Carotid sinus hypersensitivity

# Overview
The carotid sinus contains a baroreceptor that senses arterial blood pressure and allows the body to regulate blood pressure and perfusion.  Excessive sensitivity of this receptor may lead to reduced heart rate and/or blood pressure that can lead to syncope, called Carotid Sinus Hypersensitivity (CSH).     

# Historical Perspective
Weiss and Baker first described the syndrome in 15 patients in 1933. 

# Pathophysiology
The carotid sinus baroreceptor is located in the internal carotid artery sinus just above the bifurcation of the common carotid artery.  It senses arterial blood pressure and in response sends afferent signals to the sensory nucleus of the vagus, via the sinus nerve of Hering, a branch of the glossopharyngeal nerve.  Increased afferent signals lead to increased vagal tone and reduced sympathetic tone.
Two general types of CSH have been described, though patients may have mixed features:
- The slowed rate is usually due to sinus arrest or SA exit block.  Some degree of AV block may occur as well in some patients.
The mechanism of the hypersensitivity has not been fully elucidated.  Carotid sinus denervation does not always result in cure, so there may be several contributing features.  Many people believe that the defect lies in the CNS.
- One theory is increased alpha-2 adrenergic receptor mediated activity in the afferent or brainstem pathway of the carotid sinus reflex.
- Other theories include high resting vagal tone, hyperresponsiveness to acetylcholine, inadequate cholinesterase, etc. in affected patients.
The syndrome has been observed in patients with tumors or lymph node enlargement in the region of the carotid sinus. If severe atheromatous narrowing of the carotid arteries is present, compression of the carotid artery can lead to syncope due to inadequate CNS perfusion and not necessarily due to carotid sinus hypersensitivity.

# Differentiating Carotid sinus hypersensitivity from other Diseases
The differential diagnosis of syncope includes true syncope and other non-syncopal causes. Below is listed one of many classification schemes.
Neurologic-associated causes
- Neurocardiogenic syncope  due to direct cardiac inhibition
- Vasovagal syncope
- Micturition syncope
- Carotid sinus hypersensitivity
- Seizure
- Transient ischemic attack (TIA) / Stroke
- Vertigo
- Hypoglycemia
- Narcolepsy
- Psychogenic  panic attack, anxiety disorder, somatization
Cardiac syncope
- Arrhythmia
- Outflow obstruction  valve disease, myxoma, idiopathic hypertrophic sub-aortic stenosis (IHSS), pulmonary embolism
- Insufficient cardiac output  CAD/USA/Myocardial infarction (MI), cardiac tamponade
Peripheral syncope - inability to maintain peripheral vascular tone
- Volume loss  including orthostatic hypotension and blood loss
- Medication effect  common agents include anticholinergics, nitrates, diuretics, other HTN meds.
- Autonomic and peripheral neuropathies  including diabetic neuropathy and other dysautonomias
- Neurodegenerative changes with orthostatic hypotension

# Epidemiology and Demographics
CSH appears to be more common in men and in elderly patients.  Other studies have suggested that CSH is more common in patients with Coronary artery disease (CAD), Hypertension (HTN) and in those with abnormal EKGs.  Studies have also shown that CSH is more common in patients taking digitalis, alpha-methyldopa, clonidine and beta-blockers, presumably due to increased vagal activity.  Some studies have suggested that CSH is more common in patients with carotid artery atherosclerosis.
Huang, et al, found CSH in 28% of patients with unexplained syncope, and other studies have found it in up to 45% of such patients. Huang  Other studies have found CSH in up to 20% of the elderly population, though only a fraction of these individuals actually suffer from signs and symptoms of CSH. 

# Natural History, Complications and Prognosis
Many but not all patients with syncope due to CSH have recurrent symptoms if not treated.  In one study, up to 2/3 of patients did not have recurrence, so it is prudent to reserve significant interventions for those patients with recurrent symptoms. Patients with CSH have a higher rate of hip fracture, laceration and recurrent syncope than in age-matched controls. Brignole, et al evaluated 312 patients with CSH, and found similar mortality to controls. Brignole

# Diagnosis
Persons with CSH may have reproducible signs and symptoms with carotid sinus pressure, and this may be used diagnostically.  If patients have audible bruits on exam or known significant carotid atherosclerosis, carotid sinus massage (CSM) is generally not recommended.
The usual criteria for diagnosis are >3 seconds of ventricular asystole and/or a fall of SBP of >50 mmHg during carotid sinus massage.
- The massage is defined as firm steady pressure to the carotid sinus for ~5 seconds with the neck in the hyperextended position while supine.
- The carotid sinus can be found along the carotid artery medial to the sternocleidomastoid (SCM) at the level of the thyroid cartilage.
- Heart rate, blood pressure, EKG and clinical response are monitored for change.
- The procedure is then repeated on the other side.  The procedure may be repeated standing to evaluate carotid sinus vasodepressor hypersensitivity.
Patients may have either a heart rate fall, the cardioinhibitory response, or fall in blood pressure, the vasodepressor response.  Weiss and Baker also reported that some patients suffered from syncope without any fall in heart rate or blood pressure, they called the primary cerebral type. 
The test is not all-together sensitive or specific.  If normal patients are tested, some will have a fall in blood pressure and heart rate despite no history of syncope.  Other patients with history of syncope will have a fall in HR or BP but no reproducible symptoms at the time of the test.

## History and Symptoms
Many patients will give a history of syncope or near-syncope with movement of the neck, wearing of a tight collar, shaving, etc.  This history is not necessary, however, to make the diagnosis. Patients are usually upright, often standing, and develop a sudden episode of syncope, often with an associated fall.  Small convulsive movements often occur with the loss of consciousness.  The loss of consciousness is usually <30 seconds, and patients awake quickly with no clouding of consciousness.
The syndrome has been described in patients in chronic Afib, in addition to patients in normal sinus rhythm (NSR).

## Physical Examination

### Eyes
Pressing on the eyes can elicit a vagal response, but this is not part of the syndrome.

### Ear Nose and Throat
Caeful examination should be done to assess for carotid bruits and to assure that there is no obvious mass compressing the carotid (such as tumors or lymph nodes).

# Treatment

## Medical Therapy
- Anticholinergics - atropine
- Sympathomimetic meds  ephedrine.  May be particularly useful in patients with vasodepressor type.
- Volume agents  fludrocortisone
- Selective serotonin reuptake inhibitors (SSRI)  two recent studies have shown a good response to SSRIs in five patients with the vasodepressor type CSH.

## Surgery
- Patients with cardioinhibitory / vagal type CSH often do quite well with pacemaker placement. Because some patients have AV node blockade, ventricular as well as atrial pacing is often indicated.
- Carotid sinus denervation by surgery or radiation.

# 2012 ACC/AHA/HRS Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities (DO NOT EDIT)

## Permanent Pacing in Hypersensitive Carotid Sinus Syndrome and Neurocardiogenic Syncope (DO NOT EDIT)

# Sources
- The ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities