Proximal median mononeuropathies, unrelated to direct trauma or external compression, are unusual in neurophysiological practice; several syndromes have been more commonly described including the anterior interosseus syndrome, the pronator teres syndrome, and entrapments at the lacertus fibrosus or at the ligament of Struthers.1

We report the case of a 29 year old woman with an upper median mononeuropathy at an unusual lesional site. She started a new job consisting of repeatedly lifting heavy crates full of apples from the ground, carrying them for a while, and then putting them on a shelf. Fifteen days later, at the end of a working day, she began to complain of right hand weakness with numbness and tingling limited to the first, second, and third fingers; 4 weeks after the onset of symptoms neurological examination showed marked weakness of thumb abduction and opposition, forearm pronation, and flexion of the wrist and of the distal phalanges of the thumb and index finger; she had paraesthesias on the palmar side of the right thumb, index, and middle fingers without spontaneous pain, which could otherwise be strongly evoked by deep palpation of the median nerve trunk at the lower third of the right upper arm. The patient denied any kind of trauma or prolonged direct compression on that limb.

Electromyography showed fibrillation potentials and neurogenic motor units recruitment in all forearm right median innervated muscles, including the pronator teres; in thenar median innervated muscles abundant fasciculations and severely reduced recruitment patterns were the major findings, with minimal denervation; on nerve conduction studies the distal right median cMAP had a latency of 4 ms and a positive to negative peak amplitude of 25 mV; antidromic SNAP recorded at the third finger had an onset latency of 3.6 ms and a positive to negative peak amplitude of 70 μV; forearm median motor and sensory conduction velocities were 54 m/s and 60 m/s respectively; stimulation of the median nerve at more proximal sites disclosed a partial motor (fig 1 (A)) and complete sensory (fig 1 (B)) conduction block in the distal third of the upper arm; an inching technique allowed better localisation of the site of conduction block about 9 cm proximal to the elbow along the median nerve course. The rest of the neurophysiological study, performed in four limbs, was normal. Routine blood chemistry was normal, including tests for diabetes, thyroid function, and vasculitis. A study of DNA excluded the presence of the deletion at chromosome 17p11.2-p12.

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Figure 1

Neurophysiological study: (A) motor and (B) sensory inching of the right median nerve across the site of conduction block; the drop of motor and sensory potentials at latencies 9.8 and 8.3 ms respectively corresponded to a stimulation site located along the nerve course about 9 cm proximal to a line between the biceps tendon and the medial epicondyle; recordings from the abductor brevis pollicis muscle by surface electrodes in (A) and from the third finger by ring electrodes in (B).

Radiography of the right humerus ruled out a supracondylar process; an MRI study of the right upper arm using T1 weighted axial images along the nerve course showed the median nerve to be normally recognisable within the arm neurovascular bundle above the lesional site (fig 2 A); following the nerve course in more distal sections the nerve seemed to separate from the bundle as it approached the belly of the brachialis muscle (fig 2 B). In the subsequent more distal sections the nerve was no longer recognisable within the image of the brachialis muscle belly (fig 2 C). At surgical exploration of the right median nerve, the proximal segment seemed to be located within the neurovascular bundle in a deep inferolateral position. At the transition between the middle and the distal third of the arm, where the basilic vein joined the humeral vein, the nerve was found to turn deeply through the brachialis muscle belly from which it emerged about 5 cm more distal. Muscle fibres covering the nerve were sectioned allowing appraisal of a consistent reduction of the size of the nerve throughout its intramuscular course. No other abnormalities were detected along the nerve until it entered the antecubital fossa (fig 2C).

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Figure 2

Anatomical study. Top: T1 weighted MR axial images of the distal right upper arm across the lesional site; (A): proximal; (B): intermediate; (C): distal section. Left: surgical findings (D); brachialis muscle fibres covering the median nerve have been sectioned; note the anomalous deep course of the median nerve trunk (small white arrow) apart from the vascular bundle (large white arrow) and through the brachialis muscle belly (black arrow). See text for details.

Neurolysis was followed by clinical and neurophysiological improvement. Fifteen days after surgery hand function improved; 2 years later there was no weakness and EMG was consistent with a good reinnervation process.

Our patient presented a subacute upper median mononeuropathy in the distal third of the upper arm associated with an anomalous intramuscular course through the brachialis muscle; this was documented by neurophysiological investigation, imaging studies, and surgical exploration. To our knowledge this is the first report of a median nerve lesion at that site associated with such anatomical findings.

Anatomical variations involving the course or the structure of muscles and nerves and their relations are reported as not uncommon in cadaveric dissection studies of the upper limbs2; as these anatomical variations are rarely associated with peripheral nerve lesions, their pathophysiological relation remains somehow unclear.

For the proximal upper arm Bellmann and Vellander3described an isolated case of median nerve compression at its origin by an anomalous muscle band originating from the interval between the subscapularis and latissimus dorsi muscles and crossing the neurovascular bundle to reach the anterior surface of the humerus.

In the distal upper arm and elbow region the presence of a supracondilar ligament and the variable relation of the median nerve with the heads of the pronator teres muscle are indeed the most often reported situations predisposing to an entrapment.

A few authors have described even more uncommon anatomical variations at necropsy, discussing their potential role in causing a focal median nerve lesion. Dharap4 described a case showing an anomalous muscle arising from the humerus and crossing the median nerve and the brachial artery to blend with the common origin of the forearm flexor muscles. Nakatani et al 5reported a case in which a muscle slip from an anomalous four headed biceps brachii muscle joined the posterior fascia of the pronator teres forming a tunnel where the median nerve seemed to be compressed.

At the forearm level the median nerve may be compressed by an anomalous palmaris longus muscle or in the hand by anomalous lumbrical or thenar muscles6; similar cases may involve the ulnar and the superficial radial nerves.

The pathophysiology of this type of lesion is mainly mechanical: a repeated and prolonged compression may develop either directly by the contracting muscle itself or by fixation against more rigid osteoligamentous tissues; the movements performed by our patient consisted in repeated and alternated elbow flexions and extensions under loading: in this setting the contracting muscle could also have reduced or blocked the longitudinal sliding of the nerve along its bed, with subsequent stretching and friction of the nerve trunk.7

In conclusion, this case emphasises that rare anatomical variations should be considered in the differential diagnosis of proximal median mononeuropathies at uncommon lesional sites.