Rostral brachial plexus: Shoulders
Caudal brachial plexus: Hands
Lumbosacral plexus: Legs

Plexus disorders (plexopathies) are usually due to physical compression or injury:

In infants, traction during birth
In adults, usually trauma (typically, for the brachial plexus, a fall that forces the head away from the shoulder) or invasion by metastatic cancer (typically, breast or lung cancer for the brachial plexus and intestinal or genitourinary tumors for the lumbosacral plexus)

In patients receiving anticoagulants, a hematoma may compress the lumbosacral plexus. Neurofibromatosis occasionally involves a plexus. Other causes include postradiation fibrosis (eg, after radiation therapy for breast cancer) and diabetes.
Acute brachial neuritis (neuralgic amyotrophy, Parsonage-Turner syndrome) occurs primarily in men and typically in young adults, although it can occur at any age. Cause is unknown, but viral or immunologic inflammatory processes are suspected.