PMV is a rare congenital malformation of the mitral valve that mainly involves the PMs. According to the underlying anatomy, two subtypes of PMV have been described [2, 3], namely, “true” PMV, which is characterized by a single PM that receives all chordae, and PLAMV, which has two PMs with unevenly distributed chordae. PLAMV is further divided into three grades according to the degree of uneven distribution of the chordae. The most common form is Grade II, in which a few short chordae are attached to the elongated PM . Grade Ш is the most severe form . The present case was found to have a grade Ш PLAMV with two PMs, one of which received all the chordae, while the other PM was attached by its lateral side to the wall of the left ventricle with the tip located near the mitral valve annulus. Disturbed delamination of the anterior or posterior part of the trabecular ridge from the left ventricular wall, combined with underdevelopment of chordae, seems to be the cause of PLAMV . And “ture” PMV develops when the connection between the posterior and anterior part of the ridge condenses to form a single PM . Although their anatomy and embryonic development are different, there is no obvious difference in the clinical manifestations, treatment, or prognosis between PLAMV and “true” PMV [2, 5]. Therefore, most clinical researchers have considered them to be one category.
PMV usually results in congenital MS and is often associated with multiple levels of left heart obstruction, known as Shone’s syndrome. There is limited information on fetal PMV in the literature, but there have been some isolated reports of MS as a manifestation of Shone’s syndrome [1, 6]. However, congenital MS is a morphologically heterogenous lesion that is classically divided into four anatomic types : typical congenital MS, hypoplastic mitral valve, supramitral valve ring, and PMV. The different types of congenital MS require different treatment strategies and have different prognoses. Although the long-term functional outcome in children with congenital MS is satisfactory, surgical procedures for PMV are more complicated, and repeat repair may be needed [8, 9]. It is important to be able to identify the anatomic type of MS for prenatal counseling of parents with regard to treatment strategies and the prognosis.
Prenatal diagnosis of PMV by ultrasound is challenging, and the condition is usually diagnosed after birth. Fetal MS has often been described, but there are few reports on PMV [1, 6]. A restrictive opening of the mitral valve is an important clue for fetal MS. Further differential diagnosis of MS has great value in terms of prenatal consultation, but is difficult. In both children and adults, the echocardiographic characteristics of PMV include a single PM at the mid-papillary level and parachute leaflets at the basal level in the left ventricular short-axis view, doming of the elongated chords in diastole, and an enlarged left atrium in the four-chamber view [9, 10]. The short-axis view of the left ventricle and the four-chamber view are important when evaluating anomalies of the mitral valve. However, the anatomy of the PMs could not be accurately identified in the fetus . In our case, fetal echocardiography failed to demonstrate the anterolateral PM either in the short-axis view of the left ventricle or in the four-chamber view. Therefore, diagnosis of PMV by observing the anatomy of the PMs is unreliable. However, we found that observing the morphology and movement of the valve leaflets and chordae, especially in the four-chamber view, was useful for prenatal diagnosis of PMV. In PMV, all the chordae are attached to one single PM with marked reduction in the interchordal spaces, which leads to specific limitation of movement of the leaflets, especially at the tips. On ultrasound images in the four-chamber view, parts of the leaflets and converged chordae are shaped like an arched bridge, and the overall mitral valve orifice is shaped like the symbol “Ω” in diastole, which is a key diagnostic clue for PMV. The mitral valve “Ω” sign can also be used to differentiate PMV from typical congenital MS and hypoplastic mitral valve. Ultrasonographic features of typical congenital MS is characterized by thickened and rolled leaflets, thickened and shortened chords, and restrictive opening of the mitral valve . In a hypoplastic mitral valve, all components of the mitral valve are miniature versions of those of a normal valve .