Working Diagnosis:
Idiopathic lumbosacral plexus neuropathy affecting the left femoral nerve (L3 and L4 nerve roots)

Treatment:
He was referred to physical therapy to address his muscle weakness. He was treated with gabapentin, which led to improvement in his neuropathic pain and allodynia. He declined use of high dose steroids.

Outcome:
He had slow, steady improvement in his strength and sensation in the left lower extremity. He was able to return to ambulating without a cane but with some residual weakness and numbness. There has been no evidence of relapse or involvement of other nerves. He has returned to work full time.

Author's Comments:
Idiopathic Lumbosacral Plexus Neuropathy is one of a rare group of disorders of the peripheral nervous system. It is clinically similar to diabetic amyotrophy.
Clinical features include acute, asymmetric, focal onset of pain followed by weakness involving the proximal leg. Symptoms typically progressively improve over months with partial to full recovery in most patients. Patients may have persistent neuropathic pain and weakness. It can become more widespread and symmetric with time, progressing to affect the contralateral limb. It may be associated with autonomic symptoms and weight loss as well.
The diagnosis is based on clinic features and confirmed with EMG/NCS findings and/or neuroimaging.
With regards to treatment, studies have shown benefit with steroids, IVIG, plasmapheresis, and cyclophosphamide. However, the focus of treatment is primarily on symptomatic management.
(1-3)

Editor's Comments:
Lumbosacral plexus syndromes represent a less common but important cause of peripheral nerve syndromes. A thorough understanding of the lumbosacral plexus and resultant peripheral nerves helps identify these disorders. The pattern of motor involvement can help identify the location of the lesion - lesions in the lumbar plexus tend to cause weakness with hip flexion and adduction as well as knee extension. Lesions in the upper sacral plexus may cause foot drop as well as weakness with knee flexion or hip abduction.

Diagnostic work up includes the labs mentioned in the above case. A lumbar puncture for evaluation of CSF can be useful in the setting of negative lab work. MRI with contrast is the preferred imaging study for evaluation of the lumbosacral plexus. EMG/NCS can help distinguish lumbosacral plexus syndrome from a pure lumbar radiculopathy as well as help identify the location of the lesion. It can also provide information on the severity of nerve involvement.

Causes of lumbosacral plexus syndromes include diabetic amyotropathy, idiopathic lumbosacral radiculoplexus neuropathy, neoplastic invasion, radiation plexopathy, pelvic trauma, peripartum plexopathy, postoperative plexopathy, vascular causes (ischemia, hemorrhage, direct compression), infectious, inflammatory, or infiltrative processes.

Treatment is focused on the cause identified based on a thorough work up. In the case of idiopathic lumbosacral plexus neuropathy, symptomatic treatment focused on neuropathic pain control is important. Medications such as tricyclic antidepressants, gabapentin, pregabalin, duloxetine, and venlafaxine may be appropriate. Physical and occupational therapy can help with gait re-training and muscular strengthening.

Some patients will have persistent symptoms including sensory disturbances and muscle weakness despite appropriate treatment. Recurrent bouts of symptoms can occur. (1,4)

References:
1. Twydell, PT. Diabetic amyotrophy and idiopathic lumbosacral radiculoplexus neuropathy. Updated Apr 15, 2019. Accessed from UpToDate: https://www.uptodate.com/contents/diabetic-amyotrophy-and-idiopathic-lumbosacral-radiculoplexus-neuropathy.
2. Dyck, PJB, Thaisetthawatkul, P. (2014). Lumbosacral plexopathy. CONTINUUM Lifelong Learning in Neurology, 20(5), 1343-1358. https://doi.org/10.1212/01.CON.0000455877.60932.d3
3. Van Eijk J, Chan YC, Russell JW. Immunotherapy for idiopathic lumbosacral plexopathy. Cochrane Database of Systematic Reviews 2013, Issue 12. Art. No.: CD009722. DOI: 10.1002/14651858.CD009722.pub2.
4. Twydell, PT. Lumbosacral plexus syndromes. UpToDate. 17 Aug 2019.

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