lid That Maps a Long Journey

A drooping eyelid, a small pupil and a dry half-face — three quiet signs that fit in the space of one eye. Yet between them they trace the longest, strangest wiring diagram in the body: a signal that begins in the hypothalamus, dives all the way down into the chest, loops over the top of the lung and the subclavian artery, climbs back up the neck riding on the carotid artery, and finally slips into the skull to reach the eye. Interrupt that wire anywhere along its improbable length and the same three signs appear. That is the quiet genius of Horner's syndrome: the eye does not merely tell you that something is wrong — it tells you, if you know the road the nerve travelled, roughly where. A painful one warns of a torn artery in the neck. One with a wasted hand warns of a tumour at the top of the lung. The face reads like a map, and the drooping lid is the legend.

⏱ 13 min read🎯 Linked lesson: Cervical sympathetic chain & Horner's· Updated 2026-07-19

THE SCENE

A woman in her forties comes in with a headache on one side of her neck and face that started a day ago, after nothing more dramatic than a session at the hairdresser with her head tipped back over the basin. The headache is not the reason the junior doctor pauses. It is her face. Under the bright light of the cubicle one upper eyelid sits a millimetre or two lower than the other, and the pupil beneath it is distinctly the smaller of the two — a difference that becomes obvious when the lights are dimmed and the normal pupil springs wide while the small one lags behind. Her cheek on that side is dry to the touch while the other glistens faintly. Three findings, each individually easy to dismiss, together spelling one word. The doctor does not reach for an eye chart. She reaches for the phone to arrange urgent imaging of the neck vessels — because a painful Horner's syndrome, appearing over hours in an adult, is a dissection of the internal carotid artery until proven otherwise, and the drooping lid has just bought this woman the hours in which a stroke can still be prevented.

A trunk that lies where you would least look for it

The sympathetic chain of the neck is not in the carotid sheath — it is behind it. The CERVICAL SYMPATHETIC TRUNK runs vertically down each side of the neck, but it lies deeper than students expect: on the PREVERTEBRAL FASCIA, in front of the transverse processes of the cervical vertebrae and the prevertebral muscles, and BEHIND the carotid sheath that carries the common and internal carotid arteries, the internal jugular vein and the vagus nerve — the sheath whose contents are set out in the carotid and jugular. That single relationship — trunk behind, sheath in front — is why a surgeon operating on the front of the cervical spine, or a large mass in the neck, can silence the sympathetic supply to the eye without ever touching a great vessel. Now the fact that unlocks the whole subject: there are NO white rami communicantes in the neck. The sympathetic PREGANGLIONIC outflow of the entire body is THORACOLUMBAR, leaving the cord only between T1 and L2, as the whole autonomic plan in the nervous system plan describes. There is no thoracolumbar segment in the neck to give off preganglionic fibres. So every sympathetic fibre destined for the head must first climb UP into the neck from the upper thoracic cord, and the cervical trunk is essentially a long ascending relay carrying that thoracic outflow to the head — a geography that will explain, in a moment, why a tumour at the top of the lung can drop an eyelid.

Three ganglia strung on one cord

Superior, middle and stellate — and only the superior one reaches the eye. Along the cervical trunk sit three GANGLIA, the relay stations where preganglionic fibres synapse onto the postganglionic neurons that finish the journey. The SUPERIOR CERVICAL GANGLION is the largest — long and fusiform, opposite the C1–C2 vertebrae, lying behind the internal carotid artery. From it the crucial postganglionic fibres form the INTERNAL CAROTID NERVE, which climbs as a plexus wrapped around the internal carotid artery and follows it through the carotid canal into the skull; these are the fibres that supply the eye — the DILATOR PUPILLAE that widens the pupil, the SUPERIOR TARSAL MUSCLE (Müller's muscle) that helps hold the upper lid raised, and the sweat glands and blood vessels of the face, structures gathered together in the orbit and its contents. The MIDDLE CERVICAL GANGLION is the smallest and least constant, at about the C6 level near the inferior thyroid artery. The STELLATE (CERVICOTHORACIC) GANGLION is the fusion of the inferior cervical ganglion with the first thoracic ganglion, sitting at the neck of the first rib, just behind the origin of the VERTEBRAL ARTERY and in front of the C7 transverse process and the lung apex; a slender loop of fibres, the ANSA SUBCLAVIA, hangs down from it and around the subclavian artery to reach the middle ganglion. From all three, GREY RAMI COMMUNICANTES join the cervical spinal nerves to carry sympathetic fibres to the vessels, sweat glands and arrector pili of the neck and upper limb, and CARDIAC BRANCHES (superior, middle and inferior cervical cardiac nerves) descend to the cardiac plexus to speed the heart.

