Median and ulnar nerve intercommunications are very common.[1-3] The vast majority of the individuals with these anastomoses are asymptomatic and go about their daily routines without ever being aware of them. Most of the times, these anastomoses are incidentally detected during cadaveric studies.[4-11] Moreover, some of these nerve intercommunications can be incidentally noticed during surgery.[12] A number of research works on nerve conduction studies (NCS) have also demonstrated that these anomalous anastomoses can be incidentally detected during routine NCS.[9,13-16]

It has been estimated that up to 91% of upper limbs can have interconnections between the ulnar and median nerves.[3]

On average, there are about 2.8 anastomotic connections involving the median and ulnar nerves per upper limb.[3]

It has been suggested that, in the presence of conduction blocks of upper limb nerves, these intercommunications can provide an alternative route for the nerve fibers to supply the hand muscles.[17]

These anastomoses are clinically important and have resulted in misdiagnosis during the assessment of nerve injuries[18] and leprosy and entrapment neuropathies.[19-24]

Clinically, the presence of these anastomoses in the forearm and hand has been associated with anomalous innervations of muscle or skin resulting in inconsistent NCS findings observed on electrophysiological studies.[21,25-27]

There are also reports suggesting that these anastomoses can be a major factor in inducing complications during surgical procedures.[28]

Intercommunications between the median and ulnar in the upper limbs are of four predominant types. These include two in the forearm and two in the palm. They are presented in Table 1.

These intercommunications can lead to misleading clinical findings and misinterpretation of NCS; hence, knowledge of such anastomoses can facilitate in proper diagnosis and management and further prevent any iatrogenic injury.

Nerve conduction study findings of these four median-ulnar intercommunications have been elaborated on this paper.

The MGA anastomosis arises from the median nerve proximally and joins the ulnar nerve distally in the forearm region.[2,3] It was first described by Martin in 1763 and later on by Gruber in 1870, hence the name MGA.[29]

It is estimated that MGA has a prevalence of 19.5%.[2] It is believed that MGA is most often unilateral and present on the right side.[1,30] When MGA is bilateral, it has been linked to Down’s syndrome, which is trisomy of the 21^st chromosome.[2,31]

Crutchfield and Gutmann suggested that MGA can have an autosomal dominant pattern of inheritance.[32]

Anatomically speaking, in the majority of the cases, the communicating nerve arises from the anterior interosseous nerve (branch of the median nerve), then runs obliquely, and superficially to the muscles of flexor digitorum superficialis or flexor digitorum profundus. It then runs posterior to the ulnar artery before joining the ulnar nerve[2] [Figure 1].

Oh have classified MGA broadly into Three types, Type 1, Type 2, and Type 3.[3,33]

In Type 1, the communicating branch carries motor nerve fibers from the median nerve to ulnar nerve resulting in the innervation of hypothenar muscles. This type has an estimated prevalence of 17.3%. Here, on NCS, on recording from abductor digiti minimi (ADM) muscle, and stimulating ulnar nerve, compound muscle action potential (CMAP) recorded would be greater on proximal (elbow) stimulation compared to distal (wrist) stimulation.[3,33]

In Type 2, the communicating branch carries motor nerve fibers from the median nerve to ulnar nerve leading to innervation of first dorsal interosseous (FDI) muscle. This is the most common type and has a prevalence of about 31%. On NCS, while recording over FDI muscle and stimulating the median nerve, CMAP recorded would be greater on proximal (elbow) stimulation compared to distal (wrist) stimulation.[3,33]

Type 3 is the least common among the three and has a prevalence of 0.01%. Here, the communicating branch carries motor nerve fibers from the median nerve to ulnar nerve and results in the innervation of the thenar muscles. Here, on NCS, while recording over abductor pollicis brevis (APB) and stimulating the median nerve, CMAP obtained would be greater on proximal (elbow) stimulation compared to distal (wrist) stimulation.[33]

The features of the three types of MGA are summarized in Table 2.

RCA represents a neural intercommunication arising from a deep branch of ulnar nerve and joining recurrent branch of the median nerve. This anastomotic branch is seen in palm and results in variable ulnar innervation of muscles of palm[1-3] [Figure 3].

Sometimes through this anastomosis, ulnar nerve can end up supplying all the intrinsic muscles of palm. This condition has been referred to as ulnar hand.[37] In some cases, intrinsic muscles of the hand can have dual innervation by both median and ulnar nerves.[37]

RCA was described independently by Riche and Cannieu in 1897 from cadaveric studies, hence the name RCA.[1] It has a high prevalence of 55.5%.[1] It is believed that RCA has an autosomal dominant pattern of inheritance and usually occurs in combination with other median-ulnar intercommunications.[3,38]

Electrophysiological studies have shown that in patients with mild carpal tunnel syndrome (CTS), typical findings consistent with RCA can be misinterpreted as severe CTS. This, in turn, can result in unnecessary surgical interventions. In patients with prolonged median sensory and motor latency suggestive of CTS, the presence of RCA should be suspected if median motor amplitudes are worse than median sensory amplitudes.[39]

The various aspects of RCA are highlighted in Table 4.

Familiarity of these various nerve anastomoses involving median and ulnar nerves in upper limbs is essential to prevent misdiagnosis, iatrogenic injuries, and misinterpretation of NCS findings.

All figures in the manuscript were hand made by Dr. Naveen Ravi and Dr. Santosh Wakode.