Fine needle aspiration (FNA) is a type of minimally-invasive tissue sampling. It often uses ultrasound as a targeting instrument, although CT, and even MRI or fluoroscopy could potentially be used, depending on the situation. Imaging guidance is particularly useful when sampling deep or diffuse lesions. Even with a palpable, discrete lesion, imaging allows controlled sampling of different regions within the lesion.
Fine needle aspiration obtains cells for evaluation by a pathologist. The cells are disorganised and no longer maintain the spatial arrangement they originally had in the lesion. In order to preserve the spatial arrangement of the cells, which may be helpful for histopathologic analysis, a core biopsy must be obtained.
FNA is a flexible technique and can sample virtually anything. Applications currently include sampling cells in lesions from:
- non-thyroid neck, e.g. parotid, cervical lymph nodes
- lymph nodes
- gastrointestinal tract
There are few absolute contraindications to FNA, the most likely being uncorrectable coagulopathy or platelet disorder.
Relative contraindications include an inability to lie down, difficulty controlling rate and depth of respiration (more important for thoracic and abdominal biopsies), or anxiety. An inability to reach the lesion with the needle, without traversing important structures is a more or less relative contraindication, depending on the structures traversed, operator experience, and necessity of the biopsy.
- patient consent and preprocedure targeted physical exam
- if ultrasound is being used as the imaging guidance, a preprocedure targeted ultrasound exam is commonly performed
The "fine needle" in an FNA varies depending on the system being biopsied and the nature of the lesion, but is typically a 25 gauge to 27 gauge needle with a stylet. For ultrasound-guided procedures, the transducer may have a needle guide.
The FNA technique will vary depending on the system being targeted and the nature of the lesion. Techniques common to all procedures include:
- local anesthesia with 2% buffered lignocaine
- advancing the fine needle under imaging guidance until the tip is in the intended area of biopsy
- removal of the stylet
- multiple short passes through the lesion (filling the needle with cells)
- removal of the needle with appropriate disposition of the cells (e.g. slide for smear, container for cell block)
- multiple samples are usually obtained in a session
There is no standardised postprocedure care for FNA. Compression of the biopsy site with gauze is common to control minor local bleeding. If the lung is biopsied, a post procedure radiograph may be obtained
Complications are uncommon with appropriate FNA technique, usually limited to bleeding or infection. Bleeding is more of a concern with deeper abdominal aspirations since direct pressure can rarely be applied.
Pneumothorax is a risk of lung biopsies. Either a postprocedure radiograph or ultrasound may be obtained to look for this complication.
FNA success is measured in terms of aspirating a diagnostic amount of tissue and maintaining patient comfort.
Rates of success in aspirating diagnostic tissue depend on multiple factors, not the least of which include the size of the needle being used, the location of the lesion, and the type of lesion being biopsied. Larger bore "fine" needles capture more tissue per aspiration and are more likely to result in diagnostic tissue, but the benefits of minimally-invasive aspiration diminish.
If a fine needle aspiration does not return diagnostic material, it may be repeated. Alternatively, if appropriate, a core biopsy may be attempted.
Maintaining patient comfort depends on multiple factors including: appropriate administration of local anaesthesia, minimising the number of samples, and obtaining enough material to prevent having to call the patient back. The last two are often in conflict with each other.
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