Editing Sinus node dysfunction

{{Infobox medical condition
| name          = Sinus node dysfunction
| synonyms      = '''Sick sinus syndrome''' or '''sinoatrial node disease'''
| image         = ECG Sinus Pause.jpg
| caption       = Telemetry strip of a 44-year-old female with asymptomatic sinus pause found while admitted for mildly symptomatic COVID-19 pneumonia.
| field         =
| complications = Tachycardia-bradycardia syndrome
| alt           =
| treatment     =[[Pacemaker]] implantation
| diagnosis       = [[electrocardiogram]]
}}
'''Sinus node dysfunction''' ('''SND'''), also known as '''sick sinus syndrome''' ('''SSS'''), is a group of abnormal heart rhythms ([[Heart arrhythmia|arrhythmia]]s) usually caused by a malfunction of the [[sinus node]], the heart's primary pacemaker.<ref name="Kusumoto2019">{{cite journal |vauthors=Kusumoto FM, Schoenfeld MH, Barrett C, Edgerton JR, Ellenbogen KA, Gold MR, Goldschlager NF, Hamilton RM, Joglar JA, Kim RJ, Lee R, Marine JE, McLeod CJ, Oken KR, Patton KK, Pellegrini CN, Selzman KA, Thompson A, Varosy PD |title=2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society |journal=Circulation |date=20 August 2019 |volume=140 |issue=8 |pages=e382–e482 |doi=10.1161/CIR.0000000000000628 |pmid=30586772 |display-authors=6|doi-access=free }}</ref><ref name="Dobrzynski2007">{{cite journal |vauthors=Dobrzynski H, Boyett MR, Anderson RH |title=New Insights Into Pacemaker Activity: Promoting Understanding of Sick Sinus Syndrome |journal=Circulation |date=10 April 2007 |volume=115 |issue=14 |pages=1921–1932 |doi=10.1161/CIRCULATIONAHA.106.616011 |pmid=17420362 |doi-access=free }}</ref> '''Tachycardia-bradycardia syndrome''' is a variant of sick sinus syndrome in which the arrhythmia alternates between fast and slow heart rates.<ref name="Tse2017">{{cite journal |vauthors=Tse G, Liu T, LiKH, Laxton V, Wong AO, Chan YW, Keung W, Chan CW, Li RA |title=Tachycardia-bradycardia syndrome: Electrophysiological mechanisms and future therapeutic approaches (Review) |journal=International Journal of Molecular Medicine |date=March 2017 |volume=39 |issue=3 |pages=519–526 |doi=10.3892/ijmm.2017.2877 |pmid=28204831 |pmc=5360359 |display-authors=6}}</ref>

==Signs and symptoms==
Often sinus node dysfunction produces no symptoms, especially early in the disease course. Signs and symptoms usually appear in more advanced disease and more than 50% of patients will present with [[Syncope (medicine)|syncope]] or transient near-fainting spells as well as [[Bradycardia|bradycardias]] that are accompanied by rapid heart rhythms, referred to as tachycardia-bradycardia syndrome<ref>{{cite journal |vauthors=Hannibal GB, Walsh-Irwin C |title=Sick Sinus Syndrome |journal=AACN Advanced Critical Care |date=1 October 2015 |volume=26 |issue=4 |pages=376–380 |doi=10.1097/NCI.0000000000000099|pmid=26485000 |s2cid=26255262 }}</ref><ref name="Semelka20132">{{cite journal |vauthors=Semelka M, Gera J, Usman S |title=Sick sinus syndrome: a review. |journal=American Family Physician |date=15 May 2013 |volume=87 |issue=10 |pages=691–6 |pmid=23939447}}</ref> Other presenting signs or symptoms can include confusion, fatigue, [[palpitations]], [[chest pain]], [[shortness of breath]], headache, and nausea. Patients can also present with symptoms of [[Heart failure|congestive heart failure]], [[stroke]] or [[transient ischemic attack]]s due to the abnormal rhythm.<ref name="Semelka20132" />

===Complications===
[[File:Brady-tachy syndrome AV-junctional rhythm.png|thumb|[[Electrocardiogram]] from a man with tachycardia-bradycardia syndrome following [[mitral valvuloplasty]], resection of the [[left atrial appendage]], and [[Cox maze procedure|maze procedure]]. The ECG shows [[AV-junctional rhythm]] resulting in [[bradycardia]] at around 46 beats per minute.]] [[Image:Brady-tachy syndrome atrial fibrillation.png|thumb|This ECG from the same patient shows [[atrial fibrillation]] at around 126 beats per minute.]]The most common complication of sinus node dysfunction is the development of tachycardia-bradycardia syndrome with abnormal atrial rhythms such as [[atrial tachycardia]], [[atrial fibrillation]], and [[Atrial flutter|flutter]].<ref name="Semelka20132" /><ref name="Tse2017" /> These rhythms increases the risk of clot formation in the atrium, embolization, and [[stroke]].<ref name="Semelka20132" />

