ARTERIOVENOUS MALFORMATIONS

AVMs consist of direct arterial-to-venous connections without intervening capillaries; they occur in the cerebral hemispheres, brainstem, and spinal cord. Functional neural tissue does not reside within the lesion. Anomalies range from simple arteriovenous fistulas to complex tangled channels connecting enlarged feeding arteries to draining veins. AVMs are arguably the most important vascular anomaly in the nervous system in children. They are relatively common and usually require treatment, given the risk of hemorrhage (associated with a 25% risk of death with each bleeding event; they can also cause other symptoms, including seizure, focal neurologic deficits (ischemic “steal” phenomenon), and headache.

SURGICAL INDICATIONS

The treatment goal is complete removal/obliteration of the lesion. The decision to operate on an AVM is based on several factors:

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  Eloquence of cortical location (speech, motor function, and sensation)

  
  
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  Pattern of venous drainage

  
  
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  Size

  
  
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  Associated aneurysms

  
  
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  Recent hemorrhage

  
  
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  Clinical deterioration

  
  
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  Risk of complication from other modalities of therapy (for example, radiation injury to the developing brain)

Several of these factors are combined in the Spetzler-Martin AVM grading scale that incorporates eloquence of location, pattern of venous drainage, and size and is considered predictive of outcome from surgical management. The Spetzler-Martin grade helps to predict surgical risk. If a low-grade lesion (grades 1 to 3) is present, surgery should be considered. This includes incidental, asymptomatic lesions in select cases; recent data support this more proactive approach in younger patients. Higher-grade lesions (grades 4 and 5) often benefit from a multidisciplinary approach and may be considered for radiotherapy.

SURGICAL MANAGEMENT

Initial therapeutic maneuvers are dependent on the presentation of the child. For the healthy child or the child who presents with chronic symptoms (for example, seizure or developmental delay), no immediate interventions are often necessary (with the exception of antiepileptic medication if seizures are present). The following steps are warranted for the child who presents with an intracranial hemorrhage. The severity of presentation can vary greatly, and thus treatment must be individually tailored.

STABILIZATION

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  Vascular access: A large-bore (at least 2) intravenous arterial line, bladder catheter (airway intubation if unable to protect airway), and nasogastric tube if intubated can be used.

  
  
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  Blood pressure control: Antihypertensives such as labetalol or nitroprusside (Nipride) can be used to control blood pressure, with a goal of normotension for the child’s age.

  
  
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  Intracranial pressure control: An external ventricular drain can be placed if hydrocephalus is present (NB: The surgeon should avoid overdrainage of CSF to prevent rerupture, often no more than 5 ml at a time) and the head of the bed is elevated.

  
  
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  Avoidance of seizures: Antiepileptic medication can be given if concern exists regarding seizure.

PREOPERATIVE PREPARATION

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  Prepare the operating room: In addition to the steps noted in the “Stabilization” section, the operating room should be notified to prepare for surgery. Equipment should include the operating microscope, multiple suctions, bipolar electrocautery, an array of AVM/aneurysm clips, a craniotome (drill), and a retraction system.

  
  
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  Consider preoperative embolization: The surgeon should consider this in select cases to reduce the risk of bleeding.

  
  
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  Consult with the anesthesiologist: Anesthesia should be consulted, and the appropriate measures should be taken to ensure that multiple, large-bore intravenous access is present and adequate blood products are in the operating room.

  
  
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  Prepare the equipment for microsurgery: If possible, the microscope should be draped and the clips prepared before starting the case, so that quick access can be obtained if unexpected bleeding occurs during opening.

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  Position the patient according to the location of the AVM: Patient positioning is dependent on the location of the AVM and will vary widely. General principles include elevation of the head to maximize venous drainage, avoiding kinking of vessels in the neck, and maximizing access to the lesion by both the primary surgeon and the assistant.

  
  
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  Head fixation: Rigid head fixation is preferable, although in the very young child (under 2 years of age), it may be necessary to use a padded headrest.

SURGICAL APPROACH

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  Varies with location of the AVM: Given the wide variety of AVM sizes and locations, it is impossible to identify any single approach for AVM surgery. Specific approaches should be selected to maximize access to the lesion, avoid the eloquent neurologic cortex (if possible), and afford the surgeon visualization of feeding and draining vessels to permit proximal control of blood flow.

  
  
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  Opening: The approach for any AVM will vary greatly from case to case, but general principles should include a generous opening to allow access and visualization of relevant anatomy and a careful dural opening to avoid injuring AVM vessels (especially draining veins).

RESECTION TECHNIQUE

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  Cauterize feeding arteries first: A primary surgical principle for AVM resection is the obliteration of feeding arteries before occlusion of draining veins, because premature closure of outflow can lead to unexpected AVM rupture with uncontrolled bleeding.

  
  
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  Work in a circumferential pattern, maintaining even depth: AVMs are often wedge or cone shaped, and resection can be performed in a circumferential pattern, staying close to, but not entering, the nidus. The surgeon should try to maintain an even depth of resection around the lesion to avoid getting in a “hole,” and the surgeon must be careful to minimize retraction on draining vessels during dissection.

  
  
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  Watch the surrounding brain: Repeated inspection of the surrounding brain for swelling or bleeding can help the surgeon to prevent complications by early identification of poorly placed retractors or clips.

  
  
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  Clip versus coagulation of feeding vessels: AVM vessels may coagulate poorly, and the surgeon should consider clip application or gentle tamponade (if the bleeding is of small volume) if bipolar electrocautery is not working. Every attempt should be made to avoid operating within the nidus itself.

CLOSURE

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  Inspect the resection bed: Inspection of the operative cavity for residual nidus is important, and perioperative angiography can be a useful adjunct to ensure complete resection.

  
  
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  Check for brain swelling: Evidence of brain swelling at closure may indicate occult bleeding, untreated hydrocephalus, or poorly compensated redistribution of blood flow, which can result in perfusion breakthrough hemorrhage. Causes for swelling should be thoroughly investigated and treated if possible before the patient leaves the operating room.