The three-neuron road from the brain to the eye

This is the spine of the whole subject, because Horner's syndrome is localised by which neuron is hit. The oculosympathetic pathway is a chain of THREE neurons, and every serious question about Horner's syndrome is really a question about which of the three has been cut. The FIRST-ORDER (central) neuron begins in the HYPOTHALAMUS and descends, uncrossed, through the brainstem and down the lateral column of the cervical spinal cord to synapse in the CILIOSPINAL CENTRE OF BUDGE, the intermediolateral cell column at levels C8 to T2. The SECOND-ORDER (preganglionic) neuron leaves the cord through the T1 (and adjacent) ventral root and white ramus, then takes the exposed course that gives the syndrome its localising power: it arches OVER THE APEX OF THE LUNG and the SUBCLAVIAN ARTERY, passes through the stellate ganglion without synapsing, and ascends the sympathetic trunk to synapse at last in the SUPERIOR CERVICAL GANGLION. This is the segment lying against the lung apex — the segment caught by a PANCOAST TUMOUR at the top of the lung and by the same forces that can injure the lower brachial plexus described in the brachial plexus. The THIRD-ORDER (postganglionic) neuron runs from the superior cervical ganglion as the internal carotid nerve, climbing on the INTERNAL CAROTID ARTERY through the carotid canal and cavernous sinus, joining the ophthalmic division of the trigeminal and the nasociliary nerve to reach the orbit and the eye — the segment caught by an internal carotid DISSECTION or a lesion in the cavernous sinus. Three neurons, three levels, three families of disease.

THE ANALOGY

Think of a single telephone line running from a control room at the top of a building all the way down to the basement, out along a cable tray that hangs over the boiler in the plant room, and then back up a riser bolted to the outside of a water pipe to a handset on the top floor at the far end. Three long segments, one continuous wire. If the handset goes dead there is only one message — the line is broken — but where you look for the break depends on which segment failed. A fault in the control room (the brainstem) usually knocks out other lines with it. A fault down in the plant room, where the cable is draped over the hot boiler, means something has grown or burst in the plant room — the top of the lung. A fault on the outside riser strapped to the water pipe means the trouble is on that pipe — the carotid artery. The dead handset is always the same dead handset. The skill is not in noticing that it is dead; it is in knowing the route well enough to say which stretch of cable to inspect.

Horner's syndrome, read one sign at a time

Each component is a single denervated structure — name the structure and the sign explains itself. PARTIAL PTOSIS comes from loss of the sympathetically-supplied SUPERIOR TARSAL MUSCLE (Müller's muscle), a small smooth muscle that adds a couple of millimetres to the lift of the upper lid on top of the main effort of levator palpebrae superioris. Because only the accessory muscle is denervated, the droop is PARTIAL — a subtle lowering, never the complete curtain of a full third nerve palsy. There may be a matching slight elevation of the LOWER lid (an 'upside-down ptosis' from loss of the inferior tarsal muscle), which narrows the palpebral fissure from both directions and produces the apparent sinking of the eye described below. MIOSIS is a small pupil, produced not by anything actively constricting it but by the LOSS of the DILATOR PUPILLAE, leaving the parasympathetic sphincter pupillae unopposed; the tell-tale refinement is DILATATION LAG — in darkness the normal pupil dilates briskly while the Horner's pupil widens slowly and incompletely, so the anisocoria is GREATER IN DIM LIGHT. ANHIDROSIS, loss of sweating, is the component that localises, and it does so because of where the sweat fibres to the face travel: most run with the EXTERNAL carotid artery, branching away at the carotid bifurcation, while the fibres to the eye continue on the internal carotid. A lesion CENTRAL or PREGANGLIONIC, before the split, denervates sweating over the whole half of the face and neck; a POSTGANGLIONIC lesion, after the fibres to the face have already left on the external carotid, spares most facial sweating or affects only a patch of forehead and the side of the nose. Finally there is apparent ENOPHTHALMOS — the eye only looks sunken because the narrowed fissure deceives the observer; true retraction of the globe does not occur.