Developing [[sinus arrest]], [[SA block|sinus node exit block]], [[sinus bradycardia]], [[atrioventricular block]], and other types of abnormal rhythms are also common complications.<ref name="Semelka20132" /><ref name="John2016">{{cite journal |vauthors=John RM, Kumar S |title=Sinus Node and Atrial Arrhythmias |journal=Circulation |date=10 May 2016 |volume=133 |issue=19 |pages=1892–1900 |doi=10.1161/CIRCULATIONAHA.116.018011|pmid=27166347 |doi-access=free }}</ref> Sinus node dysfunction has a close association with the presence of [[atrial fibrillation]] due to their shared etiology of remodeling.<ref name="John2016" />

==Causes==
Sinus node dysfunction can be caused by intrinsic and extrinsic factors that affect the normal functioning of the sinus node. Intrinsic causes can include degeneration, dysfunction, or remodeling of the sinus node while extrinsic causes can create or worsen underlying atrial arrhythmias.<ref name="Semelka20132" /> Intrinsic causes tend to be responsible for permanent sinus node dysfunction while extrinsic causes are more commonly temporary.<ref name="Semelka20132" />

=== Intrinsic causes ===
Age-related degenerative fibrosis of the sinus node is often identified as the most common intrinsic cause.<ref name="Semelka20132" /><ref name="John2016" /> Other intrinsic causes include inherited ion channel dysfunctions, remodeling diseases such as [[heart failure]] and [[atrial fibrillation]], infiltrative diseases such as [[sarcoidosis]], [[amyloidosis]], [[Hereditary haemochromatosis|hemochromatosis]], and [[connective tissue disease]]s, inflammatory etiology such as [[rheumatic fever]], [[Chagas disease]], and [[Lyme disease]], as well as atherosclerotic and ischemic changes to the sinus node artery.<ref name="Semelka20132" /><ref name="John2016" />

Inherited sinus node dysfunction has been associated with mutations of the gene responsible for the formation of the alpha subunit of the sodium channel ([[SCN5A]]).<ref name="Semelka20132" />

=== Extrinsic causes ===
Common cardiac pharmacology such as [[Beta blocker|beta-blockers]], [[calcium channel blocker]]s, [[digoxin]], sympatholytic medication, and other antiarrhythmics can alter sinus node function to create an arrhythmia such as sick sinus syndrome. Electrolyte abnormalities such as [[hyperkalemia]], [[hypokalemia]], and [[Hypocalcaemia|hypocalcemia]] can also alter normal sinus node functioning. [[Hypothyroidism]], [[Hypoxia (medical)|hypoxia]], [[hypothermia]], and various toxins have also been associated with sinus node dysfunctions.<ref name="Semelka20132" /><ref name="John2016" />

==Diagnosis==
[[File:ECG Sinus Bradycardia 49 bpm.jpg|thumb|[[Electrocardiography|Electrocardiogram]] (ECG) of a 64-year-old female with [[sinus bradycardia]]. Heart rate 49 bpm.]]

=== Electrocardiogram ===
The primary 12-lead electrocardiogram (ECG) finding in sinus node dysfunction is inappropriate [[sinus bradycardia]].<ref name="Deponti2018">{{cite journal |vauthors=De Ponti R, Marazzato J, Bagliani G, Leonelli FM, Padeletti L |title=Sick Sinus Syndrome |journal=Cardiac Electrophysiology Clinics |date=June 2018 |volume=10 |issue=2 |pages=183–195 |doi=10.1016/j.ccep.2018.02.002 |pmid=29784479 |s2cid=29154542 }}</ref> Sinus node dysfunction can also present with sudden [[Sinoatrial arrest|sinus arrest]] with or without [[Junctional escape beat|junctional escape]], [[sinoatrial block]], prolonged asystolic period followed by tachycardias, or tachycardia-bradycardia syndrome presenting as various atrial arrhythmias such as [[atrial fibrillation]], [[Atrial flutter|flutter]], [[Sinus tachycardia|tachycardia]], or [[paroxysmal supraventricular tachycardia]].<ref name="Deponti2018" /><ref name="Semelka20132" />

