Authors

B.G.M. van Engelen, M.D., Ph.D., Professor of Neuromuscular Diseases
N. van Alfen, M.D., Ph.D.

Clinical features

Lumbosacral plexus neuropathies are relatively rare, and fairly unknown clinical syndromes, which are caused by damage to the nerve bundles in the lumbosacral plexuses.
One of the main causes for a lumbosacral plexus neuropathy is the so-called 'diabetic amyotrophy'. Another cause is the idiopathic form of lumbosacral plexus neuropathy, the lower-extremity counterpart of the more well-known Parsonage-Turner syndrome or neuralgic amyotrophy, which usually affects the brachial plexus(-es). Locally invasive tumour-growth in the pelvic region can be another cause of a lumbosacral plexopathy.

The clinical features of a lumbosacral plexopathy depend on two things: the exact parts of the plexus and nerves that are involved, and the underlying etiology. As localisation of a lesion is the first objective of any neurological evaluation, and from then on a possible etiology can be inferred, the anatomical features will first be discussed.

The lumbosacral plexus (see picture) consists of the interwoven nerves bundles coming from the 3^rd, 4^th and 5^th lumbar nerve roots, and the 1^st, 2^nd and 3^rd sacral nerve roots, with small additions from the 2^nd lumbar and the 4^rd sacral nerve root as well. Actually, the plexus consists of two separate parts, the lumbar and the sacral plexuses, that are connected by the so-called lumbosacral trunk. The lumbar part of the plexus mainly lies embedded between and in the paraspinal quadratus lumborum and psoas muscles, making it vulnerable to local trauma or compression (e.g. by a hematoma). On the other hand, the sacral plexus lies in the pelvis, with the lumbosacral trunk crossing the pelvic brim, making it vulnerable to intrapelvic pathology.

Clinically, the patient's symptoms can be attributed to a plexopathy if they are localised to the peripheral nervous system, but neither fit the distribution of one nerve root or one single peripheral nerve. For this conclusion, a careful and detailed neurological examination is required. If the symptoms are attributable to a plexus lesion, they must be further devided in mainly lumbar, mainly sacral or diffuse or patchy lesions of the whole plexus. This localisation will then aid the question of 'how' did the nerves get damaged (see: differential diagnosis).

For the patient, the symptoms of a lumbosacral plexopathy can be that of any other peripheral nerve lesion: neuropathic (severe, shooting, burning or stabbing) pains, tingling, numbness of certain skin areas or, conversely, areas that are very hypersensitive to the touch, and weakness and wasting of certain muscles in the thigh, buttock and leg region. Usually, when a lesion is acute, pain will be a marked symptom, followed by weakness and wasting. More slowly progressive lesions usually start with sensory symptoms such as numbness and tingling and increasing pain, or sometimes just slowly increasing muscle weakness.

Natural course and prognosis
Not surprisingly, the course and prognosis will largely depend on the underlying disorder causing the lumbosacral plexopathy, and especially on whether a treatable cause can be demonstrated (see Therapy section). Once a fixed plexus deficit has been established, recovery of sensory and motor function can take months to years, depending on the degree of axonal damage and the length new nerve fibers have to grow to reinnervate the affected muscles or skin parts. Unfortunately, complete functional recovery is not always the rule, and many patients remain with functional impairments, hindering daily activities such as walking or cycling.

Epidemiology
There are no data on the incidence or prevalence of lumbosacral plexopathies in the general population. For diabetics, the prevalence of a proximal diabetic neuropathy is estimated at 8 per 1000 (type I and II diabetics combined); the prevalence of type II diabetes alone in the general population is about 3% (± 500.000 people in the Netherlands). In trauma's, the incidence of a lumbosacral plexus lesion with a sacral fracture is about 2%, in other types of pelvic fractures it's about 0.8%. After major gynaecologic pelvic surgery, the incidence of lumbosacral plexus lesions was 0.16% in a series of 1200 patients. The incidence of obstetric lumbosacral trunk lesions is estimated at 1 per 2000 - 6000 deliveries. In coagulopathies, either during anticoagulant therapy or in hemophiliacs, the occurrence of an iliopsoas hematoma compressing the lumbar plexus is rare, but does occur, especially after invasive procedures. Idiopathic lumbosacral plexus neuropathy is a rare disorder, with a few hundred cases known in literature.