💡 CLINICAL PEARL

The extent of the dry skin is a ruler laid along the pathway. Ask where the face stops sweating, and the answer tells you how high the lesion sits — because the sudomotor fibres to the face desert the internal carotid at the bifurcation and ride the EXTERNAL carotid to the skin. Anhidrosis of the WHOLE half of the face and the neck means the fibres were cut BEFORE that split — a central (first-order) or preganglionic (second-order) lesion, the group that includes the sinister causes: brainstem stroke and the Pancoast tumour. Anhidrosis confined to a small patch of medial forehead and the side of the nose, or no detectable sweating loss at all, means the fibres to the rest of the face had already branched off — a postganglionic (third-order) lesion, the group that includes carotid dissection and cluster headache. So a Horner's with a bone-dry half-face and neck is a summons to image the chest and the neck; a Horner's with normal facial sweating points you higher up the carotid and into the skull. One clinical sign, palpated with the back of a finger, quietly narrows a differential that spans from the medulla to the orbit.

Locating the lesion by level — the company each sign keeps

Once you accept that Horner's syndrome is one syndrome with three possible addresses, the causes sort themselves by level. CENTRAL (first-order) lesions sit anywhere along the long descending tract from hypothalamus to C8–T2: the classic is a brainstem stroke, and specifically LATERAL MEDULLARY (WALLENBERG) SYNDROME from posterior inferior cerebellar artery territory infarction, where the Horner's arrives in the company of ipsilateral facial and contralateral body sensory loss, vertigo, dysphagia and ataxia; other central causes are SYRINGOMYELIA and demyelination. Because a central lesion damages a great deal of neighbouring tissue, the Horner's here is almost never alone. PREGANGLIONIC (second-order) lesions strike the fibres arching over the lung apex and the neck: the PANCOAST TUMOUR — an apical bronchogenic carcinoma eroding into the sympathetic chain and the lower brachial plexus, so a Horner's with wasting and pain in the hand and medial forearm is a Pancoast until proven otherwise — plus a CERVICAL RIB, a large GOITRE, and iatrogenic injury after neck surgery, carotid endarterectomy or the insertion of a central venous line; BIRTH TRAUMA to the lower plexus (KLUMPKE'S palsy) can produce a congenital preganglionic Horner's. POSTGANGLIONIC (third-order) lesions are the ones on the internal carotid and in the skull: INTERNAL CAROTID ARTERY DISSECTION, typically PAINFUL and often heralding an impending stroke; CLUSTER HEADACHE, in which a transient Horner's accompanies the attacks; and CAVERNOUS SINUS disease, where the Horner's keeps company with palsies of the third, fourth, sixth and first-division-trigeminal nerves that share that space.

◆ Two Horner's, two very different emergencies

A 38-year-old man arrives with a day of aching pain around one eye and the side of his neck after a game of squash. His upper lid is slightly ptosed, the pupil small, and — the crucial detail — the pain came before anything else and his facial sweating is normal. This is a POSTGANGLIONIC Horner's, and painful, in a young adult after exertion: INTERNAL CAROTID ARTERY DISSECTION. The torn intima has raised the alarm days before it might have thrown a clot to the brain; urgent CT or MR angiography of the neck vessels and antithrombotic treatment can prevent the stroke that would otherwise have been the presenting event. Contrast him with a 64-year-old lifelong smoker whose Horner's was noticed by his wife, painless, developing over weeks, and who mentions an ache and clumsiness in the same hand with wasting of the small muscles between the thumb and index finger. Here the sweating fails over the whole side of the face — a PREGANGLIONIC lesion — and the hand signs betray the lower brachial plexus. This is a PANCOAST TUMOUR at the apex of the lung, invading the sympathetic chain and the T1 root together. Same three ocular signs; one sends you to angiography within the hour, the other to a chest radiograph and a respiratory oncology clinic.

