Neurogenic Shock

Etiology

Neurogenic shock is a critical condition that results from the dysregulation of the autonomic nervous system following spinal cord injury, especially in the cervical and upper thoracic vertebrae above the T6 level, as a result of traumatic events. The dysregulation arises due to the lack of sympathetic tone and an unopposed parasympathetic response. Although rare, other potential causes of neurogenic shock include spinal anesthesia, Guillain-Barre syndrome, toxins affecting the autonomic nervous system, transverse myelitis, and various neuropathies involving the cervical and upper thoracic spinal cord.

Epidemiology

Approximately 8,000 to 10,000 individuals experience traumatic spinal cord injuries annually in the United States. The data obtained from the Trauma Audit and Research Network after a comprehensive analysis of 490 isolated spinal cord injury cases found that only 19.3% of cases involved typical neurogenic shock.[5] However, a retrospective study at a higher-volume level 1 trauma center found that 31% of cases (19 out of 62 patients) with high (cord level C1-C5) cervical spine injuries experienced neurogenic shock.[6] 

There are no universally defined hemodynamic parameters for neurogenic shock. However, most studies adopt the criteria of a systolic blood pressure below 90 mm Hg [7] and a heart rate below 80 bpm.[5] Evaluating the epidemiology of neurogenic shock is challenging, as it remains unclear how hemorrhagic shock and other injuries affect the hemodynamic consequences of spinal cord injury.

Pathophysiology

Neurogenic shock is a clinical condition manifested following a spinal cord injury that affects the cervical and upper thoracic spinal cord levels. This condition triggers hemodynamic alterations resulting from the disruption of descending sympathetic tracts, typically due to fractures or dislocations of vertebrae in these spinal regions. The primary onset of spinal cord injury typically occurs within minutes of the initial insult, resulting in direct damage to axons and neural membranes in the intermediolateral nucleus, lateral gray matter, and anterior root. This damage leads to a disruption of sympathetic tone.[8] 

The decline in sympathetic tone prompts the dilation of capacitance blood vessels in the lower extremities.[9] This, in turn, results in decreased cardiac filling, leading to hypotension and shock. Moreover, reduced sympathetic control over the heart leads to a situation where unopposed vagal cardiac influence causes bradycardia. Patients with neurogenic shock may also exhibit pink and warm skin due to the dilation of subcutaneous blood vessels. 

Secondary spinal cord injury occurs within hours to days after the initial trauma. This phase of injury results from vascular disturbances, electrolyte shifts, and edema development, leading to the gradual onset of central hemorrhagic necrosis within the gray matter at the injury site. Some studies have suggested that hypotension secondary to neurogenic shock exacerbates the severity of spinal cord injury.[10][11]

Neurogenic shock is underpinned by several intricate processes at the cellular level. These include the accumulation of N-methyl-D-aspartate (NMDA) and subsequent excitotoxicity, disrupted electrolyte homeostasis, mitochondrial damage, and reperfusion injury. These mechanisms collectively contribute to controlled and uncontrolled apoptosis, resulting in a complex cascade of events characterizing neurogenic shock.[7] 

History and Physical

Diagnosing neurogenic shock requires a systematic and comprehensive approach, necessitating diligent investigation by healthcare practitioners. Maintaining a high level of suspicion for neurogenic shock is crucial, especially when dealing with cases of spinal cord injury occurring above the T6 level. In such patients, healthcare practitioners should carefully assess the mechanism of injury, check for midline spinal tenderness or step-offs, consider distracting injuries that could divert attention from a spinal area, and evaluate for loss of consciousness, neurological deficits, or intoxication that might complicate the examination, as these factors can be associated with spinal trauma.

Excluding alternative causes of shock promptly is imperative in the diagnostic process. Potential factors contributing to shock must be promptly investigated in a trauma patient, including acute blood loss, pneumothorax, direct cardiac injury, and adrenal insufficiency.[12] 

Characteristic vital signs of neurogenic shock include hypotension accompanied by bradycardia. This differs from the presentation in patients with non-spinal cord injuries and experiencing hypovolemic shock, where hypotension is associated with tachycardia. Patients with neurogenic shock may exhibit warm and pink skin, contrasting with cool and pallid skin often observed in patients with hypovolemic shock. 

