Posttraumatic stress disorder (commonly referred to by its acronym, PTSD) is a severe anxiety disorder that can develop after exposure to any event which results in psychological trauma. This event may involve the threat of death to oneself or to someone else, or to one's own or someone else's physical, sexual, or psychological integrity, overwhelming the individual's psychological defenses. It has also been interpreted as a syndrome of deficient extinction ability.

PTSD is a less frequent and more enduring consequence of psychological trauma than the more frequently seen acute stress response.PTSD has also been recognized in the past as railway spine, stress syndrome, shell shock, battle fatigue, traumatic war neurosis, or post-traumatic stress syndrome.

Diagnostic symptoms include re-experiencing original trauma(s), by means of flashbacks or nightmares; avoidance of stimuli associated with the trauma; and increased arousal, such as difficulty falling or staying asleep, anger, and hypervigilance. Formal diagnostic criteria (both DSM-IV and ICD-9) require that the symptoms last more than one month and cause significant impairment in social, occupational, or other important areas of functioning (e.g. problems with work and/or relationships).

Signs and symptoms

PTSD can cause many symptoms. These symptoms can be grouped into three categories:

Re-experiencing

• Flashbacks—reliving the trauma over and over, including physical symptoms like a racing heart or sweating
• Bad dreams
• Frightening thoughts.

Re-experiencing symptoms may cause problems in a person’s everyday routine. They can start from the person’s own thoughts and feelings. Words, objects, or situations that are reminders of the event can also trigger re-experiencing.

Avoidance

• Staying away from places, events, or objects that are reminders of the experience
• Feeling emotionally numb
• Feeling strong guilt, depression, or worry
• Losing interest in activities that were enjoyable in the past
• Having trouble remembering the dangerous event.

Things that remind a person of the traumatic event can trigger avoidance symptoms. These symptoms may cause a person to change his or her personal routine. For example, after a bad car accident, a person who usually drives may avoid driving or riding in a car.

Hyperarousal

• Being easily startled
• Feeling tense or “on edge”
• Having difficulty sleeping, and/or having angry outbursts.

Hyperarousal symptoms are usually constant, instead of being triggered by things that remind one of the traumatic event. They can make the person feel stressed and angry. These symptoms may make it hard to do daily tasks, such as sleeping, eating, or concentrating.

It’s natural to have some of these symptoms after a dangerous event. Sometimes people have very serious symptoms that go away after a few weeks. This is called acute stress disorder, or ASD. When the symptoms last more than a few weeks and become an ongoing problem, they might be PTSD. Some people with PTSD don’t show any symptoms for weeks or months.

Deficient extinction capacity

People with PTSD have deficient extinction capacity relative to those that have been exposed to trauma but who have not developed PTSD. This reduced extinction capacity links to reduced activation during extinction in the hippocampus and bilateral ventromedial prefrontal cortex.

Causes

Psychological trauma

PTSD is believed to be caused by either physical trauma or psychological trauma, or more frequently a combination of both. Possible sources of trauma include experiencing or witnessing childhood or adult physical, emotional or sexual abuse. In addition, experiencing or witnessing an event perceived as life-threatening such as physical assault, adult experiences of sexual assault, accidents, drug addiction, illnesses, medical complications, or employment in occupations exposed to war (such as soldiers) or disaster (such as emergency service workers).

Traumatic events that may cause PTSD symptoms to develop include violent assault, kidnapping, sexual assault, torture, being a hostage, prisoner of war or concentration camp victim, experiencing a disaster, violent automobile accidents or getting a diagnosis of a life-threatening illness. Children may develop PTSD symptoms by experiencing bullying or sexually traumatic events like age-inappropriate sexual experiences.

Witnessing traumatic experiences or learning about these experiences may also cause the development of PTSD symptoms.

A preliminary study found that mutations in a stress-related gene interact with child abuse to increase the risk of PTSD in adults.

Neuroendocrinology

PTSD displays biochemical changes in the brain and body that differ from other psychiatric disorders such as major depression. Individuals diagnosed with PTSD respond more strongly to a dexamethasone suppression test than individuals diagnosed with clinical depression.

In addition, most people with PTSD also show a low secretion of cortisol and high secretion of catecholamines in urine, with a norepinephrine/cortisol ratio consequently higher than comparable non-diagnosed individuals. This is in contrast to the normative fight-or-flight response, in which both catecholamine and cortisol levels are elevated after exposure to a stressor.