✅ Key points

The cervical sympathetic trunk lies on the PREVERTEBRAL fascia in front of the transverse processes and BEHIND the carotid sheath. There are NO white rami in the neck: the whole sympathetic outflow is thoracolumbar (T1–L2), so head-bound fibres must ASCEND from the upper thoracic cord.
Three ganglia: SUPERIOR (largest, C1–C2, behind the internal carotid) → internal carotid nerve to the dilator pupillae, superior tarsal muscle and facial sweat glands; MIDDLE (small, ~C6); STELLATE (cervicothoracic, at the first rib neck behind the vertebral artery, with the ansa subclavia). Grey rami join the cervical spinal nerves; cardiac branches descend to the heart.
The oculosympathetic pathway has THREE neurons: 1st-order hypothalamus → ciliospinal centre of Budge (C8–T2); 2nd-order (preganglionic) over the LUNG APEX and subclavian artery, through the stellate to the superior cervical ganglion (the PANCOAST segment); 3rd-order (postganglionic) up the INTERNAL CAROTID into the orbit (dissection/cavernous sinus segment).
Horner's = partial PTOSIS (superior tarsal/Müller's, a subtle droop — NOT the complete droop of a third nerve palsy), MIOSIS (loss of dilator, unopposed sphincter, dilatation lag → anisocoria greater in the DARK), ANHIDROSIS, and apparent ENOPHTHALMOS.
The EXTENT of anhidrosis localises: whole face-and-neck if the lesion is central or preganglionic (before the sudomotor fibres split at the carotid bifurcation onto the EXTERNAL carotid); little or none if postganglionic.
Causes by level — CENTRAL: lateral medullary (Wallenberg) stroke, syringomyelia, demyelination (rarely isolated). PREGANGLIONIC: Pancoast tumour, cervical rib, goitre, neck surgery/central line, Klumpke's birth trauma. POSTGANGLIONIC: carotid dissection (painful), cluster headache, cavernous sinus disease.

The two errors: Horner's versus a third nerve palsy

Students make two opposite mistakes about the lid and the pupil — get these the right way round. A drooping lid has two great causes, and they are mirror opposites at the pupil. In HORNER'S the sympathetic supply is lost, so the droop is PARTIAL (only Müller's accessory muscle fails, not the whole levator) and the pupil is SMALL (miosis, because the dilator is gone). In a complete OCULOMOTOR (third nerve) PALSY the parasympathetic supply and the main lid elevator are lost, so the droop is COMPLETE (the whole levator palpebrae is denervated, the lid may cover the eye entirely) and the pupil is LARGE and unreactive (mydriasis, because the constricting sphincter is gone), often with the eye turned 'down and out'. So the pairing is fixed: small pupil with mild droop is sympathetic (Horner's); big pupil with heavy droop is parasympathetic (third nerve). Getting this backwards — expecting a big pupil in Horner's, or a small one in a third nerve palsy — is the single commonest error, and it matters, because a NEW third nerve palsy with a blown pupil is a posterior communicating artery aneurysm until proven otherwise, while a new painful small-pupil Horner's is a carotid dissection. Confirmation, when needed, is pharmacological and is set out in mydriatics and cycloplegics: topical apraclonidine REVERSES the anisocoria of Horner's (the denervated dilator is supersensitive, so the Horner's pupil dilates and the lid lifts), and old-fashioned cocaine drops FAIL to dilate a Horner's pupil while dilating a normal one; hydroxyamphetamine then separates a postganglionic lesion (no dilatation) from a preganglionic one (dilatation preserved).

⚠️ Common mistakes

Expecting a large pupil in Horner's syndrome. The pupil is SMALL (miosis) because the sympathetically-driven dilator is denervated. A big pupil with a heavy complete ptosis is the OPPOSITE lesion — a third nerve palsy — and demands a hunt for a posterior communicating artery aneurysm.
Calling the ptosis of Horner's 'complete'. It is PARTIAL, because only the accessory smooth muscle (superior tarsal/Müller's) is sympathetic; the main levator palpebrae superioris (oculomotor) still works. A lid that fully covers the eye is a third nerve palsy, not a Horner's.
Ignoring the extent of anhidrosis. Whether the whole half-face is dry or only a patch of forehead is not incidental — it is the sign that separates a central/preganglionic lesion (chest and neck imaging, hunting a Pancoast tumour) from a postganglionic one (carotid dissection). Skipping it discards the syndrome's best localising clue.

🎓 Questions students ask

Why does a tumour at the top of the lung drop an eyelid on the same side?

Because the second-order (preganglionic) neuron of the oculosympathetic pathway does not take a sheltered route — after leaving the cord at T1 it arches directly OVER the apex of the lung and the subclavian artery to reach the stellate ganglion and climb to the superior cervical ganglion. The lung apex and this nerve are neighbours. An apical bronchogenic carcinoma (a Pancoast tumour) growing upward erodes into the sympathetic chain and interrupts that neuron, and the eye loses its sympathetic supply: partial ptosis, miosis and anhidrosis over the whole half of the face, because the lesion is upstream of the carotid bifurcation. The same tumour usually also invades the lower trunk of the brachial plexus (C8–T1) lying alongside, so the Horner's arrives with pain and wasting in the hand — a combination that is a red flag for apical lung cancer and a reason never to treat an unexplained Horner's as trivial in a smoker.

How can I tell a Horner's pupil from a simple physiological difference in pupil size?

