Pes cavus pathogenesis in Charcot–Marie–Tooth disease type 1A
Received March 15, 2006. Accepted April 4, 2006.
Sir, We read with great interest the article by Gallardo et al. (2006)
in which the evaluation of leg and foot muscles using manual muscle testing and MRI was conducted in a selection of 11 patients aged 8–61 years with Charcot–Marie–Tooth disease type 1A duplication (CMT-1A). We commend the authors for evaluating the intrinsic foot muscles using MRI as this technique has the potential to contribute greatly to the body of knowledge concerning the pathogenesis of pes cavus in CMT-1A. Indeed, several authors have hypothesized from studies of manual muscle testing (Sabir and Lyttle, 1983; Mann and Hsu, 1992) and CT (Price et al., 1993) that weakness of the intrinsic foot muscles may contribute to the development of pes cavus in CMT-1A. However, the pathogenesis of pes cavus seen in CMT-1A has also been linked to an imbalance of the long muscles of the leg and foot. The hypothetical model proposed by various authors describes a relationship whereby a weak peroneus brevis is overpowered by a relatively stronger tibialis posterior causing adduction of the forefoot and inversion of the hindfoot (Mann and Missirian, 1988; Holmes and Hansen, 1993), in addition to a weak tibialis anterior being overpowered by a stronger peroneus longus causing plantarflexion of the first metatarsal and anterior pes cavus (Tynan et al., 1992). Clearly, our understanding of the mechanism is incomplete.
In the results of the study by Gallardo et al. (2006), we found it surprising that while all CMT-1A cases exhibited a pes cavus deformity, 55% of the sample demonstrated normal leg and foot muscle power using the Medical Research Council (MRC) scale. This finding contradicts numerous studies showing that patients with CMT-1A demonstrate leg and foot weakness in 78–93% of cases using the MRC scale (Krajewski et al., 2000) as well as other strength measures (Birouk et al., 1997; Verhamme et al., 2004).
Furthermore, we find it puzzling that despite 55% of the CMT-1A cases being recorded as having normal muscle power for ankle dorsiflexion, 82% (nine cases) found it ‘difficult’ or ‘impossible’ to heel walk. One explanation cited by the authors may be that heel walking was not possible due to shortening of the Achilles tendon and associated limited ankle dorsiflexion mobility. However, all cases were reported as demonstrating 
+5° of anatomical ankle dorsiflexion range of motion, a measure within normal limits (Moseley et al., 2001). Furthermore, 36% (four cases) had difficulty tiptoe walking, despite 75% (three of four cases) of these patients demonstrating normal muscle power for ankle plantarflexion. The authors suggest that the presence of intrinsic foot muscle atrophy, pes cavus, inversion of the calcaneus, adduction of the forefoot, clawing of toes and presence of callosity may explain an inability to tiptoe walk. However, there is no evidence or sound mechanism provided by the authors to support this unexpected result.
It is unclear why Gallardo et al. (2006) recorded normal muscle strength in more than half of their patients with CMT-1A, a disease known to cause almost universal weakness of the leg and foot muscles (Thomas et al., 1997). In our experience of numerous patients with pes cavus caused by CMT-1A, they almost invariably have weakness of the peronei and tibialis anterior muscles demonstrable by clinical and instrumented testing. We suggest that the method used to test muscle strength by Gallardo et al. (2006) may have been inaccurate. While muscle strength is commonly measured subjectively using the MRC scale (O'Brien, 2000), more recent techniques such as hand-held dynamometry are regarded as more precise, sensitive and objective (Escolar et al., 2001). Recently, hand-held dynamometry has been used to demonstrate significant lower limb weakness as well as imbalance of foot and ankle musculature in CMT-1A (Kilmer et al., 2000; Verhamme et al., 2004; Burns et al., 2005). Perhaps in addition to state-of-the-art MRI technology the authors would consider more precise and reliable means of measuring muscle strength in the future.
While weakness of the intrinsic foot muscles is a widely accepted pathological finding in CMT-1A, its role in the pathogenesis of pes cavus development is unclear. Across the various studies, broadly at least, there is support for a model in which some weakness occurs in all muscle groups, particularly in the intrinsic muscles of the foot and the anterolateral compartments of the leg. Gallardo et al. (2006) conclude that their ‘findings help to clarify the pathogenesis of pes cavus’ in CMT-1A. However, their cross-sectional study in a selection of just 11 patients and no control group using subjective and possibly imprecise manual muscle testing does not really clarify the pathogenesis of pes cavus in patients with CMT-1A. Instead longitudinal studies of the natural history of CMT-1A are needed to understand the pathogenesis of pes cavus and develop new treatments for this disabling foot deformity.
Institute for Neuromuscular Research, The Children's Hospital at Westmead Sydney, NSW, Australia
Correspondence to: Joshua Burns E-mail: joshuab2{at}chw.edu.au
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