Brain catecholamine levels are low, and corticotropin-releasing factor (CRF) concentrations are high. Together, these findings suggest abnormality in the hypothalamic-pituitary-adrenal axis.

Given the strong cortisol suppression to dexamethasone in PTSD, HPA axis abnormalities are likely predicated on strong negative feedback inhibition of cortisol, itself likely due to an increased sensitivity of glucocorticoid receptors. Some researchers have associated the response to stress in PTSD with long-term exposure to high levels of norepinephrine and low levels of cortisol, a pattern associated with improved learning in animals.

Translating this reaction to human conditions gives a pathophysiological explanation for PTSD by a maladaptive learning pathway to fear response through a hypersensitive, hyperreactive and hyperresponsive HPA axis.

Low cortisol levels may predispose individuals to PTSD: Following war trauma, Swedish soldiers serving in Bosnia and Herzegovina with low pre-service salivary cortisol levels had a higher risk of reacting with PTSD symptoms, following war trauma, than soldiers with normal pre-service levels. Because cortisol is normally important in restoring homeostasis after the stress response, it is thought that trauma survivors with low cortisol experience a poorly contained—that is, longer and more distressing—response, setting the stage for PTSD.

However, there is considerable controversy within the medical community regarding the neurobiology of PTSD. A review of existing studies on this subject showed no clear relationship between cortisol levels and PTSD. Only a slight majority have found a decrease in cortisol levels while others have found no effect or even an increase.

Neuroanatomy

In addition to biochemical changes, PTSD also involves changes in brain morphology. In a study by Gurvits et al., Combat veterans of the Vietnam war with PTSD showed a 20% reduction in the volume of their hippocampus compared with veterans who suffered no such symptoms.

In human studies, the amygdala has been shown to be strongly involved in the formation of emotional memories, especially fear-related memories. Neuroimaging studies in humans have revealed both morphological and functional aspects of PTSD.

The amygdalocentric model of PTSD proposes that it is associated with hyperarousal of the amygdala and insufficient top-down control by the medial prefrontal cortex and the hippocampus particularly during extinction. Further animal and clinical research into the amygdala and fear conditioning may suggest additional treatments for the condition.

Genetics

There is evidence that susceptibility to PTSD is hereditary. For twin pairs exposed to combat in Vietnam, having a monozygotic (identical) twin with PTSD was associated with an increased risk of the co-twin having PTSD compared to twins that were dizygotic (non-identical twins).

Recently, it has been found that several single nucleotide polymorphisms (SNPs) in FK506 binding protein 5 (FKBP5) interact with childhood trauma to predict severity of adult PTSD. These findings suggest that individuals with these SNPs who are abused as children are more susceptible to PTSD as adults.

This is particularly interesting given that FKBP5 SNPs have previously been associated with peritraumatic dissociation (that is, dissociation at the time of the trauma), which has itself been shown to be predictive of PTSD. Furthermore, FKBP5 may be less expressed in those with current PTSD.

Risk and protective factors for PTSD development

Although most people (50-90%) encounter trauma over a lifetime, only about 8% develop full PTSD. Vulnerability to PTSD presumably stems from an interaction of biological diathesis, early childhood developmental experiences, and trauma severity.

Predictor models have consistently found that childhood trauma, chronic adversity, and familial stressors increase risk for PTSD as well as risk for biological markers of risk for PTSD after a traumatic event in adulthood. This effect of childhood trauma, which is not well understood, may be a marker for both traumatic experiences and attachment problems.

Proximity to, duration of, and severity of the trauma also make an impact; and interpersonal traumas cause more problems than impersonal ones.

Schnurr, Lunney, and Sengupta identified risk factors for the development of PTSD in Vietnam veterans. Among those are:

• Hispanic ethnicity, coming from an unstable family, being punished severely during childhood, childhood asocial behavior and depression as pre-military factors
• war-zone exposure, peritraumatic dissociation, depression as military factors
• recent stressful life events, post-Vietnam trauma and depression as post-military factors

They also identified certain protective factors, such as:

• Japanese-American ethnicity, high school degree or college education, older age at entry to war, higher socioeconomic status and a more positive paternal relationship as pre-military protective factors
• Social support at homecoming and current social support as post-military factors. Other research also indicates the protective effects of social support in averting and recovery from PTSD.

