Frostbite is a localized cold thermal injury, as a result of tissue exposure to temperatures below freezing point (typically −0.55°C, but can occur as high as 2°C) for a prolonged period of time [1, 2]. Typically involves young men between 30 to 50 years old firstly cited in the military and in countries with extreme temperatures. Later in rural areas, it has been described among homeless, malnutrition,smoking, alcoholics, substance abusers, psychiatric patients, and recently in those individuals who practice winter sports [1,2,3]. Systemic diseases such as peripheral vascular disease like the Raynaud phenomenon, vasculitis, diabetes mellitus, heart failure are predisposed factors for individuals to be affected by frostbite [3,4]. Cold injuries affect extremities in the greatest proportion: hands and feet predominantly with ears, cheek, nose and genitalia reported in a fewer rate [2,3]. The severity of frostbite injury depends on environmental temperature, the wind chill factor and the length of exposure being the duration of exposure the utmost important factor [2, 5].
The spectrum of injury is wide, thus frostbite injuries may have devastating effects with the possibility of losing part or whole extremities. Therefore, it is paramount for medical staff to have the proper knowledge of the benefits of imaging techniques to provide a certain diagnostic approach. Magnetic resonance and bone scintigraphy are superior to X-rays and allow early intervention in cases of severe frostbite, thus preventing secondary infection [6,7].
Multiphase technetium-99m-methylenediphosphonate (99mTc-MDP) bone scintigraphy may hasten clinical management of frostbite injuries as it furnish high and precise clinical-imaging correlation by determining the extent of injury and can accurately predict the level of amputation if required (figure 4) [2,6].
Pathogenesis
Basically, frostbite injuries occur due to the formation of ice crystals intracellularly and extracellularly which provokes microcirculatory dysfunction, osmotic gradient alterations and inflammatory reactions [2, 8]. Intracellular ice crystal formation stimulates a cascade of deleterious effects resulting in the production of prostaglandins, cytokines and oxygen free radicals as an intent to remove ice crystals and preserve cells. Notwithstanding, these products impoverish the cell environment and produce detrimental effects, such as hypoxia and microthrombosis, creating a vicious circle where the production of more substances is stimulated, resulting in cell death [2,9].
Imaging
Radiology
Radiography is commonly the first diagnostic technique available and it may be used to examine the underlying bone, however lacking pivotal information as it is the vasculature examination. X-rays may identify radiopaque foreign bodies or occult fractures [3]. Radiographic findings include nonspecific changes. In an initial stage, soft tissue changes as oedema or marked soft-tissue swelling with tissue distortion [3]. In late stages, radiography may show osteopenia and osteoporosis [3, 8].
Magnetic resonance (MR) angiography
MR may be used in the initial evaluation of frostbite injuries. On T1 and T2-weighted sequences, show increased signal intensity in all muscle groups in initial stages and signal absent in later stages. MR angiography is a noninvasive tool to assess vasculature of occluded vessels and has a higher soft-tissue resolution. MR angiography may have limited clinical utility in helping to define persistently occluded vessels and demarcate the soft-tissue ischemic border in patients who present more than 24 hours after injury [3, 7].
Bone scintigraphy
Multiphase technetium-99m-methylenediphosphonate (99mTc-MDP) bone scintigraphy assesses tissue viability in an effort to allow early debridement of soft tissue and early coverage of ischaemic bony structures with an almost unerring accuracy: specificity 99%, sensitivity 96% [5,6] .
Bone scintigraphy is commonly used with bisphosphonates labeled with 99mTc, which is a compound that binds to hydroxyapatite crystals present in the surface of viable bone. Another key point is that uptake depends on vascularization and the activity of new bone formation or blast activity. Consequently, non-viable areas of bone due to freezing injuries are visualized as photopenic or cold areas with little or no vascularization in early blood flow and pool images.
Bone scintigraphy is a non-invasive technique with low radiation exposure and easy access. Thus, it is the elective diagnosis procedure particularly given that contraindications and side-effects are practically non-existent or relative (pregnancy).
Ideally, an initial bone scan is appropriate within the 2 to 4 day after exposure albeit, the transfer of the patient from the site of exposure may be delayed particularly in mountaineers. A second bone scan should be performed between the day 7 to 10 to follow the evolution of areas of low tracer uptake [3, 6].
Imaging protocol includes a three phase scan. Firstly, an early tissue phase or planar blood flow phase immediately after intravenous administration of 20-30 mCi 99mTc-MDP is useful to identify the vasculature and to delimit the level of vascularization (figure 2). Secondly, planar soft-tissue phase images (known as blood pool phase images) are acquired in 3-5-minute intervals for up to 10 minutes after tracer injection which allows the evaluation of soft tissues (figure 3). Lastly, bone phase is taken approximately between 2-4 hours later which enables bone perfusion and viability. Particularly in our center, we have decided to extend the last phase up to 4-6 hs to properly assess bone viability (figure 3).
Bone scans can be used not only to predict tissue activity, but also to assess the response of the lesion to treatment. Reduced perfusion (not absent) indicates the possibility that viable tissue exists and needs specific therapy such as thrombolytic treatment [8].On the contrary, the possibility of amputation is high when the tissue uptakes are absent (photopenic or cold areas) [3, 8].