Horner syndrome Anatomy Pathophysiology Diagnosis and treatment

 Horner syndrome Anatomy Pathophysiology Diagnosis and treatment 

A rare disruption of the sympathetic nerves serving the eye and facial areas, Horner syndrome or oculosympathetic paresis has another name. The classical triad in Horner syndrome would be ptosis, the drooping eyelid; miosis, the constrictive pupil; and, thirdly, anhidrosis, or the inhibition of sweating. There can be many basic conditions that cause Horner syndrome. The anatomy of the sympathetic nervous pathway must be appreciated to understand the pathophysiology, as this syndrome often serves as a signpost to potentially serious underlying conditions.

 Anatomy and Pathophysiology

The sympathetic nervous system, responsible for "fight-or-flight" responses, has fibers that travel through a three-neuron pathway to reach the eye. Any interruption along this pathway can cause Horner syndrome:

1. First-order neuron**: It originates in the hypothalamus, descends through the brainstem to the spinal cord at the level of C8-T2, also known as the ciliospinal center of Budge. Lesions involving the first-order neuron may be due to stroke, brainstem tumors, or cervical spinal cord trauma.

2. Second-order neuron**: Projects from the spinal cord to the superior cervical ganglion. Lesions in this area may be related to Pancoast tumors (lung apex tumors), thyroid or neck surgeries, or other chest and neck trauma.

3. Third-order neuron**: Proximal to the superior cervical ganglion, close to the internal carotid artery, and goes up into the eye muscles. Lesions in this area could be due to carotid artery dissection, cluster headaches, or sinus infections.

This is a challenging and long pathway; areas that include the brainstem spreading across to the chest, down the neck, and then crossing into the eye, thus exposing the pathway to possible damage at different levels. The clinical characteristics of Horner syndrome as well as their manifestations have sometimes even enabled an approximated idea about where in the pathway the lesion may locate.

### Clinical Features of Horner Syndrome

Horner syndrome mainly comprises the following:

1. **Ptosis**: Drooping of the upper eyelid occurs as a result of loss of innervation to the Müller muscle that is present within the eyelid. The Ptosis tends to be quite mild with Horner syndrome at a level of around 1-2 mm as this is a subsidiary lifting mechanism that is mediated through the levator palpebrae superioris, and its pathway goes via cranial nerve III.

2. Miosis: The affected pupil is smaller because of loss of sympathetic input to the dilator muscle of the iris. Miosis in Horner syndrome classically results in a smaller pupil that is still reactive to light, because the parasympathetic control is preserved. This is a distinguishing feature from other causes of anisocoria.

3. **Anhidrosis**: It may be a bilateral total half-face anhidrosis, or unilateral when partial half-face anhidrosis results from the interruption of sympathetic innervation of sweat glands, thus providing an important localizing clue when present.

4. **Enophthalmos**: This apparent "sinking" of the eye into the orbit is a less common sign and can be misleading. It's thought to be due to the relaxation of the muscles that normally help to protrude the eyeball. However, true enophthalmos is rare.

5. Loss of ciliospinal reflex This is a reflex whereby, upon pain or other stimuli on the neck or face, pupils dilate. In Horner syndrome, this response will not occur on the side due to disruption of the sympathetic pathway.

 Horner Syndrome Diagnosis

Clinical Examination

The diagnosis follows a very careful clinical observation. A combination of ptosis, miosis, and anhidrosis can be sufficiently suggestive of Horner syndrome. Several diagnostic tests can be performed to distinguish it from other forms of anisocoria.

1. **Cocaine test**: Cocaine blocks the reuptake of norepinephrine at sympathetic nerve endings, causing dilation of a normal pupil. In Horner syndrome, the pupil on the affected side does not dilate with cocaine, confirming the diagnosis.

2. **Apraclonidine test**: The pupil becomes mydriatic after use of apraclonidine due to denervation hypersensitivity, which is reversible with ptosis in patients affected by Horner syndrome. This test is gradually becoming popular because of the availability of apraclonidine more readily than cocaine.

3. Hydroxyamphetamine test. It tells the location of the lesion in the sympathetic pathway. Hydroxyamphetamine is given to release pre-formed norepinephrine in the post-ganglionic neurons. This way if the lesion happens at third-order neuron the pupil remains dilated whereas the involvement of first- and second- order neurons pupils becomes dilated.