There may also be an attitudinal component; for example, a soldier who believes that they will not sustain injuries may be more likely to develop symptoms of PTSD than one who anticipates the possibility, should either be wounded. Likewise, the later incidence of suicide among those injured in home fires above those injured in fires in the workplace suggests this possibility.

Diagnosis

The diagnostic criteria for PTSD, per the Diagnostic and Statistical Manual of Mental Disorders IV (Text Revision) (DSM-IV-TR), may be summarized as:

A. Exposure to a traumatic event

B. Persistent reexperience (e.g. flashbacks, nightmares)

C. Persistent avoidance of stimuli associated with the trauma (e.g. avoidance of experiences that they fear will trigger flashbacks and reexperiencing of symptoms fear of losing control)

D. Persistent symptoms of increased arousal (e.g. difficulty falling or staying asleep, anger and hypervigilance)

E. Duration of symptoms for more than 1 month

F. Significant impairment in social, occupational, or other important areas of functioning (e.g. problems with work and relationships.)

Notably, criterion A requires that "the person’s response involved intense fear, helplessness, or horror." The DSM-IV-TR criterion differs substantially from the previous DSM-III-R stressor criterion, which specified the traumatic event should be of a type that would cause "significant symptoms of distress in almost anyone," and that the event was "outside the range of usual human experience."

Since the introduction of DSM-IV, the number of possible PTSD traumas has increased and one study suggests that the increase is around 50%.Various scales exist to measure the severity and frequency of PTSD symptoms.

Prevention

In recent history, catastrophes (by human means or not) such as the Indian Ocean Tsunami Disaster may have caused PTSD in many survivors and rescue workers. Today relief workers from organizations such as the Red Cross and the Salvation Army provide counseling after major disasters as part of their standard procedures to curb severe cases of post-traumatic stress disorder.

In the United States

In part through the efforts of anti Vietnam war activists and the anti war group Vietnam Veterans Against the War and Chaim F. Shatan, who worked with them and coined the term post-Vietnam Syndrome, the condition was added to the DSM-III as posttraumatic stress disorder.

A review of the provision of compensation to veterans for PTSD by the United States Department of Veterans Affairs began in 2005 after the VA had noted a 30% increase in PTSD claims in recent years. This led to a backlash from veterans'-rights groups, and to some highly-publicized suicides by veterans who feared losing their benefits, which in some cases constituted their only income. In response, on November 10, 2005, the Secretary of Veterans Affairs announced that "the Department of Veterans Affairs (VA) will not review the files of 72,000 veterans currently receiving disability compensation for post-traumatic stress disorder..."

The diagnosis of PTSD has been a subject of some controversy due to uncertainties in objectively diagnosing PTSD in those who may have been exposed to trauma, and due to this diagnosis' association with some incidence of compensation-seeking behavior.

Many veterans of the wars in Iraq and Afghanistan returning home have faced significant physical, emotional and relational disruptions. In response the United States Marine Corps has instituted programs to assist them in re-adjusting to civilian life - especially in their relationships with spouses and loved ones - to help them communicate better and understand what the other has gone through. Similarly, Walter Reed Army Institute of Research (WRAIR) developed the Battlemind program to assist service members avoid or ameliorate PTSD and related problems. In the UK there has been some controversy that National Health Service is dumping veterans on service charities like Combat Stress.

Canadian veterans

Veterans Affairs Canada offers a new program that includes rehabilitation, financial benefits, job placement, health benefits program, disability awards and family support.

Management

Early interventions

Some benefit has been found from early access to cognitive behavioral therapy, as well as from some medications such as propranolol. Effects of all these prevention strategies is modest.

Critical incident stress management (CISM) has been used to attempt to reduce effects of a potentially traumatic incident, and to attempt to prevent a full-blown occurrence of PTSD. However, recent studies regarding CISM seem to indicate iatrogenic effects.Six studies have formally looked at the effect of CISM, four finding no benefit for preventing PTSD, and the other two studies indicating that CISM actually made things worse. Hence this is not a recommended treatment.

Psychotherapeutic interventions

Many forms of psychotherapy have been advocated for trauma-related problems such as PTSD. Basic counseling practices common to many treatment responses for PTSD include education about the condition and provision of safety and support.

