Abnormal Reflexes: What Brisk or Absent Reflexes Mean

How reflexes work

A deep tendon reflex is a simple circuit. When a doctor taps a tendon, the muscle stretches. A sensory nerve carries the signal to the spinal cord, which sends a signal straight back through a motor nerve, making the muscle contract. This happens without involving the brain. But the brain constantly dampens these reflexes through pathways in the spinal cord.

That arrangement explains the two main patterns:

• Damage to the reflex arc (sensory nerve, nerve root, motor nerve or muscle) weakens or abolishes the reflex.
• Damage to the brain or spinal cord above the reflex removes the dampening, so reflexes become exaggerated.

Doctors grade reflexes from 0 (absent) to 4+ (very brisk, with clonus), where 2+ is normal.

Upper vs lower motor neuron patterns

The Babinski sign is when stroking the sole of the foot makes the big toe extend upward instead of curling down. It is normal in babies up to about 1 to 2 years, but in older children and adults it signals a brain or spinal cord problem.

Causes of brisk reflexes (hyperreflexia)

• Stroke: sudden one-sided weakness with brisk reflexes on that side (often after the first few days).
• Multiple sclerosis: episodes of numbness, vision loss or weakness, with brisk reflexes.
• Spinal cord compression: from neck arthritis (cervical myelopathy), a disc, tumor or injury. Clumsy hands, unsteady walking and brisk reflexes below the level of compression.
• ALS (motor neuron disease): a mix of brisk reflexes with muscle wasting and twitching.
• Brain tumors, cerebral palsy, and head injury.
• Overactive thyroid, anxiety, and stimulant use: symmetrical brisk reflexes without other neurological signs.
• Serotonin syndrome: very brisk reflexes and clonus, agitation and fever after serotonin-raising medicines.
• Pre-eclampsia: brisk reflexes in late pregnancy with high blood pressure and headache.

Causes of reduced or absent reflexes (hyporeflexia)

• Peripheral neuropathy: ankle reflexes are lost first, usually with numbness or burning in the feet. Diabetes, alcohol and vitamin B12 deficiency are common causes.
• Nerve root compression: a herniated disc in the lower back can reduce one ankle or knee reflex, with sciatica.
• Guillain-Barre syndrome: rapidly progressive weakness with absent reflexes, often after an infection.
• Underactive thyroid: reflexes relax slowly ("hung-up" reflexes).
• Early spinal cord injury (spinal shock): reflexes are absent at first, then become brisk weeks later.
• Muscle diseases in later stages, and low potassium or high magnesium levels.
• Aging: ankle reflexes are often reduced or absent in healthy people over 70.

How doctors investigate abnormal reflexes

Reflexes are interpreted alongside the rest of the neurological exam: strength, tone, sensation, coordination and gait. The doctor asks about weakness, numbness, bladder and bowel symptoms, pain, and how quickly symptoms developed.

Tests depend on the pattern:

• MRI of the brain or spine: for upper motor neuron signs.
• Nerve conduction studies and EMG: for suspected neuropathy, nerve root or muscle problems.
• Blood tests: blood sugar, vitamin B12, thyroid function, electrolytes, kidney and liver function.
• Lumbar puncture: in suspected Guillain-Barre syndrome or multiple sclerosis.

Treatment

Treatment targets the underlying cause: urgent stroke treatment, surgery to relieve spinal cord or nerve root compression, disease-modifying therapy for MS, immunotherapy for Guillain-Barre syndrome, thyroid hormone replacement, or controlling diabetes and correcting vitamin deficiencies. Physical therapy helps with weakness and spasticity, and medicines can reduce muscle stiffness.

Frequently asked questions

What do abnormal reflexes mean?

They suggest that something is affecting the nerve pathways. Brisk reflexes usually point to the brain or spinal cord, and weak or absent reflexes point to nerves, nerve roots or muscles. Mild changes on both sides with no other symptoms are often normal.

Can anxiety cause brisk reflexes?

Yes. Anxiety, an overactive thyroid and stimulants such as caffeine can make reflexes symmetrically brisk. This is not a concern if the reflexes are equal on both sides, there is no clonus or Babinski sign, and the rest of the exam is normal.

Is it normal to have no knee reflex?

Some healthy people have reflexes that are hard to obtain, and ankle reflexes fade with age. Doctors use reinforcement, such as clenching the teeth or hooking the fingers and pulling, to bring them out. Reflexes that are absent along with numbness or weakness need investigation.

What is clonus?

Clonus is a series of rhythmic, involuntary jerks when a muscle is suddenly stretched, most often tested at the ankle. A few beats can be normal, but sustained clonus usually indicates a brain or spinal cord problem.