Echinococcosis (“Cyst in the cyst pattern”) [Dietrich CF], Case of the month 


The increased knowledge of the natural course of cystic echinococcosis has also improved our understanding of typical imaging features (“cysts in the cyst pattern”) which can be nicely shown by imaging modalities, e.g., ultrasound. In the very early stages of echinococcosis without calcifications ultrasound can depict the specific intralesional morphology in detail which might be of advantage.
Gharbi et al first introduced 1981 the widely used and most cited classification of hydatic disease which has been modified many times. Gharbi type I cysts consist of pure fluid; Gharbi type II have a fluid collection with a split wall; Gharbi type III cysts contain daughter cysts (with or without degenerated solid material); Gharbi type IV have a heterogeneous echo pattern; and Gharbi type V have a calcified wall. It is of importance that lesions may show different stages of hydatid evolution.
Ultrasound and contrast enhanced ultrasound (CEUS) is helpful in recognising echinococcosis in all stages. Most useful is the combination of morphological criteria and the biological behaviour (fertile cysts with viable protoscoleces [Gharbi type I, II], transitional phase [Gharbi type III] and inactive cysts which have lost their fertility [Gharbi type IV, V]). Characteristics on ultrasound that are suggestive of an inactive lesion include a collapsing, flattened elliptical cyst (corresponds to low pressure within the cyst), detachment of the germinal layer from the cyst wall ("water lily sign"), coarse echoes within the cyst, and calcification of the cyst wall.
The use of a combined classification including imaging criteria and biological evidence of viable parasites will enable clinicians to perform the correct clinical procedures for the different cyst types. 

Type I: The most common type I lesion (50 – 80 %) represents an anechoic smooth, round pure fluid collection without hydatid sand and septa. containing usually fertile cysts with viable protoscoleces, which can be difficult to distinguish from a benign cyst. The roundish lesion with well defined borders is most importantly characterised by an irregular localised thickened wall (initial stage of splitting the wall) which should be carefully sought by high frequency and therefore high resolution probes (7 – 15 MHz) and contrast enhanced ultrasound which delineates the nodular appearance very early in the course of the disease. 

Type II: Splitting of the wall (infoldings of the inner cyst wall resulting in floating membrane, so-called water lily sign) is typically for type II fluid filled lesions also containing fertile cysts and seems to be the most important and pathognomonic sign of echinococcosis. It is of interest that splitting of the wall to the outer margin (“outfoldings”) implies the typical fine nodular appearance of contrast enhancement. When the liver cyst contains membranes and “sand”, mixed echoes will appear that can be confused with an neoplasm or abscess. The term "hydatid sand," reflects a complex image which consists predominantly of parts of protoscolices (hooklets and scolexes). This finding of mobile “sand” may be overt and visible turning patient's position, e.g. into the standing position. 

Type III: When daughter cysts are present, characteristic internal septation results. Type III is characterised by septa resulting in a honeycomb appearance with infolding membranes [Figure and Video]. This stage has been described as transitional where the integrity of the cyst has been compromised either by the host or by medical treatment. The “cyst in the cyst sign” by separation of the hydatid membrane from the wall, “hydatid sand” in combination with fine nodular appearance of contrast enhancement are pathognomonic of echinococcosis. Neoplasia can be ruled out by exclusion of contrast enhancement within the lesion. In the very early stages of echinococcosis without calcifications ultrasound can depict the specific intralesional morphology in detail much earlier than any other imaging modality. 

Type IV: Type IV lesions are characterised by a heterogenous echo pattern. The echogenicity of the lesion might be more hypoechoic but can also be hyperechoic due to regressive changes. This stage is more unspecific compared to the pathognomonic II and III stages.
Type V: The type V pattern reflects a solid heterogeneous mass which is difficult to differentiate from tumours. An identifiable thickened hyperechoic calcified wall suggests echinococcal cyst. Cysts with a calcified rim may have typically an "eggshell" appearance. Type IV and V lesions represent inactive cysts with degeneration which have lost their fertility. Calcification, which usually requires five to ten years to develop, occurs most commonly with hepatic cysts. Calcifications in pulmonary or bone cysts are more rarely encountered. Total calcification of the cyst wall suggests that the cyst is nonviable.
References:
· Dietrich CF, Mueller G, Beyer-Enke S. Cysts in the cyst pattern. Z Gastroenterol 2009; 47(12):1203-1207.

Figure 1 Echinococcosis. Echinococcosis stage in this patient is characterised by multiloculated cysts in a honeycomb appearance with infolding membranes. Calcifications are difficult to recognise.

Video 1 Echinococcosis. Contrast enhanced ultrasound did not show any enhancement within the cyst.