The psychotherapy programs with the strongest demonstrated efficacy include cognitive behavioral programs, variants of exposure therapy, stress inoculation training (SIT), variants of cognitive therapy (CT), eye movement desensitization and reprocessing (EMDR), and many combinations of these procedures.

The British Journal of Psychiatry has recommended EMDR or trauma-specific cognitive behavioral therapy as first-line treatments for trauma victims. A meta-analytic comparison of EMDR and cognitive behavioral therapy found both protocols indistinguishable in terms of effectiveness in treating PTSD.

Cognitive behavioral therapy

Cognitive Behavioral Therapy (CBT) is a psychotherapeutic approach that aims to change the patterns of thinking and/or behavior that are responsible for a trauma victim’s negative emotions and, in doing so, change the way they feel and act. CBT has been proven to be an effective treatment for PTSD, and is currently considered the standard of care for PTSD by the Department of Defense . In CBT, individuals learn to identify thoughts that make them feel afraid or upset, and replace them with less distressing thoughts. The goal is to understand how certain thoughts about trauma cause stress and make symptoms worse.

Eye movement desensitization and reprocessing

Eye Movement Desensitization and Reprocessing (EMDR) is specifically targeted as a treatment for PTSD. Based on the evidence of controlled research, the American Psychiatric Association and the U.S. Department of Veterans Affairs and Department of Defense, have placed EMDR in the highest category of effectiveness and research support in the treatment of trauma. Several international bodies have made similar recommendations.

Exposure therapy

Exposure involves assisting trauma survivors to therapeutically confront distressing trauma-related memories and reminders in order to facilitate habituation and successful emotional processing of the trauma memory. Most exposure therapy programs include both imaginal confrontation with the traumatic memories and real-life exposure to trauma reminders.

Indeed, the success of exposure-based therapies has raised the question of whether exposure is a necessary ingredient in the treatment of PTSD. Some organizations have endorsed the need for exposure.

Interpersonal psychotherapy

Other approaches, particularly involving social supports, may also be important. An open trial of interpersonal psychotherapy reported high rates of remission from PTSD symptoms without using exposure. A current, NIMH-funded trial in New York City is now comparing interpersonal psychotherapy, prolonged exposure therapy, and relaxation therapy /www.columbiatrauma.org/>.

Medication

Medications have shown benefit in reducing PTSD symptoms, but "there is no clear drug treatment for PTSD".

Standard medication therapy useful in treating PTSD includes SSRIs (selective serotonin reuptake inhibitors) and TCAs (tricyclic antidepressants). Positive symptoms (re-experiencing, hypervigilance, increased arousal) generally respond better to medication than negative symptoms (avoidance, withdrawal).

Tricyclics tend to be associated with greater side effects and lesser improvement of the three PTSD symptom clusters than SSRIs. SSRIs for which there are data to support use include: citalopram, escitalopram, fluvoxamine, paroxetine and sertraline.

There are data to support the use of "autonomic medicines" such as propranolol (beta blocker) and clonidine (alpha-adrenergic agonist) if there are significant symptoms of "over-arousal". These may inhibit the formation of traumatic memories by blocking adrenaline's effects on the amygdala, has been used in an attempt to reduce the impact of traumatic events, or they may simply demonstrate to an individual that the symptoms can be controlled thereby assisting with "self efficacy" and helping the person remain calmer.

There are also data to support the use of mood-stabilizers such lithium carbonate and carbamazepine if there is significant uncontrolled mood or aggression. Risperidone is used to help with dissociation, mood and aggression, and benzodiazepines are used for short-term anxiety relief.

Recently the anticonvulsant lamotrigine has been reported to be useful in treating some people with PTSD.

There is some evidence suggesting that administering glucocorticoids immediately after a traumatic experience may help prevent PTSD. Several studies have shown that individuals who receive high doses of hydrocortisone for treatment of septic shock or following surgery have a lower incidence and fewer symptoms of PTSD. Additionally, post-stress high dose corticosterone administration was recently found to reduce 'PTSD-like' behaviors in a rat model of PTSD. In this study, corticosterone impaired memory performance, suggesting that it may reduce risk for PTSD by interfering with consolidation of traumatic memories. The neurodegenerative effects of the glucocorticoids, however, may prove this treatment counterproductive.