Evaluation

Following the Advanced Trauma Life Support (ATLS) protocol, the initial survey prioritizes a sequential assessment approach known as "ABCs,' which stands for airway, breathing, circulation, and hemorrhage control.[13][14] A concise neurological examination should be performed only after evaluating and addressing crucial elements. Spine immobilization must be maintained during this phase until spinal cord injury is ruled out.

After the initial assessment and stabilization, the focus shifts toward additional diagnostic procedures. High-quality computed tomography (CT) imaging of the spine is recommended to evaluate for fractures and alignment issues, particularly if suspicion of spinal cord injury persists. Once the patient's condition has been stabilized, magnetic resonance imaging (MRI) of the spine may be performed to help obtain a more detailed view of the ligamentous structures. MRI scans can also assess for any abnormal cord signal changes and the presence of compressive or herniated discs. 

By following this systematic approach, medical practitioners can efficiently assess patients with suspected neurogenic shock, enhancing their care and implementing the necessary diagnostic and therapeutic measures. The diagnosis of neurogenic shock involves a combination of radiographic imaging, hemodynamic monitoring, and clinical examination.[4][15][16][17][18]

Treatment / Management

The primary objective in the initial management of neurogenic shock is centered on achieving hemodynamic stability. The treatment of hypotension in patients with neurogenic shock should be conducted in the intensive care unit (ICU). Vigilant monitoring of hemodynamics and cardiac function is crucial, and the prompt correction of hypotension is vital to improve patient outcomes.[10] (B3)

The first-line treatment for hypotension involves intravenous fluid resuscitation. This approach is designed to address the vasogenic dilation typical of neurogenic shock. However, exercising caution and avoiding overly aggressive fluid resuscitation is essential, as excessive administration can lead to complications.

The second-line treatment involves using vasopressors and inotropes in cases where hypotension continues even after euvolemia. However, no universally recommended agent exists for all patients. Phenylephrine, being a pure α-1 agonist, is frequently utilized to induce peripheral vasoconstriction and mitigate the loss of sympathetic tone. However, phenylephrine lacks β-activity, so it can cause reflex bradycardia, further intensifying the preexisting unopposed vagal tone, especially in cervical and upper thoracic spinal cord injuries.

Norepinephrine is preferred due to its dual α- and β-activity, effectively addressing hypotension and bradycardia. Epinephrine may also be considered in rare cases of persistent refractory hypotension, although it is seldom necessary.

Maintaining a mean arterial pressure within the 85 to 90 mm Hg range for the initial 7 days is recommended to optimize spinal cord perfusion. Caution should be exercised when using vasopressors, as their vasoconstrictive effects could worsen co-existing injuries.[19][20][21][22] Managing bradycardia in neurogenic shock involves a targeted approach to oppose vagal tone and restore appropriate heart rate. Atropine and glycopyrrolate are administered to counteract excessive vagal stimulation and alleviate bradycardia. This is particularly relevant before procedures such as suctioning that might trigger a vagal response.(A1)

Isoproterenol is utilized when a targeted enhancement of heart rate is required by inducing a pure chronotropic effect. Methylxanthines such as theophylline and aminophylline have been cited in cases of refractory bradycardia.[23] These agents stimulate the central nervous system and can assist in mitigating persistent bradycardia. 

The initial immobilization of the cervical spine is crucial to prevent further injury. This can be achieved using devices such as the Miami J or Philadelphia collar. Although methylprednisolone and corticosteroids demonstrated efficacy in animal models [24], clinical trials have not yet substantiated their effectiveness. Moreover, steroid use is not recommended for managing spinal cord injury as it increases the risk of complications, including infections.[25] In some cases, surgical intervention may be necessary to alleviate ongoing neural compression and enhance the treatment of neurogenic shock. Notably, symptoms of neurogenic shock have been documented to persist for up to 4 to 5 weeks.(B3)

Refractory cases of neurogenic shock can also be managed using resuscitative endovascular balloon occlusion of the aorta (REBOA). Although primarily used in cases of hemorrhagic shock, REBOA has shown effectiveness in bolstering central aortic pressure crucial for cerebral, coronary, and spinal cord perfusion in patients with neurogenic shock.[1]

Neurogenic shock remains a diagnosis of exclusion in trauma patients. As outlined by the ATLS protocol, it is imperative to initially exclude hemorrhagic shock, which is the common cause of hypotension.[13] After the meticulous management of hemorrhagic shock, the evaluation for the potential presence of neurogenic shock should be initiated.[26]

Differential Diagnosis

Shock signifies the inadequate perfusion of vital organ tissues. The distinct causes of shock can be discerned by their influence on essential hemodynamic parameters, including systemic vascular resistance, cardiac output, and volume status. 

