Anatomy
Lower Limb
A 23 year old horse rider is brought to the ED following a fall during a show jumping competition. She complains of pain in her lumbar spine. On examination you note loss of sensation in the S5 dermatome. The S5 dermatome is best tested at which of the following landmarks:
Answer:
The S4/S5 dermatome is best tested in the perianal area, less than one cm lateral to the mucocutaneous junction.Dermatomal Supply of Lower Limb
Anatomy / Lower Limb / Innervation of Lower Limb
Last Updated: 29th January 2019
Table: Dermatomal Supply of the Lower Limb
| Dermatome | Landmark |
|---|---|
| L1 | Upper Anterior Thigh |
| L2 | Mid Anterior Thigh |
| L3 | Medial Femoral Condyle |
| L4 | Medial Malleolus |
| L5 | Dorsum 3rd MTP Joint |
| S1 | Lateral Heel |
| S2 | Popliteal Fossa |
| S3 | Ischial Tuberosity |
| S5 | Perianal Area |
![By Grant, John Charles Boileau (An atlas of anatomy, / by regions 1962) [Public domain], via Wikimedia Commons](https://mrcemsuccess.com/wp-content/uploads/2016/12/Dermatome-Body-Map-946x1024.png)
Dermatome Map. (Image by Grant, John Charles Boileau (An atlas of anatomy, / by regions 1962) [Public domain], via Wikimedia Commons)
The T12 dermatome is best tested at the midclavicular line, over the midpoint of the inguinal ligament.
The L1 dermatome is best tested on the upper anterior thigh, at a point midway between the key sensory points for T12 and L2.
The L2 dermatome is best tested on the anteromedial thigh, at the midpoint drawn on an imaginary line connecting the midpoint of the inguinal ligament and the medial femoral condyle.
The L3 dermatome is best tested at the medial femoral condyle above the knee.
The L4 dermatome is best tested over the medial malleolus.
The L5 dermatome is best tested on the dorsum of the foot at the third metatarsophalangeal joint.
The S1 dermatome is best tested on the lateral aspect of the calcaneus.
The S2 dermatome is best tested at the midpoint of the popliteal fossa.
The S3 dermatome is best tested over the ischial tuberosity or infragluteal fold (depending on the patient their skin can move up, down or laterally over the ischii).
The S4/S5 dermatome is best tested in the perianal area, less than one cm lateral to the mucocutaneous junction.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
