Horner’s syndrome

Horner’s syndrome is a neurological disorder resulting from disruption of the oculosympathetic pathway. It is characterized by a combination of ocular, facial, and autonomic signs. Understanding its anatomy and clinical features is essential for accurate diagnosis and management.

Anatomy and Physiology of the Sympathetic Pathway

Sympathetic Nervous System Overview

The sympathetic nervous system is part of the autonomic nervous system responsible for the body’s fight or flight responses. It controls functions such as pupil dilation, eyelid elevation, sweating, and blood vessel constriction. Sympathetic fibers run in a three-neuron pathway from the brain to the eye and face.

Oculosympathetic Pathway

The oculosympathetic pathway consists of three main neurons:

• First-order (central) neurons: Originate in the hypothalamus and descend through the brainstem to the spinal cord at levels C8 to T2.
• Second-order (preganglionic) neurons: Exit the spinal cord and travel over the apex of the lung to the superior cervical ganglion.
• Third-order (postganglionic) neurons: Travel along the internal carotid artery to the eye and along the external carotid artery to the sweat glands of the face.

Key Anatomical Structures

The main anatomical structures involved in the oculosympathetic pathway include:

• Hypothalamus: Origin of first-order sympathetic neurons.
• Brainstem and cervical spinal cord: Pathway for descending fibers to the thoracic spinal cord.
• Superior cervical ganglion: Relay station for preganglionic to postganglionic fibers.
• Carotid artery and related structures: Pathway for postganglionic fibers to reach the eye and face.
• Eye and facial structures: Target tissues for sympathetic innervation, including the iris, eyelids, and sweat glands.

Etiology

Central Causes

Central causes of Horner’s syndrome involve lesions affecting the first-order neurons of the sympathetic pathway:

• Brainstem stroke affecting the lateral medulla (Wallenberg syndrome).
• Brainstem tumors or demyelinating lesions.
• Spinal cord lesions at levels C8 to T2, such as trauma or syringomyelia.

Perganglionic Causes

Perganglionic causes involve the second-order preganglionic neurons as they exit the spinal cord and travel toward the superior cervical ganglion:

• Apical lung tumors (Pancoast tumor) compressing the sympathetic chain.
• Neck trauma or surgical procedures affecting the thoracic outlet.
• Thoracic or cervical spinal surgery complications.

Postganglionic Causes

Postganglionic causes affect the third-order neurons traveling from the superior cervical ganglion to the eye and face:

• Carotid artery dissection causing interruption of sympathetic fibers.
• Cluster headache or migraine-related sympathetic dysfunction.
• Iatrogenic injury during carotid or neck surgery.

Pathophysiology

Horner’s syndrome results from disruption of sympathetic innervation to the eye and face. The loss of sympathetic input leads to characteristic clinical features:

• Pupil constriction (miosis): Due to unopposed parasympathetic activity on the iris sphincter muscle.
• Mild ptosis: Drooping of the upper eyelid caused by paralysis of the superior tarsal muscle.
• Anhidrosis: Loss of sweating on the affected side of the face when postganglionic fibers are involved.
• Apparent enophthalmos: Slight sunken appearance of the eye due to loss of sympathetic tone in orbital muscles.

Clinical Features

Ocular Signs

• Ptosis: Mild drooping of the upper eyelid due to paralysis of the superior tarsal muscle.
• Miosis: Constricted pupil on the affected side caused by unopposed parasympathetic activity.
• Anisocoria: Unequal pupil sizes, more noticeable in dim light.
• Apparent enophthalmos: Slight sinking of the eyeball in the orbit due to loss of sympathetic tone.

Facial and Other Signs

• Anhidrosis: Loss of sweating on the affected side of the face when postganglionic fibers are involved.
• Flushing: Facial redness on the affected side may occur due to vasodilation.
• Temperature changes: Skin on the affected side may feel cooler due to impaired sympathetic vasoconstriction.

Diagnostic Evaluation

Clinical Examination

• Observation of ocular signs including ptosis, miosis, and anisocoria.
• Assessment of facial sweating and temperature differences.
• Pharmacological testing using cocaine or apraclonidine drops to confirm sympathetic denervation.

Imaging Studies

• MRI or CT: Evaluation of brain, neck, and chest to detect central or preganglionic lesions.
• Angiography: Assessment of carotid artery dissection or other vascular causes affecting postganglionic fibers.

Differential Diagnosis

Several conditions can mimic the features of Horner’s syndrome. It is essential to differentiate these disorders to ensure accurate diagnosis and management:

• Other causes of ptosis, such as myasthenia gravis or oculomotor nerve palsy.
• Physiological anisocoria, which is a normal variation in pupil size.
• Adie’s tonic pupil, which presents with a dilated pupil and slow light response.
• Traumatic or iatrogenic injury to the eyelids or orbit that may cause ptosis or miosis.

Management

Treatment of Underlying Cause

Management of Horner’s syndrome primarily involves addressing the underlying pathology:

• Surgical or medical treatment of tumors compressing the sympathetic pathway, such as Pancoast tumors.
• Vascular interventions for carotid artery dissection or other vascular lesions.
• Management of central lesions such as stroke or demyelinating disorders according to standard neurological protocols.

Symptomatic Management

In cases where the underlying cause cannot be fully corrected, symptomatic measures may be considered:

• Cosmetic correction of ptosis using eyelid crutches or surgery if function is impaired.
• Eye care for dryness or irritation due to mild ocular involvement.
• Patient education regarding avoidance of further trauma or exposure that may worsen symptoms.

Prognosis

The prognosis of Horner’s syndrome largely depends on the underlying cause and the extent of sympathetic pathway damage. Key considerations include:

• Central causes such as brainstem stroke may result in permanent deficits depending on lesion size and location.
• Perganglionic causes like tumors may improve if the lesion is treated early, but long-standing compression can lead to irreversible changes.
• Postganglionic causes such as carotid artery dissection often recover partially or fully with timely vascular intervention.
• Early diagnosis and treatment of the primary condition generally improve functional outcomes and reduce the risk of persistent ocular or facial signs.

Prevention

Preventive strategies for Horner’s syndrome focus on minimizing the risk of injury or disease affecting the sympathetic pathway:

• Careful surgical techniques in the neck, chest, and skull base to avoid iatrogenic injury to sympathetic fibers.
• Prompt detection and treatment of tumors or vascular abnormalities that may compress the sympathetic chain.
• Protective measures during activities that carry a risk of trauma to the cervical or thoracic regions.
• Regular monitoring for high-risk patients, such as those with a history of carotid artery disease or apical lung tumors, to detect early signs of sympathetic disruption.

References

1. Standring S. Gray’s Anatomy. 42nd ed. London: Elsevier; 2020.
2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2020.
3. Wray SH, Shapiro R. Horner’s Syndrome: Clinical Manifestations and Etiologies. Neurology. 2018;91(12):e1122-e1131.
4. Neubauer AS, Safran M. Oculosympathetic Pathway Disorders. Curr Opin Ophthalmol. 2017;28(5):505-511.
5. Wilson D, Hogg JP. Horner’s Syndrome: Diagnostic Approaches and Management. J Clin Neurol. 2016;12(1):1-9.
6. Gundersen VE, Olesen J. Pharmacologic Testing in Horner’s Syndrome. Acta Ophthalmol. 2015;93(4):305-312.
7. Keane JR. Horner’s Syndrome: Etiology and Clinical Correlation. Arch Neurol. 2010;67(5):551-556.
8. Ropper AH, Samuels MA. Adams and Victor’s Principles of Neurology. 11th ed. New York: McGraw-Hill; 2021.