=== Clinical diagnosis ===
Diagnosing sinus node dysfunction requires clinical symptoms as well as ECG abnormalities. If ECG findings cannot be identified, prolonged cardiac monitoring should be pursued either with a [[Holter monitor]] in an outpatient setting or telemetry while inpatient, due to the transient nature of abnormal ECG findings.<ref name="Semelka20132" /> If Holter or telemetry monitoring fails to identify ECG changes and suspicion of sinus node dysfunction remains high due to severe symptoms or episodes of syncope, [[implantable loop recorder]]s should be considered for extended monitoring up to 24 months.<ref name="Deponti2018" />

[[Exercise stress test]]s can be utilized to identify intrinsic causes of sinus node dysfunction. [[Tilt table test]]s can be used to discriminate bradycardia caused by dysfunction of the [[autonomic nervous system]].<ref name="Semelka20132" /><ref name="Deponti2018" />

==Treatment==
The primary reason for considering treatment is the presence of symptoms.<ref name="Kusumoto2019" /> [[Artificial cardiac pacemaker|Pacemaker]] implantation is the primary treatment modality of symptomatic sinus node dysfunction.<ref name="Semelka20132" /><ref name="Deponti2018" /><ref>{{cite journal |vauthors=Drago F, Silvetti MS, Grutter G, De Santis A |title=Long term management of atrial arrhythmias in young patients with sick sinus syndrome undergoing early operation to correct congenital heart disease |journal=EP Europace |date=1 July 2006 |volume=8 |issue=7 |pages=488–494 |doi=10.1093/europace/eul069 |pmid=16798761 |doi-access=free }}</ref> The goal of this treatment modality is to relieve symptoms associated with sinus node dysfunction and improve quality of life.<ref name="Semelka20132" /> Dual chamber pacemakers are preferred due to the possibility of developing [[atrioventricular block]]<ref name="Semelka20132" /> as well as long term cost-effectiveness relative to single-chamber atrial pacemakers.<ref>{{cite journal|vauthors=Edwards SJ, Karner C, Trevor N, Wakefield V, Salih F|date=August 2015|title=Dual-chamber pacemakers for treating symptomatic bradycardia due to sick sinus syndrome without atrioventricular block: a systematic review and economic evaluation|url=https://doi.org/10.3310%2Fhta19650|journal=[[Health Technology Assessment (journal)|Health Technology Assessment]]|volume=19|issue=65|pages=1–210|doi=10.3310/hta19650|pmc=4781212|pmid=26293406}}</ref>

In tachycardia-bradycardia syndrome, medication-based management can treat atrial tachyarrhythmias. However, these medications may exacerbate underlying bradyarrhythmia. Therefore, a dual-chamber pacemaker capable of managing atrial tachyarrhythmias as well as bradyarrhythmias is implanted before drug therapy is begun.<ref name="Deponti2018" />

==Epidemiology==
Overall incidence of sinus node dysfunction increases with age<ref name="Jensen2014">{{cite journal |vauthors=Jensen PN, Gronroos NN, Chen LY, Folsom AR, deFilippi C, Heckbert SR, Alonso A |title=Incidence of and Risk Factors for Sick Sinus Syndrome in the General Population |journal=Journal of the American College of Cardiology |date=August 2014 |volume=64 |issue=6 |pages=531–538 |doi=10.1016/j.jacc.2014.03.056 |pmid=25104519 |pmc=4139053 |display-authors=6}}</ref> with 1 in 1000 in adults over 45 years old<ref name="John2016" /> and 1 in 600 cardiac patients over 65 years old.<ref name="Semelka20132" /> Sinus node dysfunction is the primary indication for approximately 30%-50% of all pacemaker implantation in the United States.<ref name="Jensen2014" /> Sinus node dysfunction is a relatively uncommon syndrome in the young and middle-aged population.{{cn|date=February 2021}}

== References ==
{{Reflist}}

== External links ==
{{Medical resources
|  DiseasesDB     = 12066
|  ICD10          = {{ICD10|I|49|5|i|49}}
|  ICD9           = {{ICD9|427.81}}
|  ICDO           =
|  OMIM           =
|  MedlinePlus    = 000161
|  eMedicineSubj  =
|  eMedicineTopic =
|  MeshID         =
}}
{{Heart diseases}}

[[Category:Cardiac arrhythmia]]
[[Category:Syndromes affecting the heart]]