Abstract
Many causes of fibular (peroneal) neuropathy have been identified in the literature making it the most common entrapment neuropathy of the lower extremity. As the anatomical pathway is reviewed, one can appreciate the vulnerabilities along the course of the nerve. From a functional standpoint, foot drop is the most common manifestation. This chapter will include an approach to the physical evaluation to distinguish between a fibular neuropathy and those of a more proximal nature, through electrodiagnostic assessment and imaging. Rehabilitation commonly consists of the use of an ankle foot orthosis, and a discussion of when to consider this intervention will aid in clinical decision-making and management. Additionally, available surgical options will be reviewed when the expected clinical and functional improvement does not occur with conservative treatment.
Keywords
fibular mononeuropathy, fibular (peroneal) palsy, footdrop palsy, lateral popliteal neuropathy, peroneal mononeuropathy
| Synonyms | |
| |
| ICD-10 Codes | |
| G62.9 | Polyneuropathy, unspecified |
| G57.90 | Mononeuropathy of unspecified lower limb |
| G57.91 | Mononeuropathy of right lower limb |
| G57.92 | Mononeuropathy of left lower limb |
| M21.371 | Foot drop, right foot |
| M21.372 | Foot drop, left foot |
| M21.379 | Foot drop, unspecified foot |
Definition
Fibular (peroneal) neuropathy, the most common entrapment neuropathy of the lower extremity, is a compromise of any portion of the fibular nerve. This can be from its origin within the sciatic nerve, in which it remains distinct from the tibial portion, throughout the course of the sciatic nerve, to its termination in the leg and foot. The common fibular nerve completely separates from the tibial nerve in the upper popliteal fossa and then traverses laterally to curve superficially around the fibular head. Before the fibular head, the lateral cutaneous nerve of the calf branches off to supply cutaneous sensation to the upper lateral leg. Near the fibular head, the common fibular nerve bifurcates into the superficial fibular nerve and deep fibular nerve, which describes their relative locations as they wrap around the fibular head. Because the deep portion is immediately adjacent to the hard bony surface, it is more susceptible to compression injuries at the fibular head, which is the most common site of fibular nerve compromise.
The common fibular nerve provides the lateral cutaneous nerve of the calf and the motor branch to the short head of the biceps femoris above the fibular head. The superficial fibular nerve is predominantly sensory, providing cutaneous sensation to the lateral lower leg and most of the dorsum of the foot. The superficial fibular nerve also innervates the foot evertors, peroneus longus and brevis. The deep fibular nerve is predominantly motor, innervating the ankle and toe dorsiflexors, but it has a small cutaneous representation at the first dorsal web space of the foot. A common anatomic variant arising from the superficial fibular (peroneal) nerve is the accessory deep fibular (peroneal) nerve, which has a prevalence of 18.8%, and will give motor supply to the extensor digitorum brevis. Predisposing factors for fibular mononeuropathy at the fibular head, which is the most common site of compromise, include weight loss, diabetes, peripheral polyneuropathy, positioning, and localized prolonged pressure, (e.g., habitual leg crossing or prolonged squatting). A history of such sustained compression should be elicited.
Fig. 75.1 shows the most common causes of acute and nonacute lesions. Iatrogenic causes include anesthesia for surgery, leading to immobility and possible positioning issues ; surgery about the hip, knee, or ankle ; prolonged bed rest with decreased sensorium due to sepsis or coma ; compression and casting and, ironically, ankle-foot orthoses. A history of severe inversion ankle sprain or blunt trauma to the ankle, leg, or fibular head can be helpful in identifying likely pathophysiologic mechanisms.
Stretch injury commonly occurs at the hip region and may be associated with hip surgery (e.g., total hip arthroplasty, especially if the limb is lengthened) or traumatic hip dislocation. The fibular portion of the sciatic nerve is more susceptible to stretch injuries than the tibial portion because of its lateral position, and the shorter distance between the piriformis and the fibular head than between the piriformis and the tarsal tunnel (i.e., the sites of relative fixation of these two nerves). Distal superficial fibular nerve stretch injury can also occur at the point where it passes through the peroneus longus muscle. In addition, fibular nerve injury occurs after stroke by equinovarus footdrop posturing.
Symptoms
Fibular neuropathy typically is manifested with acute footdrop, but this can sometimes occur insidiously over the course of several days to weeks. This footdrop can be complete or partial, often with increased tripping, stumbling, or falls as the primary complaint. Numbness or dysesthesias frequently occur in the lower lateral leg and dorsum of the foot, while pain is uncommon. When pain is present, it is usually localized around the knee and noted as deep and ill-defined. Also, when pain is prominent and neuropathic in character, then a stretch injury of the fibular portion of the sciatic nerve should be considered.
Physical Examination
The examination should be guided by a close understanding of the relevant anatomy, with focused study of the elements of each component of the fibular nerve.
Sensory deficits in the upper lateral leg ( Fig. 75.2 ) suggest a lesion proximal to the fibular head. Testing of foot inversion, to rule out concomitant tibial nerve compromise and therefore sciatic nerve as the likely site of injury, must be performed with the foot passively slightly dorsiflexed for optimal strength testing since inversion is normally weak when the foot is relatively plantar flexed. At rest, the foot will typically be in a plantar flexed position during the examination due to the existing footdrop. With the long head of the biceps femoris intact, knee flexion strength will appear normal despite a weakened short head of the biceps femoris. Palpation may reveal a lack of tissue tensing where the short head of the biceps femoris should be located. However, this is challenging to discern and helpful only in the case of acute complete proximal fibular nerve injury with relative sparing of the tibial-innervated hamstring muscles. The function and innervation of the long and short heads of the biceps femoris can be more accurately determined with electrodiagnostic testing. If both are compromised, knee flexion will be weak, as will plantar flexion and toe flexion, suggesting a sciatic nerve lesion. Hip abduction strength testing has been found helpful in distinguishing fibular neuropathy from L5 radiculopathy in patients with footdrop. Muscle stretch reflexes will usually be normal unless the sciatic nerve is severely compromised, in which case the medial hamstring and Achilles reflexes could be reduced or absent.
Sensory deficit or dysesthesia in the lower lateral leg and over most of the dorsum of the foot suggests involvement of the superficial fibular nerve or this portion of the sciatic nerve (see Fig. 75.2 ). Foot eversion weakness is consistent with superficial fibular nerve compromise. If the superficial fibular nerve lesion is isolated, then Achilles, quadriceps, and medial hamstring muscle stretch reflexes will be normal.
If eversion is strong but dorsiflexion is very weak, a focal deep fibular nerve compromise is suggested. There may be sensory deficits or dysesthesias along the isolated area of the dorsum of the first web space of the foot on the affected side ( Fig. 75.3 ). A combination of deep and superficial fibular nerve branch compromise often occurs, usually affecting the deep branch more severely than the superficial branch, especially with lesions at the fibular head.