Turn the lights down. A Horner's pupil is small because its dilator is denervated, so its abnormality is greatest in the DARK, where the normal pupil dilates briskly and the Horner's pupil lags — the anisocoria INCREASES in dim light, and you can often watch the 'dilatation lag' over a few seconds. Physiological (simple) anisocoria, by contrast, stays roughly the same in light and dark and both pupils react and dilate normally. Then look at the lid and the sweating: a true Horner's usually has the accompanying partial ptosis and, in central or preganglionic lesions, reduced facial sweating — features a benign size difference never shows. If doubt remains, topical apraclonidine reverses the anisocoria of a Horner's (the supersensitive denervated dilator over-responds, so the smaller pupil becomes the larger) while doing little to a normal pupil, confirming the diagnosis pharmacologically before any thought of imaging.

A baby is born with one blue eye and one brown eye and a slightly droopy lid. Is that relevant?

Yes — it is congenital Horner's syndrome. Sympathetic innervation is needed for the normal pigmentation of the iris melanocytes in early life, so an oculosympathetic lesion present at or near birth leaves the affected iris paler: HETEROCHROMIA IRIDIS, a lighter iris on the side of the ptosis and miosis. The commonest cause is birth trauma to the lower brachial plexus and its neighbouring sympathetic fibres — the same stretch injury that produces Klumpke's palsy — but a congenital Horner's in an infant also warrants exclusion of a neuroblastoma of the cervical or upper thoracic sympathetic chain, so it is investigated rather than dismissed. In an adult, by contrast, iris colour is already fixed and a new Horner's does not change it; heterochromia therefore quietly dates the lesion to childhood. It is a lovely example of how a sign the family noticed as a curiosity — two different-coloured eyes — is really a fingerprint of the sympathetic pathway laid down years before.

Test yourself

A 60-year-old smoker has a right partial ptosis, a right miosis that is more obvious in a darkened room, absent sweating over the whole right side of the face, and wasting of the small muscles of the right hand. At which level of the oculosympathetic pathway is the lesion, and what is the most likely cause?

🫁 In one breath

The cervical sympathetic trunk lies on the prevertebral fascia BEHIND the carotid sheath. There are no white rami in the neck — the sympathetic outflow is thoracolumbar (T1–L2) — so fibres for the head ASCEND from the upper thoracic cord and relay in three ganglia: superior (C1–C2, → internal carotid nerve to the eye and face), middle (~C6) and stellate (first rib neck, behind the vertebral artery, with the ansa subclavia).
The oculosympathetic pathway is three neurons: 1st-order hypothalamus → ciliospinal centre of Budge (C8–T2); 2nd-order (preganglionic) over the LUNG APEX to the stellate then superior cervical ganglion (the Pancoast segment); 3rd-order (postganglionic) up the INTERNAL CAROTID into the orbit (dissection / cavernous sinus segment). Horner's is localised by which neuron is hit.
Horner's = PARTIAL ptosis (superior tarsal/Müller's — not the complete droop of a third nerve palsy), MIOSIS (denervated dilator; anisocoria greater in the DARK, with dilatation lag), ANHIDROSIS and apparent enophthalmos. The EXTENT of anhidrosis localises: whole face-and-neck if central/preganglionic, little or none if postganglionic (the facial sweat fibres branch off onto the external carotid).
Causes by level and their clinical company: central — Wallenberg stroke, syringomyelia; preganglionic — Pancoast tumour (with hand wasting/pain), cervical rib, goitre, neck surgery, Klumpke's; postganglionic — carotid dissection (PAINFUL, warns of stroke), cluster headache, cavernous sinus disease. A big pupil with a heavy droop is the OPPOSITE lesion (third nerve palsy); congenital Horner's gives a paler iris (heterochromia). Confirm and localise pharmacologically (apraclonidine/cocaine, then hydroxyamphetamine); a stellate ganglion block is used for pain.

📚 Sources

Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Head and Neck: the cervical sympathetic trunk, its ganglia and the sympathetic supply to the head.
Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — The sympathetic trunk in the neck; the oculosympathetic pathway and Horner's syndrome.
Netter FH. Atlas of Human Anatomy — Autonomic nerves of the neck; cervical sympathetic ganglia and the stellate ganglion.
Snell RS. Clinical Anatomy by Regions — The autonomic nervous system in the neck; Horner's syndrome and its localisation.
Standring S (ed). Gray's Anatomy — The cervical part of the sympathetic system; ciliospinal centre and postganglionic course on the internal carotid.
TeachMeAnatomy — The Sympathetic Nervous System; Horner's Syndrome.