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Journal of the Anatomical Society of India

An Unusual Origin of the Nerve To The Thenar Muscles - A Case Report

Author(s): Udyavar, A.

Vol. 50, No. 1 (2001-01 - 2001-06)

Department of Anatomy, Kasturba Medical College, Manipal. INDIA

For Reprints, request the first author.

Abstract

During the work on the dissertation the following variation was encountered in a cadaver. The branch of the median nerve to the thenar muscles was given off in the lower part of the forearm, about 2 cm above the proximal margin of the flexor ratinaculum. It had a superficial course to the superficial head of the flexor pollicis brevis, which it entered. It then supplied the other thenar muscles. In the literature, although there are reports of the median nerve dividing in the distal forearm, there is no mention of a branch entering the thenar muscles. The variation described is significant in many ways. Such a nerve may be injured in wounds of the wrist region; the possibility of such a nerve is to be remembered in surgery of this region; presence of such a nerve may result in sparing of the thenar muscles in carpal tunnel syndrome.

Key words: Thenar Muscles, Median nerve.

Introduction:

The thenar muscles (muscles of the thumb) are usually innervated by a motor or recurrent branch of the median nerve given off at the level of the distal margin of the flexor retinaculum. There are reports of higher origins of the recurrent branch but the following variation is extremely rare.

Observation:

During the work on my dissertation ["A Study of the Intrinsic Muscles of the Hand and Their Innervation"] the following variation was encountered in the left upper limb of a cadaver (of a middle aged man).

The branch of the median nerve to the thenar muscles was given off in the lower part of the forearm about 2cm proximal to the proximal margin of the flexor retinaculum.

It then had a downward, superficial and slightly lateral, course to the thenar muscles. It entered the superficial head of flexor pollicis brevis and then supplied the other thenar muscles. The usual ‘recurrent' branch of the median nerve was absent, but the other branches of the median nerve were as usual. (See figure)

Discussion:

In the literature, although there are reports of the median nerve dividing in the distal forearm (Kessler; 1969; Schultz et al, 1973; Crandall & Hamel 1979) none of these mention a branch entering thenar muscles directly from the distal part of the forearm. Papathanassiou (1968) reported the motor branch arising high up in the carpal tunnel and piercing the flexor retinaculum to enter the thenar muscles. Although Bergman et al (1988) mention the possibility of the forearm origin of the thenar branch, they term it, rather erroneously, the ‘recurrent' branch itself. Tubiana (1981) mentions that rarely there may be a proximal branch running into the thenar muscles from the median nerve, before the latter enters the carpal tunnel.

The variation described is significant in many ways. In such a person, there is sparing of the thenar muscles in carpal tunnel syndrome. In surgery of this region, e.g. carpal tunnel decompression, the possibility of such a nerve is to be remembered. Such a nerve may also be injured in incised wounds of the wirst region or a trauma like a fall on the outstretched hand.

References:

  1. Anson B.J. Morris' Human Anatomy. 2nd Edn. McGraw Hill, New York, (1966).
  2. Bergman, Thompson, Affif et al Compendium of Human Anatomic Variation. Urban and Schwarzenberg, Baltimore-Munich, (1988).
  3. Crandall R.C.; Hamel A.L. (1979); Bipartite median nerve at the wrist. Journal of Bone & Joint Surgery 61:311.
  4. Hollinshead W.H.: Anatomy for Surgeons. Vol.3, 3rd Edn. Harper and Row, Philadelphia, (1982).
  5. Kessler 1. (1969) Unusual distribution of the median nerve at the wrist. Clinical orthopedics 67: 124-6.
  6. Papathanassior, B.T. (1968) A variant of the motor branch of the median nerve in the hand Journal of Bone & Joint Surgery 50B : 156-7.
  7. Romanes G.J. (Ed.): Cunningham's Testbook of Anatomy, 11th Edn. Oxford University Press, London, (1972).
  8. Schultz R.J., Endler P.M. and Huddleston H.D. (1973): Anomalous median nerve and an anomalous muscle belly of the first lumbrical associated with carpal tunnel syndrome. Journal of Bone & Joint Surgen 55: 1744-46.
  9. Spinner M (Ed.): Kaplan's Functional and Surgical Anatomy of the Hand. 3rd Edn I.B. Lippincot Co., Philadelphia, (1984).
  10. Tubiana R.: The Hand. Vols. 1, 2 and 3 W.B. Saunders, Philadelphia (1981).
  11. Thorek P.: Anatomy in Surgery, 2nd Edn. J.B. Lippincot, Co. Philadelphia. (1962).
  12. Williams P.L., Warwick R. et al Gray's Anatomy. 37th Edn. Churchill Livingstone, London (1993).

J. Anat. Soc. India 50(1) 45 (2001) Opp. 45 Unusual Nerve to Thenar Muscles

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Fig.1. An Unusual Origin of Nerve to Thenar Muscles

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