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S1 Nerve Root Examination | OrthoFixar 2024

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S1 Nerve Root Examination

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S1 Nerve Root, a part of sacral plexus, exits between S1 and S2 vertebra. It is responsible for plantar flexion, ankle eversion and hip extension.

Sacral Nerve Roots anatomy
Sacral Nerve Roots anatomy
See Also: Lumbar Spine Nerve Roots

S1 Nerve Root Examination

Primarily plantar flexion is done by gastrosoleus complex, with assistance from toe flexors. Plantar flexion can be screened by asking the patient to toe walk.

Assessing S1 motor function - toe walking
Assessing S1 Nerve Root motor function – toe walking

With the patient seated by the side of the table with his knees flexed, the examiner stabilizes the patient’s ankle with one hand and instructs the patient to passively plantar flex the ankle. The patient is told to maintain this position while the examiner attempts to force the ankle back into dorsiflexion by pressing upward on the patient’s forefoot with the examiner’s other hand.

Assessing S1 motor function -gastrosoleus
Assessing S1 Nerve Root motor function -gastrosoleus

The main evertors of the foot are peroneus longus and brevis. The examiner stabilises the patient’s leg with one hand and asks the patient to rotate the foot outward. The examiner may have to passively place the patient’s foot in eversion to communicate the desired position. The patient is then instructed to maintain the foot in the everted position while the examiner attempts to invert the foot by pressing inward on the lateral aspect of the foot.

Assessing S1 motor function - peromeus longus and brevis
Assessing S1 Nerve Root motor function – peromeus longus and brevis

The gluteus maximus is the main extensor of the hip. To test it, the patient is asked to lie prone on the examination table and to flex the knee on the side being tested. The patient is then instructed to raise the thigh off the table. Finally, the examiner presses downward on the thigh with both hands while asking the patient to maintain the position of hip extension. In a normal patient, the examiner is able to overcome the patient’s effort with some difficulty.

Assessing S1 motor function - gluteus maximus
Assessing S1 Nerve Root motor function – gluteus maximus

S2, S3 and S4 Nerve Roots

S2, S3 and S4 Nerve Roots are the principal supply for the bladder and they also supply intrinsic muscles of the feet.

The motor function of the sacral nerve roots is, therefore, usually tested by performing a rectal examination. When normal function is present, the examiner should note fairly firm resistance as the examining finger enters the rectum. The patient is then instructed to try to squeeze the examiner’s finger, thus contracting the external anal sphincter. This should produce a strong, readily palpable feeling of constriction around the examiner’s finger.

Sensory Examination

The approximate areas of sensory innervations from the lumbar and sacral nerve roots are shown in the figure.

spinal nerve dermatomes

Reflex Examination

Achilles’ Tendon Reflex (S1):

With the patient by the side of the table with the legs dangling, the examiner gently dorsiflexes the foot to place the Achilles tendon under tension, and then strikes the Achilles tendon 3 cm above the calcaneum.

In a normal patient, this produces visible plantar flexion of the ankle and contraction of the gastrosoleus. Undue briskness of the Patellar and Achilles tendon reflex should raise the suspicion of an upper motor lesion.

Achilles tendon reflex
Achilles tendon reflex

Anal Reflex:

The examiner tests the superficial anal flex by touching the perianal skin with a pointed object. A normal result is shown by contraction of the anal sphincter muscles (S1–S4).

References

  1. Berry JA, Elia C, Saini HS, Miulli DE. A Review of Lumbar Radiculopathy, Diagnosis, and Treatment. Cureus. 2019 Oct 17;11(10):e5934. doi: 10.7759/cureus.5934. PMID: 31788391; PMCID: PMC6858271.
  2. Figliuzzi A, Alvarez R, Al-Dhahir MA. Achilles Reflex. [Updated 2021 Jul 31]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459229/
  3. Previnaire JG, Alexander M. The sacral exam-what is needed to best care for our patients? Spinal Cord Ser Cases. 2020 Jan 7;6:3. doi: 10.1038/s41394-019-0252-2. PMID: 31934355; PMCID: PMC6946808.
  4. Sarvdeep S. Dhatt, Sharad Prabhakar – Handbook of Clinical Examination in Orthopedics. An Illustrated Guide-Springer Singapore.
  5. Dutton’s Orthopaedic Examination, Evaluation, And Intervention 3rd Edition.
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