Hypovolemic Shock

Hypovolemic shock predominantly results from acute blood loss (hemorrhagic) or volume depletion from diarrhea, vomiting, burns, or dehydration. Hemodynamically, hypovolemic shock is characterized by reduced cardiac output, volume status, and increased systemic vascular resistance. Clinical presentation encompasses hypotension, tachycardia, and the appearance of pale and cool skin. 

Cardiogenic Shock

Cardiogenic shock can develop after acute myocardial infarction, cardiac tamponade, tension pneumothorax, or a massive pulmonary embolus. Hemodynamically, cardiogenic shock is characterized by a decline in cardiac output, a surge in systemic vascular resistance, and an elevation in volume status, which is measured by jugular venous or pulmonary capillary wedge pressure. Clinical manifestations of cardiogenic shock include hypotension, tachycardia, and the appearance of pale and cool skin. 

Septic Shock

Septic shock is characterized by hypotension induced by sepsis, leading to hypoperfusion. Hemodynamically, septic shock is identified by decreased cardiac output and volume status and increased systemic vascular resistance. Clinical signs of septic shock comprise hypotension, tachycardia, fever, or hypothermia, often accompanied by a suspected site of infection.  

Specific hemodynamic features distinguish neurogenic shock, including a decline in cardiac output, systemic vascular resistance, and volume status. Notably, it is crucial to differentiate between spinal shock and traditional shock. Spinal shock refers to the acute loss of motor, sensory, and reflex functions below the injury level. This phenomenon is closely related to neurogenic shock, which differs from conventional shock characterized by inadequate tissue perfusion.  

Prognosis

The prognosis in cases of neurogenic shock is influenced by various factors, with 2 critical elements mentioned below.

Spinal cord injury: As assessed through the American Spinal Injury Association (ASIA) scale, the severity of spinal cord injury significantly influences the overall prognosis.

Treatment response: The individual's response to treatment and management strategies significantly influences the expected outcomes.

In addition to the factors mentioned above, an individual's prognosis in cases of neurogenic shock can be further influenced by several significant factors, as provided below.

Neurological deficits: The existence and severity of neurological deficits at the initial assessment can have a pivotal role in shaping the overall prognosis.

Age: Age is a determining factor, as younger patients often display a more favorable prognostic potential for recovery and adaptation.

Concomitant organ injuries: Concurrent injuries affecting other organs can impact overall recovery and prognosis.

Glasgow Coma Scale (GCS) score: The GCS score, which assesses consciousness and neurological function, contributes to understanding the patient's potential for recovery.

Individuals who have experienced neurogenic shock have a unique prognostic landscape determined by these factors.[19][27]

Complications

Neurogenic shock can lead to complications, including prolonged and severe hypotension, which may require vasopressor therapy. This hypotension might persist for an extended period, spanning several weeks following the initial injury. Individuals with neurogenic shock may encounter additional complications, such as autonomic dysreflexia, in the long term.

Autonomic dysreflexia occurs due to the disruption of normal sympathetic spinal cord control resulting from central nervous center interference. This phenomenon was first observed by Anthony Bowlby in 1890 and subsequently described by Guttmann and Whitteridge in 1947.[11] Autonomic dysreflexia presents as an exaggerated and imbalanced sympathetic response to stimuli below the level of spinal cord injury. Contributing factors to this condition include the distention, stimulation, or manipulation of the bladder or bowel.[28]

Patients with spinal cord injuries are at an increased risk of developing deep vein thrombosis (DVT). Research has shown that the preoperative incidence of DVT among patients with spinal fractures is 13.10%.[29] Furthermore, advanced age is correlated with an increased susceptibility to DVT. Other factors contributing to DVT occurrence include venous stasis in the extremities due to muscle paralysis, venodilation, and a hypercoagulable state. 

Patients may experience disruptions in electrolyte balance, including the potential for developing conditions such as hypokalemia. This electrolyte imbalance can be attributed to hypotension and hypovolemia, which can elevate plasma aldosterone levels. Gastrointestinal dysfunction, particularly paralytic ileus, may be associated with spinal cord injuries. This occurrence results from mesenteric vascular dysfunction accompanying the spinal cord injury.[30]