Imaging Studies

Imaging is important in establishing the cause and exact location of the lesion especially since Horner syndrome can be due to serious conditions. These may include:

1. **Magnetic Resonance Imaging (MRI)**: This is particularly useful for scanning the brain, brainstem and spinal cord and often considered if a central or first-order neuron lesion is suspected.

2. CT: A chest CT may demonstrate a Pancoast tumor or another lung apical mass that compresses the second-order neuron. A carotid artery dissection may be imaged using a CT angiogram.

3. Carotid Doppler ultrasound: This is a noninvasive modality and may be used in the investigation of the carotid artery if there is suspected dissection.

 Causes of Horner Syndrome

Causes of Horner syndrome due to central, or first-order neuron, include the following:

1. **Stroke**: A stroke in the lateral medullary or pontine region.

2. Brainstem tumors: Primary and metastatic tumors can destroy the first-order neuron pathway.

3. Syringomyelia: A cyst in the spinal cord that can affect the sympathetic pathway at C8-T2.

Multiple sclerosis (MS): May be linked with lesions in the brainstem resulting in demyelinating and involve sympathetic pathways

2. Pre-ganglionic (second-order neuron) causes:

    Pancoast tumor: Cancer at the apex of the lung compresses the sympathetic chain

    Neck trauma or surgery: Neck surgeries and injuries cause damage to second-order neurons.

    Thoracic aortic aneurysm: This could stretch or compress sympathetic fibres.

3. **Postganglionic (third-order neuron) causes**:

   Carotid artery dissection: The carotid artery is one of the arteries supplying the brain that can tear inside its wall, which results in an inner lining defect. That can cause Horner syndrome along with a grave risk that may result in stroke. Cluster headaches The mechanism is obscure, but it has some association with the transient development of Horner syndrome. Middle ear infections or tumors These may impinge sympathetic fibers within the skull.

Differentiating Horner syndrome from other conditions that may have similar presentations is important. Some of the conditions that must be considered include the following:

- **Third cranial nerve palsy**: This condition can cause ptosis but is typically accompanied by a dilated pupil, which is opposite of what occurs in Horner syndrome.

- **Physiological anisocoria**: This is a benign condition characterized by slight asymmetry of the pupils with no other symptoms.

- **Other causes of anisocoria**: Like Adie's pupil-a tonic pupil or pharmacologic agents, for example eye drops.

Management of Horner Syndrome

The treatment of Horner syndrome is aimed at managing the underlying cause. Treatment for carotid dissection is immediate with regards to anticoagulation or surgery. Pancoast tumors are managed as a combination of surgery and radiation therapy and chemotherapy may be involved. Central causes due to stroke or multiple sclerosis are treated based on that underlying neurological condition.

Partial or complete resolution may be noted in some of the patients if the disease process is well controlled. Symptoms can persist in such patients, especially when there is extensive nerve damage.

Surgical intervention may be considered when ptosis poses a threat to the ability to use the vision normally or in appearance.

 Prognosis and Complications

The prognosis varies with the cause; if benign or self-limiting, the outlook remains optimistic. However, a fatal cause of Horner syndrome, such as that caused by carotid artery dissection, Pancoast tumor, and stroke, depends upon early recognition and proper management of its causative factor.

Other Horner syndrome complications may involve chronic ptosis and can affect the visual fields as well as appearance. Other complications due to Horner syndrome from carotid dissection are a heightened risk for stroke, so these patients usually need to be evaluated and treated promptly.

 Case Studies and Examples

 Case 1: Carotid Artery Dissection

A 45-year-old man presents with acute neck pain and drooping of the left eyelid after what he characterizes as a minor injury to the neck. He presents with ptosis and miosis on the left, which is consistent with Horner syndrome. A CT angiogram confirms a dissection of the carotid artery. The patient is placed on anticoagulation therapy and carefully monitored for complications to avoid them.

Case 2: Pancoast Tumor

A 60-year-old smoker presents with complaints of shoulder pain and drooping eyelid on the right side. Horner syndrome is confirmed on examination, and chest CT scan reveals a Pancoast tumor at the lung apex. The patient undergoes surgery and chemotherapy, and there is partial relief of symptoms after removal of the tumor.