Differential diagnosis

The most important aspect in the differential diagnosis is establishing the existence of a plexus lesion, by demonstrating symptoms that cannot be explained by a lesion of a single nerve root or peripheral nerve. However, there are some disorders that can mimic this clinical picture. For example, a lumbar spinal stenosis can cause compressive lesions of multiple nerve roots, which causes pain, sensory symptoms and sometimes paresis in a 'plexus' distribution. Usually, the additional symptoms of lower back pain and provocation of symptoms by certain activities (e.g. walking) give a clue to this diagnosis, and when in doubt, imaging of the lower spine should be performed. Also, an EMG examination will reveal intact sensory conduction in the presence of sensory symptoms, localising the lesion proximal to the spinal sensory ganglion. Another mimic of a lumbosacral plexopathy can be lower-limb motor neuron disease, for example idiopathic (such as ALS) or infectious in endemic areas (e.g. poliomyelitis). A previously undiscovered tethered spinal cord can also present with patchy paresis and sensory symptoms in the legs. Finally, if the patient suffers from a patchy peripheral nerve disorder such as a mononeuritis multiplex, the symptoms can resemble a plexus lesion.

Once the clinical diagnosis of a lumbosacral plexopathy is established, one should consider the following etiological categories (see table):

TABLE: Causes of lumbosacral plexopathy

Vascular:
proximal diabetic neuropathy / diabetic amyotrophy
vasculitis in connective tissue disease (PAN, RA, SLE)

Trauma:
pelvic ring disruption
obstetric complications
pelvic surgery

Compressive:
tumor retroperitoneal or pelvic
abdominal aortic or iliac aneurysm
retroperitoneal hematoma or abscess
invasion of nerves by metastatic tumour growth

Idiopathic:
lumbosacral plexus neuropathy
neuralgic amyotrophy

Hereditary:
hereditary neuralgic amyotrophy (HNA)
hereditary neuropathy with liability to pressure palsies (HNPP)

Infectious:
Herpes simplex (type II) or Herpes zoster
Borrelia

Most frequently, the lumbosacral plexus is damaged: in diabetics, by direct or indirect pelvic trauma, by compression, or obstetric complications. Tumours, aneurysms and idiopathic or hereditary neuropathies are more infrequent causes of a lumbosacral plexopathy. This also means that usually the cause of the plexopathy is relatively obvious or easily diagnosed once the clinical diagnosis has been made. Two less obvious causes of a lumbosacral plexopathy will be discussed below.

Diabetic amyotrophy or proximal diabetic neuropathy is a syndrome which classically occurs in older type II diabetics, who already have some signs of a diabetic symmetric distal polyneuropathy, but not yet an overt vasculopathy. It starts with a subacute, burning neuropathic pain in the leg, thigh, buttock, or lower back, with weakness in the proximal leg muscles, usually mainly in the distribution of the femoral or obturator nerve, and dysesthesias. Some patients suffer from considerable weight loss. The neuropathic pain can last for as long as months, and gradually decreases, while strength returns in months to years. The cause is probably ischemia, induced by a metabolically induced focal microvasculitis [Dyck ea, 2002].

Idiopathic lumbosacral plexus neuropathy is characterized by extreme neuropathic pain at onset, rapidly followed by a patchy paresis and striking atrophy of muscles in the thigh and leg. In about 30%-50% of the patients there are also sensory symptoms such as paresthesias and subsequent hypesthesia in skin regions innervated by the lumbosacral plexus.[van Alfen ea, 1997] Although the name of the disorder implies that all parts of the lumbosacral plexus can be involved (and they sometimes are), the symptoms are usually localized to the lumbar plexus region. The severe pain usually lasts for days to weeks, and recovery of strength can take months to years.

Ancillary investigations

Depending on whether there's an obvious or plausible cause for the lumbosacral plexopathy, there will be a need for limited or more extensive ancillary investigations.

Imaging studies are warranted whenever any structural cause of the plexopathy is suspected.

