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Tumors of the nail unit may be difficult to diagnose because of the anatomic peculiarity of this organ: the growth and appearance of the tumor may be modified by the screening effect due to the nail plate. Lesions originating deeply in the posterior cul-de-sac are covered by the proximal nail fold and thus may only express by a nail dystrophy such as a longitudinal groove. Therefore, any suspicious lesion of the nail unit should undergo medical imaging and biopsy. Standard X-rays remain essential for the study of the distal phalanx in cases suggestive of bone involvement (fracture, subungual exostosis, i.e.) MRI is well known for its high contrast (500 times more than CT scan) of soft tissues. Its spatial resolution is however poorer than CT scan (0.75 mm for CT scan vs. 1.15 mm for MRI). Use of MRI of the nail apparatus started in the early nineties with the available of small surface coils. When associated with most powerful whole-body magnetic resonance (> 20mT/m), they provide a clinically suitable image of the nail unit. Routine examination shows in T1 a morphologic and anatomical study of the region and T2 corresponds to the characterization of the tissue using the signals emitted by the tumor. These are always compared to the dermis, a reference tissue chosen for its homogeneity of its signal. T1 sequences after intravenous injection of gadolinium may be valuable in some instances. Remarkable experimental work has been performed by J.L. Drape these last years. In our University, we have been routinely using MRI for more than three years. The indications of MRI for pathologies of the nail unit are still under investigation and we performed a lot of MRI in order to define its limits.
Glomas tumor
Glomas tumor appears as the main indication for MRI of the nail unit. It shows a high signal on T2 weighted images and a strong enhancement after gadolinium injection. However, MRI may show some slight differences due to the histological nature of the tumor, as already described by Masson in 1924 (vascular, cellular or mucoid nature). A cortical erosion is very often depicted (82%). MRI may help in detecting the multiple glomus tumors in the same fingertip.
Myxoid cysts
Myxoid cysts are mostly clinically obvious. MRI of these lesions may however demonstrate the existence of a peduncle between the cyst and the interphalangeal joint in about 80% of cases; it may also highlight satellite cysts that may be responsible for the high recurrence rate after surgery, especially in the sub matricial location. Myxoid cysts show a low signal on T1 weighted images and a very high signal on T2 weighted images. Injection of gadolinium shows early faint peripheral enhancement that is different to that of the glomus tumor.
Implantation epidermoid cysts
Implantation epidermoid cysts may develop after trauma or surgery. MRI shows a regular mass with heterogeneous content in T1 weighted and T2 weighted imaged and heterogeneous enhancement after gadolinium injection. Bone erosion is very common and is highly detected with MRI even when subtle.
Onychomatricoma
Onychomatricoma is rare matrix tumor that was first described by Baran in 1992. The filamentous extensions of the tumor present a high signal on T2 weighted images due to a mucoid stroma.
Exostoses
Exostoses are by far the most frequent benign tumor of the extremities. X-rays easily demonstrate the lesion. MRI may however show the cartilaginous cap of the lesion that may help in differentiating exostosis from osteochondroma.
Osteoid osteoma
Osteoid osteoma is rare on the distal phalanx. MRI is as accurate as CT scan to detect the tiny nidus if contiguous 1mm slices are acquired.
It seems that this imaging technique is mostly valuable for vascular lesions and liquid cysts, such as glomus tumors and ganglion cysts, as advocated by Drape. We found it very helpful in the differential diagnosis of erythronychia (glomus tumors, subungual filamentous tumor i.e.). In case of solid lesion, such as epithelial tumors, MRI may anyway allow the accurate location of the lesion and help the surgeon when planning its removal.
References
1. Goettmann, S, Drape J-L, Idy-Peretti I.., Bittoun J., Thelen P., Arrive L., Belaich S. Magnetic Resonance Imaging; a new Tool in the Diagnosis of Tumors of the Nail Apparatus, Br J Dermatol, 1994; 130:701-710.
2. Drape J-L., Idy-Peretti I., Goettmann S., Guerin-Surville H. et al. Standard and High Resolution MR Imaging of Glomus Tumors of Toes and Fingertips. J Am Acad Dermatol, 1996; 35:550-555.
3. Drape J-L, Idy-Peretti I., Goettmann S., Salon A., Abimelec et al. MR Imaging of digital Mucoid Cysts. Radiology, 1996;200:531-536.
© 2001 Dermatology Online Journal
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