RICERCA ITALIANA     

Clinical-radiological correlation in patients with inherited miopathies.

Università Cattolica del Sacro Cuore

Abstract

The recent advances achieved in the knowledge of muscle inherited disorders highlighted the clinical, histopathological and genetical eterogeneity in the various forms. It is therefore important to identify other markers that can help in the diagnostic workup. Recently several studies suggested that muscle imaging techniques may help to identify specific patterns of involvement in patients with genetically defined forms of muscle inherited disorders. The technique mainly used is the muscle MRI, but studies on muscle ultrasound and CT are available. Muscle ultrasound has the advantage of being less expensive and easily available, but the disadvantage of being operator-dependent but has not been sistematicallyused to evaluate possible patterns of muscle involvement in neuromuscular diseases. Muscle CT , that has proved to be able to detect patterns of muscle involvement, has the disadvantage of using ionising radiations.
Magnetic Resonance Imaging (MRI) is much more expensive than ultrasound and CT but has the advantage of not using ionising radiations. MRI provides excellent information on normal and abnormal signal in individual muscles and has been used to recognise specific patterns of muscle involvement in various forms of hereditary neuromuscular disorders. The aim of our study is to evaluate if and how the use of multiple sequences such as T1, T2 e STIRS may help not only to identify abnormal signal but also to differenziate between various components such as fat or fibrotic tissue in the diseased muscle.

We also aim to establish sensibility and sensitivity of msucle ultrasound in identifying patterns of muscle involvement as observed on muscle MRI.

Another area of potential diagnostic interest is the use of Magnetic Resonance Spectroscopy (MRS). This technique can considerably increase our knowledge on the mechanisms underlying muscle energy metabolism in the different forms of neuromuscular disorders.

We will try to establish how this integrated approach may help to obtain more information on phenotype-genotype correlation in genetically distinct forms of inherited disorders, providing some help in targeting appropriate genetic investigations.

to identify new phenotypes with distinct clinical, imaging and istopathological features that are not associated to known genes and may constitute the basis for seaching new genes. <<<

Principal Investigator

Eugenio Maria Mercuri Università Cattolica del Sacro Cuore

Research Objectives

The aim of the study is to develop an integrated approach, including clinical, hystopathological, genetic and imaging techniques, to be used in the diagnosis of inherited neuromuscular disorders.
The research will try to establish not only the value of each technique, but also if and how the combination of the information achieved using the individual techniques may help to identify and classify specific phenotypes.

More specifically, the aims of the research will be:

- to recognise specific patterns of muscle involvement using a simple protocol (transverse T1 weighted sequences of lower limbs), easily applicable to children or non collaborant patients, to be used in patients with various forms of inherited neuromuscular disorders. These will include both genetically distinct forms of ineherited disorders (i.e. Emery-Dreifuss muscular dystrophy, congenital muscular dystrophies, distrophinopathies etc) and forms for which the genetic defect is not yet known.

- to use, in collaborative patients, a more detailed protocol, including other sequences such as T2 and STIRS, in an attempt to gain more information on the areas of abnormal signal observed on T1 sequences and more generally, on the mechanisms underlying different forms of neuromuscular disorders.

- to compare muscle MRI findings with standardised measures of muscle function and strength in order to establish the relationship between the severity of involvement of individual muscles or groups of muscles on MRI and their functional activity and strength. This will allow to establish whether the abnormal signal observed on muscle MRI is always associated with abnormal function.

- to evaluate patterns of energy production in the skeletal muscle in both genetically identified forms of neuromuscular disorders and in those in which the genetic defect has not been yet identified.

- to establish sensibility and sensitivity of msucle ultrasound in identifying patterns of muscle involvement as observed on muscle MRI. Muscle MRI and ultrasound findings obtained in the same day will be analysed separately and subsequentely correlatedin order to establish the value of ultrasound scans, that can be more easily used as a routine investigation, as a diagnostic tool for the identification of specific patterns in genetically distinct forms of inherited disorders.

Finally, we will try to establish how this integrated approach may help to obtain more information on phenotype-genotype correlation in genetically distinct forms of inherited disorders, providing some help in targeting appropriate genetic investigations.

to identify new phenotypes with distinct clinical, imaging and istopathological features that are not associated to known genes and may constitute the basis for seaching new genes. <<<

Timescale

24 months

National and international background

The field of inherited muscle disorders is becoming increasingly complex: the level of genetic heterogeneity is far greater than initially appreciated. If we consider, for example, congenital muscular dystrophies (CMD) that has been considered as a single entity for many years, is now recognised as an heterogeneous group of conditions. There are already 9 genetically distinct forms of CMDs and an at least equal number of distinct phenotypes for which the genetic defect has not yet been established. Similar heterogeneity has also been found in other groups of disorders such as limb girdle muscular dystrophies or in congenital myopathies.
The possibility to reach a precise diagnosis in these patients is not always easy as genetically different conditions often share similar clinical and histopathological phenotypes and not all the forms have a specific hystopathological marker on muscle biopsy. As the numbers of proteins and genes that can be potentially screened in these forms are numerous and expanding, additional useful markers for selecting the appropriate genetic and biochemical investigations are required by the clinician. Several studies have reported the diagnostic value of muscle imaging in identifying specific patterns of muscle involvement in patients with distinct forms of muscle disorders, including various forms of muscular dystrophies or of congenital myopathies. The patterns observed appear to be consistent among patients affected by the same form and are different from those observed in patients affected by other conditions who may share a similar phenotype. The imaging techniques used so far include muscle ultrasound, CT and MRI.