If there is a direct or indirect trauma to the plexus, imaging of the region by CT or MRI scanning, sometimes by abdominal or pelvic ultrasound, will help to clarify the extent of the lesion and identify treatable causes (such as a hematoma).

An electrophysiological examination, including needle examination (EMG) and nerve conduction studies (ENG) can help to confirm the clinical diagnosis of a plexopathy, and identify the nerve structures involved. ENG will show reductions in the sensory nerve action potential amplitudes in different lumbosacral plexus branches, excluding a purely radicular problem. Motor nerve conduction studies are usually not very informative unless the paresis is severe, in which case the compound motor action potentials will be reduced in affected nerves. ENG can also help in the differential diagnosis by evaluating the presence or absence of pressure palsies, making HNPP as an underlying cause more or less likely. Needle examination will identify neurogenic abnormalities in affected muscles, assess the severity of the lesions, and show if there are signs of recovery. This also makes it an important tool in predicting the functional outcome.

Laboratory investigations or blood test can be helpful if for example a hematoma during anticoagulation therapy is likely, by measuring the bleeding time or INR, or when an auto-immune or infectious cause is suspected. In these cases, a lumbar puncture should also be considered, for identification of an auto-immune or infectious response in the cerebrospinal fluid which could affect the nerve roots.  Ofcourse a blood glucose will help to evaluate the diabetic patient; but the chance that a proximal diabetic neuropathy is the first sign of diabetes mellitus is very small.

If an underlying genetic disorder (HNA or HNPP) is suspected because of a family history with similar signs and symptoms,  a DNA sample can be tested for a PMP22 deletion or point mutation which will identify patients with HNPP. For HNA, DNA testing in individuals is not  yet available.

Therapy

For space-occupying lesions, relieving the compression of the plexus by tumour debulking or irradiation, hematoma evacuation or abscess drainage, should be the first goal of therapy. For diabetic and idiopathic lumbosacral plexopathy, an early treatment with high-dose corticosteroids could be helpful, by mitigating the inflammatory response in the nerves. Ofcourse, when an infectious process is suspected, treatment should be aimed at eradicating that infection, e.g. by antibiotics for Borrelia, or aciclovir in case of a Herpes infection.

If neuropathic pain is present, an attempt should be made to alleviate this, first by a trial of  a long-acting NSAID, such as diclofenac slow-release 100 mg twice a day, combined with a strong opiate such as morphine in a slow-release formula twice or three times a day. If this does not provide adequate relief, the best option is probably the use of a co-analgesic from the group of anti-epileptic drugs, such as gabapentin or carbamazepine. As these are not 'ordinary' painkillers, one needs to carefully instruct patients on their (prolonged) use and side-effects. We start with an increasing dose of gabapentin, until a maintenance dosage of 2400 mg per day (in 2 or 3 gifts) is reached, and evaluate its effect after 4-6 weeks. If the side-effects are acceptable, but the pain not sufficiently suppressed, the dosage is increased to a maximum of 3600 mg per day.

The musculoskeletal pain that can ensue from altered mechanical stability of the hip, knee or ankle joint, is best treated by a combination of physiotherapy for maintaining a normal posture and mobility, and NSAID's if necessary. Some patients find the use of a TENS apparatus helpful. During physiotherapy, there is little sense in pure strength training, as this is a near-impossible task for denervated muscle groups. However, the patient should be encouraged to use the limb as fully as possible. There is no indication that inadvertent overexertion worsens the nerve damage.

For many patients, the advice of a rehabilitation doctor can be very useful in helping them overcome practical difficulties in everyday life, and finding their way back to their work or occupation. Because the recovery is usually slow and gradual, it can take up to 2 or 3 years before the final residual symptoms and impairments can be assessed. This should be taken into account for example, when advising patients on how to deal with occupational disability. In the Netherlands, the first assesment of any permanent occupational disability is made after six months, and after one year it is decided if, and for which part or percentage, the patient is unable to work ('WAO', or 'wettelijk arbeidsongeschikt'). For patients with a fixed lumbosacral plexopathy, the timing of this decision can just be too early.