Combination therapies

PTSD is commonly treated using a combination of psychotherapy and medications.

Clinical trials evaluating methylenedioxymethamphetamine (MDMA, "Ecstasy") in conjunction with psychotherapy are being conducted in Switzerland and Israel. A clinical trial is also examining the efficacy of hydrocortisone in conjunction with exposure therapy for PTSD symptoms.

Co-morbid substance dependence as an inhibitor of recovery

Recovery from posttraumatic stress disorder or other anxiety disorders may be hindered, or the conditions worsened, by alcohol or benzodiazepine dependence. Treating co-morbid substance dependences particularly alcohol or benzodiazepine dependence can bring about a marked improvement in an individuals mental health status and anxiety levels. Recovery from benzodiazepines tends to take a lot longer than recovery from alcohol but people can regain their previous good health. Symptoms may temporarily worsen however, during alcohol withdrawal or benzodiazepine withdrawal.

Epidemiology

[[Image:Post-traumatic stress disorder world map - DALY - WHO2002.svg|thumb|Disability-adjusted life year rates for post-traumatic stress disorder per 100,000 inhabitants in 2002.

]]There is debate over the rates of PTSD found in populations, but despite changes in diagnosis and the criteria used to define PTSD between 1997 and 2007, epidemiological rates have not changed significantly.

United States

The National Comorbidity Survey has estimated that the lifetime prevalence of PTSD among adult Americans is 7.8%, with women (10.4%) twice as likely as men (5%) to have PTSD at some point in their lives.

The United States Department of Veterans Affairs estimates that 830,000 Vietnam War veterans suffered symptoms of PTSD. The National Vietnam Veterans' Readjustment Study (NVVRS) found 15.2% of male and 8.5% of female Vietnam Vets to suffer from current PTSD at the time of the study. Life-Time prevalence of PTSD was 30.9 for males and 26.9 for females. In a reanalysis of the NVVRS data, along with analysis of the data from the Matsunaga Vietnam Veterans Project, Schnurr, Lunney, Sengupta, and Waelde found that, contrary to the initial analysis of the NVVRS data, a large majority of Vietnam veterans suffered from PTSD symptoms (but not the disorder itself). Four out of five reported recent symptoms when interviewed 20–25 years after Vietnam.

International PTSD rates

The United Nations' World Health Organization publishes estimates of PTSD impact for each of its member states; the latest data available are for 2004. Considering only the 25 most populated countries,ranked by overall age-standardized Disability-adjusted life year (DALY) rate, the top half of the ranked list is dominated by Asian/Pacific countries, the USA, and Egypt. Ranking the countries by the male-only or female-only rates produces much the same result, but with less meaningfulness, as the score range in the single sex rankings is much reduced (4 for women, 3 for men, as compared with 14 for the overall score range), suggesting that the differences between female and male rates, within each country, is what drives the distinctions between the countries.

Age-standardized]] Disability-adjusted life year (DALY) rates for PTSD, per 100,000 inhabitants, in 25 most populous countries, ranked by overall rate (2004)

Region	Country	PTSD DALY rate, overall	PTSD DALY rate, females	PTSD DALY rate, males
Asia / Pacific	Thailand	59	86	30
Asia / Pacific	Indonesia	58	86	30
Asia / Pacific	Philippines	58	86	30
Americas	USA	58	86	30
Asia / Pacific	Bangladesh	57	85	29
Africa	Egypt	56	83	30
Asia / Pacific	India	56	85	29
Asia / Pacific	Iran	56	83	30
Asia / Pacific	Pakistan	56	85	29
Asia / Pacific	Japan	55	80	31
Asia / Pacific	Myanmar	55	81	30
Europe	Turkey	55	81	30
Asia / Pacific	Viet Nam	55	80	30
Europe	France	54	80	28
Europe	Germany	54	80	28
Europe	Italy	54	80	28
Asia / Pacific	Russian Federation	54	78	30
Europe	United Kingdom	54	80	28
Africa	Nigeria	53	76	29
Africa	Dem. Republ. of Congo	52	76	28
Africa	Ethiopia	52	76	28
Africa	South Africa	52	76	28
Asia / Pacific	China	51	76	28
Americas	Mexico	46	60	30
Americas	Brazil	45	60	30

History

Earliest reports

Reports of battle-associated stress reactions appear as early as the 6th century BCE. One of the first descriptions of PTSD was made by the Greek historian Herodotus. In 490 BCE he described, during the Battle of Marathon, an Athenian soldier who suffered no injury from war but became permanently blind after witnessing the death of a fellow soldier.

