An Approach to the Examination of the Lumbar Plexus for Neurosurgical Residents: A Video Manuscript

The lumbar plexus provides innervation to the lower limbs and is essential in enabling motor movement and sensation in the lower limbs. Some of its branches also innervate the muscles in the pelvic girdle. Compared to the brachial plexus in the upper limbs, the lumbar plexus appears to garner less recognition among physicians and surgeons. However, it is important to understand the anatomy of the lumbar plexus and its branches along with the innervation they enable, as injury to them can cause plexopathies and pathologies that should be recognised by any treating clinician. Lumbar disc herniation, trauma and entrapment by muscles or hypertrophic ligaments are common causes of lumbar plexus or nerve injuries. A video was produced to demonstrate the examination techniques explained in this article. To provide comprehensive examination of the lower limbs, the sciatic nerve and its branches are also included in the examination video.


Introduction
The examination of the lumbar plexus is underrepresented compared to that of its counterpart, the brachial plexus.This paper will present a new approach to the examination of the lumbar plexus.A link to a video demonstrating this approach is provided at the end of the article.

Anatomy of the Lumbar Plexus
The T12-L4 nerve roots are the main source of the lumbar plexus's branches (1).The lumbar plexus is located in the retroperitoneum, below and inside the psoas muscle.Thus, pathologies within the retroperitoneum, such as haematomas or abscesses, are a common cause of lumbar plexopathy (2).The iliohypogastric (L1), ilioinguinal (L1), genitofemoral (L1-L2) and lateral cutaneous nerves in the thigh (L2-L3) are primarily responding for enabling sensory input in the lower limbs.
The rootlets from L2 to L4 comprise the anterior and posterior divisions of the lumbar plexus.The anterior division forms the obturator nerve, which innervates the adductor muscles in the thigh and the skin in the medial thigh.The posterior division forms the femoral nerve, from which motor branches extend to innervate the psoas muscle, the iliacus muscle and pectineus muscle prior to entering the upper leg.
In the upper leg, the anterior division extends sensory branches to the medial part of the thigh via the medial cutaneous and intermediate cutaneous nerves in the thigh.The posterior division of the femoral nerve then proceeds to supply the quadriceps muscles before producing the terminal branch, called the saphenous nerves, which provide sensory supply to the skin of the medial side of the leg and foot.
The medial and intermediate cutaneous nerves in the thigh and saphenous nerves, respectively, innervate the quadriceps muscle after they pass beneath the inguinal ligament and supply feeling to the medial thigh and lower leg.The obturator nerve is formed by contributions from L2-L4, while the femoral nerve is formed by the posterior division of these contributions.
The medial thigh skin and adductor muscles in the thigh are both innervated by the obturator nerve.The psoas and iliacus muscles are proximally innervated by the femoral nerve.Figure 1 illustrates the nerve roots and its division, while Figure 2 illustrates the branches of the femoral nerve.Table 1 shows the anatomy of the lumbar plexus, the supply, it's innervation and it's clinical correlation.Table 2 will show clinical signs and symptoms in relation to dorsal root ganglia.

Inspection
Both lower limbs must be properly exposed and in a natural physiological position to assess the attitude of the limbs and observe muscle bulk, atrophy, asymmetry, scars and signs of injury (3).Each limb must be compared to the other during every step of the examination.

Muscle Tone
Muscle tone is defined as 'the tension in the relaxed muscle' or 'the resistance, felt by the examiner during passive stretching of a joint when the muscles are at rest' (4).Meanwhile, spasticity was described by Lance JW in 1980 as 'a motor condition characterised by a velocity dependent increase in tonic stretch reflexes (muscle tone) with accentuated tendon jerks.'During assessments of tone, patients need to be relaxed and cooperative.Small talk may help.
Muscle tone can be graded using O'Sullivan's scale:

Reflex
Reflex is an involuntary response to a sensory stimulus.It has afferent and efferent components.i) Cresmatic reflexes (ilioinguinal and genitofemoral nerves, L1-L2 and cutaneous reflex), can be assessed by lightly scratching or pinching the skin on the upper, inner aspect of the thigh.The expected response is a contraction of the cresmatic muscle with a quick elevation of the ipsilateral testicle.This can be absent in elderly males or individuals with hydrocele, torsion, orchitis or epididymitis.
ii) Gluteal reflex (inferior gluteal nerve, L4, S2 and cutaneous reflex) -a contraction of the gluteal maximus muscle after stroking the skin over the buttock.
iii) Patellar reflex (femoral nerve, L3-L4 and deep tendon reflex) -a contraction of the quadriceps resulting in extension of the knee.iv) Adductor reflex (L2-L4 and deep tendon reflex) -with the thigh in slight abduction, tap either the medial epicondyle of the femur in the vicinity of the adductor tubercle or the medial condyle of the tibia, resulting in contraction of the adductor muscle of the thigh and inward movement of the limb.v) Medial hamstring reflex (L5 and deep tendon reflex) -the semitendinosus and semimembranosus muscles are struck; the leg is abducted and slightly externally rotated, and the knee is flexed.The fingers are placed over the tendon on the medial posterior aspect of the knee and tapped with a reflex hammer to observe the contraction of the muscle.

Sensory Examination
Sensation -such as a light touch, pain, temperature, vibration, 2-point discrimination, kinaesthesia, proprioception or stereognosiscan be assessed by localising the lesion, i.e. at the level of the spinal nerves, spinal cord or cortical.The sensory level then can be graded and charted using the American Spinal Injury Association (ASIA) chart (7) according to the relevant dermatome, refer to Figure 3.Moreover, in the examination of peripheral nerves, stocking-glove distribution also needs to be examined, especially in cases of diabetic neuropathy.

Autonomic System
The lumbar plexus is partly involved in bladder function.It supplies the internal urethral sphincter at the neck of the bladder.It is supplied by the intermediolateral column at the T12-1L1 level via the sympathetic prevertebral plexus and hypogastric nerve.If any lesion affects the sympathetic ganglia, the function of this muscle will be affected, causing urinary loss (1).Any reduced sweating of the nerve distribution also needs to be taken into consideration during examination.
Together with this article, attached the link to the video of examination of lumbar plexus examination, which lasted for 15 min.YouTube link for lumbar plexus examination: https://youtu.be/2IdLe2i-Zxc

Conclusion
We hope that this comprehensive evaluation of the lumbar plexus will be helpful in localising and identifying the extent of injuries that patients may have.

Figure 1 .Figure 2 .
Figure 1.The lumbar plexus Patient lies on face and flexes leg at the knee against resistance (continued on next page) Patient lies on back with leg extended and plantar-flexes foot against resistance To detect slight weakness, the patient should be asked to stand on one foot, raise the heel from the ground and maintain the position Soleus Tibial nerve (S1 and S2) Plantar-flexes foot Patient lies on back with limb flexed at the hip and knee and plantar-flexes the foot

Table 1 .
Anatomy of lumbar plexus and its clinical correlations

Table 2 .
Clinical signs and symptoms in relation to dorsal root ganglia

Table 3 .
Motor examination of lumbar plexus