In the early 80’s Heckmatt and Dubowitz reported the value of muscle ultrasound to detect the presence of muscle involvement , identifying different patterns of involvement in patients with primary muscle disorders and in those with neurogenic disorders such as spinal muscular atrophy (Heckmatt et al. 1980, 1982). The typical changes observed in primary myopathies include increased echogenicity in the individual muscles, while an increased ratio between subcutaneous tissue and muscle belly characterises denervation atrophy. The same authors subsequently reported the possibility to guide muscle biopsies by selecting relatively spared muscles (Heckmatt and Dubowitz 1985), and suggested that in patients with inherited disorders not all the muscles were equally affected. Muscle ultrasound has subsequently been used in other studies and has proved to be a valuable tool in the routine assessment to identify muscle involvement but its diagnostic value in identifying genetically distinct forms has not been systematically investigated. This may be partly due to the fact that although ultrasound scans are low cost and easily accessible, the technique is highly operator-dependent. Another limitation is that in patients with severe muscle involvement in whom the muscles closer to the probe are severely affected, it is not always easy to visualize the involvement in the deeper muscles.
On the contrary various studies used CT in the diagnostic workup with good results, but the use of this technique has been mainly limited by the use of ionizing radiations (Liu et al. 1993b).
The advent of muscle MRI has however almost completely replaced the use of CT. The main advantage of MRI compared to CT is not only the lack of ionizing radiations but also the ability to perform multiplanar scanning, which is particularly useful for those patients with severe limb contractures who can not lie in the correct position during examination.
Furthermore, comparative studies using both CT and MRI techniques have shown that MRI has a higher sensitivity than CT for identifying early fatty replacement in muscles (Schedel et al. 1992) and also provides better anatomical details. Several papers have recently reported how a brief protocol, using T1 transverse images, can help to identify specific patterns of msucle involvement in genetically distinct forms of muscle disorders (Mercuri et al. 2002a, Taylor et al. 2001). This information has been thought to be useful to select, in association with clinical and histopathological findings, the most appropriate genetic investigations. It has been shown that patients with genetically different forms of muscular dystrophies but wit similar phenotypes, such as those affected by the dominant and the X-linked form of Emery Dreifuss muscular dystrophy, show different patterns of muscle involvement on muscle MRI (Mercuri 2002c). Similar findings have been found in other dystrophies, e.g. in the forms of CMD with rigid spine and early respiratory involvement (Ullrich CMD and RSMD1) with overlapping phenotype but distinct muscle MRI patterns, in distrophinopathies, in limb girdle muscular distrophies etc. (Mercuri et al. 2002b, d, 2003, 2005a). Further studies have been pubblished highlighting the role of MRI in describing the muscle involvement in dystrophinopathies, limb girdle muscular dystrophy and myotonic dystrophy (Lodi et al. 1997, Fisher et al. 2005, Liu et al. 1993b, Mercuri et al. 2005b, Nago et al. 1991, Schedel et al. 1992, Schneider-Gold et al. 2004, Lamminen et al. 1990).
All the studies published in the last few years are strongly supporting the value of muscle MRI, but there are several aspects that still need to be further explored. No systematic study has been performed to correlate the sensitivity and the value of different MRI sequences in providing more detailed information on muscle damage. Muscle biopsy studies have reported a variable presence of oedema or other inflammatory signs and of fibrotic and adipose tissue in different forms of 'chronic' miopathies and muscular dystrophies. This information is however limited to the muscle sampled for muscle biopsy and a detailed protocol, using T2 and STIRS sequences may help to identify the different components in all the muscles examined by imaging. This protocol is routinely used in adults with inflammatory miopathies but it has not systematically applied to inherited disorders.

Similarly only few studies have attempted to correlate data obtained with muscle MRI to that obtained with other muscle imaging techniques (Lodi et al. 1997, Schedel et al. 1992, Schneider-Gold et al. 2004, Cea et al. 2002) and to explore the use of muscle ultrasound in the diagnosis of muscle diseases (Bonnemann et al. 2003, Heckmatt et al. 1982, Topaloglu et al. 1992, Zuberi et al. 1999), technique that has the advantage of being more easily available and much less expensive.

Another area of potential diagnostic interest is the use of Magnetic Resonance Spectroscopy (MRS). Proton MRS (1H MRS) allows to study the relative concentration of the fibro-adipose tissue, differentiating the intramyocellular (IMCL) from the extramyocellular lipids (EMCL). Phosphorus MRS (31P MRS),in contrast, allows to investigate in vivo the skeletal muscle energy metabolism. More specifically, 31P MRS helps to evaluate the energetic status of skeletal muscle and its possible alterations, measuring the concentrations of ATP, Phosphocreatine (PCr), inorganic Phosphate (Pi) and intracellular pH. It is therefore possible to evaluate the functionality of different metabolic pathways such as oxidative phosphorilation, glycolysis, glycogenolysis, phosphate transport and proton efflux. Moreover, a general and indirect index of the energetic status can also be obtained by measuring the pH level, which varies in relation to the changes of muscle energy request. This technique can therefore increase our knowledge on the mechanisms underlying muscle energy metabolism in the different forms of neuromuscular disorders. So far this technique has been mainly used in metabolic and inflammatory myopathies (Argov et al. 2000, Bendahan et al. 2002, Cea et al. 2002, Trenell et al. 2006). Few studies focused on the use of spectroscopy in patients with Duchenne, Becker and limb girdle muscular dystrophies (Felber et al. 2000, Ikehira et al. 1995, Kemp et al. 1993, Lodi et al. 1997, Younkin et al. 1987), whereas it has not been systematically used in other forms of neuromuscular disorders whose genetic background has been recently described. <<<