|Type||Link nerve, anatomical variation.|
|From||Arises at L4 most commonly|
|To||Links the lumbar and sacral plexuses. Its fibres branch out to be part of femoral and obturator nerves, in addition to the lumbosacral trunk|
The furcal nerve (furcal = fork) is an independent nerve that forms a link between the lumbar and sacral plexuses. It has its own ventral and dorsal branches (rootlets). It is most commonly found at the L4 level. Atypical presentations of sciatica are common. They may be due to the patient having two lesions, variations of cutaneous supply of the dorsal rami, nerve root anomalies, and the presence of a furcal nerve. The furcal nerve is the most common cause of double nerve root contribution in unilateral radiculopathies.
The lumbar plexus is formed by the ventral and dorsal rami of L1-L4 nerve roots, with a contribution from the subcostal nerve. It is located within the psoas muscle anterior to the transverse process. The lumbosacral trunk is formed with a branch from the ventral ramus of L4 with the ventral ramus of L5. The sacral plexus is formed by the lumbosacral trunk and anterior divisions of the first to third sacral nerves. The plexus can be pre-fixed (L3 contribution) or postfixed (L5 contribution).
The furcal nerve is an independent nerve, and it has its own ventral and dorsal rootlets. It is most commonly found to arise at the L4 level (93%). It tends to be found superior and ventral to the L4 nerve roots, and runs beside it in the intervertebral foramen. The L3 level is the second most common site. Double furcal nerves exist but are rare. They may be part of conjoined nerve roots, and combined with intradural anomalies.
The furcal nerve connects the lumbar and sacral plexuses. It's dorsal root has a separate dorsal root ganglion. It supplies branches to the femoral nerve, lumbosacral trunk, and obturator nerve
There are two sub-types of furcal nerve - the intra-foraminal, and extra-foraminal variants. The majority bifurcate extra-foraminally.
The furcal nerve causes disparity between imaging and clinical presentations. The patient may have motor weakness and sensory disturbance at levels not explained by standard imaging. Transitional anatomy if present, can also contribute to variable clinical presentations.
Diagnostic single nerve root block can help with diagnosis. Meticulous reading the the imaging is required.
See Harshavardhana et al for an open access review of the topic.