In the early 1800s military medical doctors started diagnosing soldiers with "exhaustion" after the stress of battle. This "exhaustion" was characterized by mental shutdown due to individual or group trauma. As in the present time, soldiers during the 1800s were not supposed to be scared or show any fear in the midst of battle. The only treatment for this "exhaustion" was to bring the afflicted to the back for a bit then send them back into battle. During the intense and frequently repeated stress, the soldiers became fatigued as a part of their body's natural shock reaction.

One-tenth of mobilized American men were hospitalised for mental disturbances between 1942 and 1945, and after thirty-five days of uninterrupted combat, 98% of them manifested psychiatric disturbances in varying degrees.

Although PTSD-like symptoms have also been recognized in combat veterans of many military conflicts since, the modern understanding of PTSD dates from the 1970s, largely as a result of the problems that were still being experienced by US military veterans of the war in Vietnam.

Terminology

The term post-traumatic stress disorder or PTSD was coined in the mid 1970s. Early in 1978, the term was used in a working group finding presented to the Committee of Reactive Disorders. The term was formally recognized in 1980. (In the authoritative DSM-IV, the spelling "posttraumatic stress disorder" is used. Elsewhere, "posttraumatic" is often rendered as two words — "post-traumatic stress disorder" or "post traumatic stress disorder" — especially in less formal writing on the subject.)

Society and culture

In recent decades, with the concept of trauma and PTSD in particular becoming just as much a cultural phenomenon as a medical or legal one , artists have engaged the issue in their work. Many movies, such as the Bourne films, First Blood, Birdy, Born on the Fourth of July, Brothers, Coming Home, The Deer Hunter, Heaven & Earth, In the Valley of Elah, The War at Home, Gran Torino and Halloween II deal with PTSD. It is an especially popular subject amongst "war veteran" films, often portraying Vietnam war veterans suffering from extreme PTSD and having difficulties adjusting to civilian life . Military-themed videogames have also begun touching on the subject; Metal Gear Solid 4: Guns of the Patriots presents the idea of a quick-fix technology that stops soldiers from experiencing an emotional reaction to combat, thus negating PTSD, and the ensuing trauma soldiers under this system suddenly find themselves with when the technology fails and leaves them without any built-up tolerance. Several characters in the game are revealed to have experienced extreme trauma at a young age, and PTSD over these events influences them into their adult lives .

In more recent work, an example is that of Krzysztof Wodiczko who teaches at MIT and who is known for interviewing people and then projecting these interviews onto large public buildings. Wodiczko aims to bring trauma not merely into public discourse but to have it contest the presumed stability of cherished urban monuments . His work has brought to life issues such as homelessness, rape, and violence . Other artists who engage the issue of trauma are Everlyn Nicodemus of Tanzania and Milica Tomic of Serbia.

General Romeo Dallaire mentions his PTSD in his book, Shake hands with the Devil, which was adapted into a film.

Targets: Reporters in Iraq is a film made by Maziar Bahari that deals with journalists suffering PTSD in Iraq.

Posttraumatic stress disorder is the central subject of the Israeli film Waltz with Bashir, in which a former soldier struggles to cope with his traumatic memories of the 1982 Lebanon War twenty years later. The narrative of the film itself becomes structured entirely around the nature of PTSD recollection. In one scene, a specialist on post-traumatic stress disorder directly addresses his trauma.

In other species

There have been reports of elephants suffering from posttraumatic stress disorder:

• http://www.elephants.com/ptsd/Bradshaw&Lindner_PTSD-rev.pdf , in captive elephants
• http://www.elephants.com/media/Elephant_breakdown_2005.pdf , in wild elephants, from seeing herd fellows shot by hunters

See also

• Acute stress reaction
• Complex post-traumatic stress disorder
• Emotional dysregulation
• Media violence research
• Post-traumatic embitterment disorder
• Psychogenic amnesia
• Psychoneuroimmunology
• Survivor syndrome
• Thousand-yard stare
• Trauma model of mental disorders

References

External links